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Logo: Institut für Meteorologie und Klimatologie/Leibniz Universität Hannover
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Logo: Institut für Meteorologie und Klimatologie/Leibniz Universität Hannover
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Laufende Projekte

Forschungsprojekte

Von der Schallquelle zur psycho-akustischen Bewertung (WEA-Akzeptanz)

 

Leitung:

Günter Groß, Siegfried Raasch

Bearbeitung:

Christoph Knigge

Laufzeit:

2017-2019

Förderung durch:

BMWi

Kurzbeschreibung:

Ziel dieses Projektes ist die Entwicklung eines Gesamtmodells, das die Vorhersage der Schallemission und -ausbreitung einer Windenergieanlage (WEA) bereits vor dem Bau ermöglicht, um Störungen von Anwohnern zu verhindern.

 

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Model-based city planning and application in climate change (MOSAIK)

Bild zum Projekt Model-based city planning and application in climate change (MOSAIK)

Leitung:

Björn Maronga, Günter Groß, Siegfried Raasch, Gunther Seckmeyer

Bearbeitung:

Tobias Gronemeier, Matthias Sühring, Farah Kanani-Sühring, Robert von Tils, Michael Schrempf

Laufzeit:

2016-2019

Förderung durch:

BMBF

Kurzbeschreibung:

The goal of the project is the development of a new innovative urban climat model that is able to simulate the microclimate in densely-populated cities like Berlin with a spatial resolution of less than 10 m. The model will be developed as a user-friendly tool, which can be applied by both scientists and urban planners.

 

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A new LES-based system for short-range forecasting of near-surface high impact weather at airports (ALERT)

Bild zum Projekt A new LES-based system for short-range forecasting of near-surface high impact weather at airports (ALERT)

Leitung:

Björn Maronga, Siegfried Raasch

Bearbeitung:

Helge Knoop

Laufzeit:

2016-2019

Förderung durch:

Deutscher Wetterdienst (DWD)

Kurzbeschreibung:

The project aims to improve the local forecast of atmospheric surface layer processes and quantities for airport areas with a focus on the precise forecast of critical events such as radiation and advection fog as well as wind gusts by using high-resolution large-eddy simulations (LES).

 

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HD(CP)2 (2nd phase): Flux heterogeneity and boundary layer circulations

Bild zum Projekt HD(CP)2 (2nd phase): Flux heterogeneity and boundary layer circulations

Leitung:

Siegfried Raasch, Björn Maronga

Bearbeitung:

Katrin Scharf

Laufzeit:

2016-2019

Förderung durch:

BMBF

Kurzbeschreibung:

In nature, atmospheric state variables are modified by surface flux variations at all scales, which are induced by dynamic (vegetation growth, land use management) surface heterogeneities, and which feedback on larger scale atmospheric boundary layer (ABL) fluxes. ABL circulations driven by these heterogeneities modify the surface fluxes resulting in two-way feedbacks and impacts on even extended circulations. Sub grid-scale heterogeneity of land surfaces in climate models is usually tackled by the tile approach, which assumes a homogeneous atmosphere interacting with a heterogeneous land surface. In this approximation, the atmosphere is solely driven by grid-averaged fluxes at the surface. This project will quantify the effect of climate model sub-grid flux variability including induced boundary circulations within the scale of the climate model grid cell and their feedbacks on the fluxes on the uncertainty of regional climate predictions. The main intention is to explicitly resolve the near-surface processes both in order to gain a better physical understanding, and to quantify errors in the existing weather forecasting model ICON surface parameterizations and suggest improvements.

 

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High-resolution numerical studies on the effect of turbulence on nocturnal radiation fogs

Bild zum Projekt High-resolution numerical studies on the effect of turbulence on nocturnal radiation fogs

Leitung:

Björn Maronga

Bearbeitung:

Johannes Schwenkel

Laufzeit:

2015-2017

Förderung durch:

DFG

Kurzbeschreibung:

In this project, high-resolution large-eddy simulations (LES) will be used to investigate the effect of turbulence on nocturnal radiation fogs. The LES model PALM will be used at very high resolution in the order of 1 m with both an Eulerian bulk cloud physics scheme and an embedded Lagrangian particle model that allows for explicitly resolving aerosols and fog droplets will be employed. This innovative approach allows for studying fog droplet-turbulence interactions for the first time with LES. The aim of this study is to achieve a comprehensive view on the key parameters that determine the life cycle of radiation fog as well as its three-dimensional macro- and microstructure. Moreover, the effect of a nocturnal fog layer on the morning transition and the daytime boundary layer will be studied. The effect of surface heterogeneity on nocturnal radiation fog will be investigated by means of LES with prescribed idealized regular and observed irregular surface heterogeneities.

 

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Turbulenzauflösende Simulationen zur meteorologischen Standortbewertung von Windkraftanlagen in komplexem Gelände

 

Leitung:

Siegfried Raasch

Bearbeitung:

Lennart Böske, Christoph Knigge

Laufzeit:

2015-2017

Kurzbeschreibung:

Insbesondere in komplexem Gelände stellt atmosphärische Turbulenz eine Herausforderung für die Planung neuer Windenergieanlagen dar. Orographisch induzierte Turbulenz kann zu deutlich erhöhten Lasten an den Anlagen führen und so deren Lebensdauer erheblich verkürzen. Für Fragen der Standortbewertung eingesetzte RANS-Modelle, in denen Turbulenz vollständig parametrisiert wird, können diese Effekte gerade in komplexem Gelände oft nicht richtig erfassen. Ziel des Projektes ist es daher, den Einsatz des turbulenzauflösenden LES-Modells PALM für die Standortbewertung von Windenergieanlagen zu demonstrieren und die Vorteile dieser Methode gegenüber RANS-Modellen aufzuzeigen. Hierfür werden räumlich hochauflösende Simulationen des turbulenten atmosphärischen Windfeldes für einen existieren Windenergiestandort im komplexen Gelände durchgeführt und die Ergebnisse mit Messdaten validiert. Für eine zusätzliche Validierung der Modelldaten anhand von Windkanalergebnissen wird die Überströmung eines idealisierten Berges simuliert. Unter anderem werden diese Simulationen genutzt, um das numerische Setup für den operationellen Einsatz zu optimieren.

 

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Entrainment von Aerosolen und ihre Aktivierung in flachen Kumuluswolken -- Grobstruktursimulationen mit einem eingebetteten Lagrangeschen Partikelmodell

 

Leitung:

Siegfried Raasch

Bearbeitung:

Fabian Hoffmann

Laufzeit:

2014-2017

Förderung durch:

DFG

Kurzbeschreibung:

Flache Kumulus-Bewölkung beeinflusst das Klima, z.B. durch die Reflexion extraterrestrischer, kurzwelliger Strahlung (Albedo). Dieser Einfluss wird durch die Anzahl und spektrale Verteilung der Tropfen, also auch durch die in der Atmosphäre vorhandenen Aerosole bestimmt. Jedoch können nur aktivierte Aerosole als Kondensationskeim für Wolkentropfen dienen. Primäre Aktivierung von Aerosolen geschieht an der Wolkenbasis, wo durch die adiabatische Abkühlung in aufsteigenden Luftpaketen eine zur Aktivierung ausreichende Übersättigungen erzeugt wird. Allerdings werden Aerosole auch oberhalb der Wolkenbasis aktiviert (sekundäre Aktivierung), wofür zwei Erklärungen existieren: (i) die Aktivierung sehr kleiner Aerosole, für die die Übersättigung an der Wolkenbasis nicht ausreicht, und (ii) die Aktivierung von Aerosolen, die durch laterales Entrainment in die Wolke gemischt und dort aktiviert werden. Das grundlegende Ziel dieses Projektes ist die Verbesserung unseres Verständnis derjenigen Prozesse, die zur Aktivierung von Aerosolen führen. Dieses Ziel soll durch die Anwendung von Large-Eddy Simulationen (LES) und einem Lagrangeschen Partikelmodell (LPM) für die Parametrisierung der Wolkenphysik erreicht werden. Dazu wird eine bestimmte Anzahl realer Wolkentropfen/Aerosole gleicher Eigenschaft durch einzelne Lagrangesche Partikel (sogenannte Supertropfen) repräsentiert. Diese Lagrangesche Perspektive der Wolkenphysik erlaubt es uns, einzelne Tropfen/Aerosole zu verfolgen und so direkte Entrainment-Raten zu berechnen. Dies ist nötig, um den Beitrag der einzelnen Prozesse die zur Aktivierung von Aerosolen führen können, und die damit die Anzahl und spektrale Verteilung der Wolkentropfen bestimmen, zu quantifizieren. Diese Prozesse sollen im Lebenszyklus einzelner Wolken, in einem Wolken-Ensemble (d.h. in Abhängigkeit von morphologischen Eigenschaften wie der Wolkenhöhe) und in Abhängigkeit von verschiedenen Aerosol-Konzentrationen untersucht werden.

 

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Investigation on reflection properties of snow (IREPS )

Bild zum Projekt Investigation on reflection properties of snow (IREPS )

Bearbeitung:

Ansgar Stührman, Michael Schrempf, Stefan Riechelmann, Gunther Seckmeyer

Laufzeit:

03/2014 - 02/2016

Förderung durch:

Bundesministerium für Bildung und Forschung (BMBF)

Kurzbeschreibung:

Untersuchungen von Reflektionseigenschaften von Schnee in den Anden und der Antarktis in Kooperation mit der Universität Santiago de Chile. Hierzu soll ein Messgerät entwickelt werden um die vom Himmel und von Schneeoberflächen ausgehende spektrale Strahldichte innerhalb eines Wellenlängenbereichs von 280 nm – 600 nm aus etwa 120 Richtungen zeitgleich aufzunehmen.

 

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4D-Images (4Dimensional Imaging Spectroradiometer)

Bild zum Projekt 4D-Images (4Dimensional Imaging Spectroradiometer)

Leitung:

Prof. Dr. Seckmeyer

Bearbeitung:

Ansgar Stührman, Stefan Riechelmann, Michael Schrempf, Gunther Seckmeyer

Laufzeit:

01/03/2013 -

Förderung durch:

BMWI

Kurzbeschreibung:

Entwicklung eines neuartigen streulichtarmen 4D-Imaging Spektralradiometers (2D im Bild, 1D in der Wellenlänge, 1D in der Zeit) für einen Wellenlängenbereich von 280nm bis 400nm im ultravioletten Spektralbereich und hoher räumlicher Auflösung zusammen mit der Gigaherz-Optik GmbH aus München.

 

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Novel Method for Exposure Calculation

Bild zum Projekt Novel Method for Exposure Calculation

Bearbeitung:

Michael Schrempf, Anna Wieczorek, Kathrin Graw, Gunther Seckmeyer

Laufzeit:

2011 -

Kurzbeschreibung:

Im Rahmen dieses Projekts wird vom IMuK eine neue Methode zur Berechnung der tatsächlichen Exposition eines Menschen entwickelt.

 

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Development of a multidirectional spectroradiometer (MUDIS)

Bild zum Projekt Development of a multidirectional spectroradiometer (MUDIS)

Bearbeitung:

Stefan Riechelmann, Michael Schrempf, Ansgar Stührman, Gunther Seckmeyer

Laufzeit:

2010 - 2014

Förderung durch:

Fakultät für Mathematik und Physik

Kurzbeschreibung:

Am IMuK wurde der Prototyp eines multidirektionalen Spektralradiometers (MUDIS) entwickelt, welches in der Lage ist die spektrale Strahldichte des Himmels in einem Wellenlängenbereich von 300 bis 600 nm in 113 Richtungen simultan in weniger als zwei Sekunden zu erfassen (Riechelmann et al., 2013).

 

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Masterarbeiten / Diplomarbeiten

Modellierung des Jahresgangs der Grasvergilbung

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Annika Bollmann

Laufzeit:

01.11.2015-31.10.2016

Kurzbeschreibung:

Innerhalb des DWD wird ein Messnetz zur Beobachtung der jahreszeitlichen Vegetationsentwicklung betrieben. Erfasst werden einerseits definierte phänologische Phasen ausgewählter Pflanzen, andererseits aber auch Vergilbungsanteile von Grasflächen, die für die Feuergefährdung offener Freiflächen eine Rolle spielen. Ziel ist es, die jahreszeitliche Grün-/Gelbwelle einer wildwachsenden Grasfläche mittels üblicher Standarddaten (Temperatur, Strahlung, Niederschlag, …) zu simulieren, um im Rahmen operationeller Grasbrandwarnungen zu einer realistischen Einschätzung der Feuergefährdung zu kommen. Nach australischen/US-amerikanischen/kanadischen Feuerausbreitungsexperimenten sind Flächen mit Vergilbungsanteilen > 90 % extrem gefährdet, Flächen mit weniger als 50 % Gelbanteil dagegen wenig gefährdet. Grundlage der Simulation wird ein vorliegendes SVAT-Modell sein, welches allerdings für Grasflächen noch angeeicht werden muss. Hierzu stehen u.a. Lysimeterdaten zur Verfügung.

 

 

Anwendbarkeit der ConWX-Reanalysedaten auf die Ertragsprognose von Windenergieanlagen

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Jill Johnson

Laufzeit:

01.10.2015-31.09.2016

Kurzbeschreibung:

Für die Ertragsprognosen von Windenergieanlagen ist die Kenntnis der Windverhältnisse am geplanten Standort von großer Bedeutung. Da nicht an jedem Standort Windmessungen vorhanden sind, wird auf Reanalysedaten zurückgegriffen. Im Rahmen dieser Masterarbeit soll die Qualität der ConWX-Reanalysedaten untersucht werden. Dazu werden im ersten Schritt die Reanalysedaten mit Messwerten des Deutschen Wetterdienstes verglichen. Im zweiten Schritt sollen, mit Hilfe ausgewählter Referenz-Windenergieanlagen, Ertragsprognosen jeweils aus Reanalysedaten und den Messwerten des Deutschen Wetterdienstes erstellt und mit dem tatsächlich registriertem Ertrag der Referenz-Windenergieanlage verglichen werden. Die zu erwartenden Unterschiede aus diesen Gegenüberstellungen sollen Hinweise auf zukünftige Verbesserungsmöglichkeiten hinsichtlich der Ertragsporognose geben.

 

 

Validierung von Klimaprojektionen im Cuvelai-Etosha-Basin

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Merle Glomb

Laufzeit:

01.09.2015-31.08.2016

Kurzbeschreibung:

Die Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) beschäftigt sich im Rahmen des SASSCAL-Projektes unter anderem mit der Grundwasserneubildung im Cuvelai-Etosha-Basin (Nordnamibia/Südangola). Für die Abschätzung der Grundwasserneubildung in der Zukunft werden möglichst exakte Prognosen von meteorologischen Daten, wie Temperatur und Niederschlag benötigt. Im Rahmen dieser Masterarbeit werden die regionalen Ergebnisse verschiedener Klimamodelle für RCP 4.5 und RCP 8.5 miteinander verglichen.. Um eine Abschätzung der Güte der einzelnen Klimamodelle zu ermöglichen, findet im Vorfeld eine Validierung der historischen Klimamodelldaten statt. Anschließend soll in der Masterarbeit ein Vergleich zwischen der Temperatur- und Niederschlagsentwicklung mit der Entwicklung der Grundwasserneubildung bis 2100 gezogen werden.

 

 

Untersuchungen zur optimierten Vorhersage des Flüssigwassergehalts in COSMO-EU

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Jasmin Siedler

Laufzeit:

01/01/2014 -

Kurzbeschreibung:

Im Rahmen dieser Masterarbeit werden Fallstudien durchgeführt, die auf eine Optimierung der Vorhersage des Flüssigwassergehalts (LWC) im COSMO-EU hinauszielen. Weitere Untersuchungen umfassen einen Vergleich mit Satellitendaten sowie die Überprüfung der Sensitivität in ADWICE.

 

 

Staubtransport durch Staubteufel – Eine LES-Studie

 

Leitung:

Siegfried Raasch

Bearbeitung:

Veit Rautmann

Kurzbeschreibung:

Dieses Projekt zielt darauf ab, den durch Staubteufel verursachten Eintrag von Staubpartikeln in die Atmosphäre anhand von LES zu untersuchen.

 

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Abgeschlossene Projekte

Forschungsprojekte

Study of air ventilation performance of high density cities under neutral and unstable atmospheric conditions using advanced large-eddy simulations

 

Leitung:

Siegfried Raasch

Bearbeitung:

Tobias Gronemeier

Laufzeit:

2015-

Kurzbeschreibung:

In former numerical studies air ventilation of high density cities has been analyzed for neutral atmospheric conditions only. This method gives a sufficient estimation of the ventilation as long as a high background wind is present. Under such conditions, shear generated turbulence has a main impact on the ventilation, and thermal effects can be neglected. In the present study, however, we will focus on weak wind situations in Hong Kong, where thermal effects on turbulence generation cannot be neglected anymore. Comparisons between neutral and unstable cases will allow us to quantify the thermal effect on the ventilation in Hong Kong.

 

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KURAS - Konzepte für urbane Regenwasserbewirtschaftung und Abwassersysteme

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

M.Sc. Robert Günther

Laufzeit:

01.06.2013-31.04.2016

Förderung durch:

Bundesministerium für Bildung und Forschung

Kurzbeschreibung:

Im Verbundforschungsvorhaben KURAS soll modellhaft untersucht werden, wie durch intelligent gekoppeltes Regenwasser- und Abwassermanagement die zukünftige Abwasserentsorgung, die Gewässerqualität, das Stadtklima und die Lebensqualität einer Stadt verbessert werden kann. www.kuras-projekt.de

 

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Weiterentwicklung des Warnsystems ADWICE zur Vorhersage und Diagnose von Gebieten mit Vereisungsrisiko

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Katharina Roloff

Laufzeit:

01/07/2012 - 31/03/2014

 

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Simulation postfrontaler Schauer

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Stephan Himmelsbach

Laufzeit:

01/01/2010 - 18/06/2015

 

 

Health at Risk through UV induced Skin Cancer in the Context of a Changing Climate

Bild zum Projekt Health at Risk through UV induced Skin Cancer in the Context of a Changing Climate

Leitung:

Dr. Stana Simic, Prof. Dr. Seckmeyer

Bearbeitung:

Michael Schrempf

Laufzeit:

01/09/2011-31/08/2013

Förderung durch:

Klima und Energie Fonds Austria, (ACRP)

Kurzbeschreibung:

Im Projekt UVskinrisk wurden Untersuchungen zur UV-Variabilität in der Vergangenheit, zur Einschätzung von Zukunftsszenarien, dem Einfluss des Klimawandels auf das Auftreten von Hautkrebs durchgeführt.

 

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MET4ATM

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Patrick Hupe, Manuela Sauer

Laufzeit:

01/10/2011 - 30/06/2015

Förderung durch:

Austro Control

Kurzbeschreibung:

In diesem Projekt wurde die Beeinflussung des Luftverkehrs und insbesondere die Zunahme des Arbeitsaufwandes für Fluglotsen durch gefährdendes Wetter, im Speziellen durch eine Squall Line Situation, untersucht. Projektpartner waren Austro Control und die Universität Salzburg.

 

 

Erfassung, Qualitätssicherung, Aufbereitung und Bereitstellung von Pilotenbeobachtungen (PIREPs/AIREPs)

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Patrick Hupe, Jasmin Siedler

Laufzeit:

17/01/2013 - 17/10/2014

Förderung durch:

Deutscher Wetterdienst (DWD)

Kurzbeschreibung:

Seit dem 14. Dezember 2000 werden Pilotenwettermeldungen (PIREPs) am IMuK archiviert. Im Rahmen dieses Projekts werden Meldungen über Vereisung ausgewertet und dem Deutschen Wetterdienst (DWD) bereitgestellt. Ziel ist der Aufbau eines Beobachtungsarchivs, damit die auf den PIREPs basierenden Informationen auf Seiten des DWD für die Belange der Luftfahrt verwendet werden können.

 

 

Bereitstellung von hochaufgelösten LES-Turbulenzdaten für verschiedene meteorologische Bedingungen zur Validierung des DLR-Codes THETA

 

Leitung:

Siegfried Raasch

Bearbeitung:

Farah Kanani-Sühring

Laufzeit:

2014 - 2015

Förderung durch:

BMWi

Kurzbeschreibung:

Das Ziel des BMWi-Projekts SimBA (Simulationswerkzeug für die hochgenaue und effiziente aero-dynamische Berechnung und Auslegung von Windenergieanlagen), welches unter Leitung des DLR Göttingen bearbeitet wird, ist die Entwicklung eines deutschen Forschungscodes für die numerische Simulation von Strömungen an Windenergieanlagen. Mithilfe dieses Werkzeugs soll die Effizienz von Windenergieanlagen unter verschiedensten meteorologischen Situationen erhöht werden. Für die Validierung des entwickelten Forschungscodes sollen in einem Unterauftrag für das Projekt SimBA mithilfe von PALM hochaufgelöste Turbulenzdaten für verschiedene meteorologische Bedingungen generiert werden. Diese Turbulenzdaten werden dann über eine instationäre Randbedingung in das DLR-Modell THETA eingefüttert, um darin mit der Windenergieanlage im durch THETA simulierten Strömungsgebiet zu interagieren.

 

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A new turbulent inflow method for PALM and the effect of tall buildings on the urban boundary layer

 

Leitung:

Manabu Kanda (Tokyo Institute of Technology), Siegfried Raasch

Bearbeitung:

Tobias Gronemeier

Laufzeit:

2014-2015

Förderung durch:

DAAD

Kurzbeschreibung:

The research project, which is a collaboration between the research group of Prof. Kanda from Tokyo Institute of Technology (Tokyo, Japan) and the PALM group, focuses on creeating a new inflow method for PALM when no cyclic boundary conditions are desired. This new inflow method produces turbulence at the inflow based on prescribed length scales and Reynolds stress tensor of the desired turbulent flow (synthetic turbulence generator). With this synthetic turbulence generator a turbulent flow can be created without cyclic boundaries. This is very important for simulating an isolated obstacle or a real topography like a city. Also with using the synthetic turbulence generator there is no need to simulate a prerun, like it is the case with the recycling method, and still the domain size can be kept small. Later on this inflow method will be used to discover the effect of tall buildings on the development of the urban boundary layer.

 

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High-resolution large-eddy simulations of the atmospheric boundary layer in Macau

 

Leitung:

Siegfried Raasch

Bearbeitung:

Marius Keck

Laufzeit:

2012-2013 (phase one), 2014-2016 (phase two)

Kurzbeschreibung:

As one of the most densely populated cities in the world Macau has to face particular difficulties in terms of creating a comfortable living environment. Narrow streets and high-density districts significantly reduce the natural ventilation. Especially in Macau's humid subtropical climate the natural ventilation should be used to decrease thermal stress. As an aggravating factor, Macau's population is still growing. In order to create new habitable space, five reclamation areas are currently being built around Macau's present coasts. This study investigates the pedestrian level ventilation in Macau using the parallelized large-eddy simulation model PALM. The aims of the study are a) to evaluate the impact of the planned reclamation areas on the ventilation within the present city districts, b) to investigate how urban morphology quantities, e.g. the surface cover fraction, affect the ventilation in cities and c) to show the advantages of urban LES over RANS simulations.

 

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Untersuchung von Wirkung von Immissionsschutzpflanzungen in der Umgebung von Stallanlagen

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

Dipl.-Met. Viola Weniger, M.Sc. Stefanie Zander

Laufzeit:

01.09.2013-28.02.2016

Förderung durch:

Landwirtschafskammer Niedersachsen, Deutsche Bundesstiftung Umwelt

Kurzbeschreibung:

Das Projekt beruht sowohl auf einer experimentellen Messkampagne des LWK in den Stallungen und der nahen Umgebung eines Rindviehbetriebs, als auch auf numerischen Simulationen des IMuK mit dem mikroskaligen Strömungsmodell ASMUS und dem Partikel-Ausbreitungsmodell PARAT.

 

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Anflugroutensimulation für den Flughafen Hongkong

 

Leitung:

Prof. Dr. T. Hauf

Bearbeitung:

Manuela Sauer, Patrick Hupe

Laufzeit:

01/06/2013 - 31/08/2015

Kurzbeschreibung:

Dieses Projekt diente der Identifikation möglicher Hilfestellungen für Fluglotsen im Anflugbereich. Der Ausweichalgorithmus DIVMET wurde dahingehend weiterentwickelt, dass es möglich ist, individuelle Flugzeiten für wetterbeeinflusste Routen vorherzusagen, um die Eingliederung eines Flugzeugs in den Arrival Manager (AMAN) zu erleichtern.

 

 

The impact of entrainment and mixing on the fine-structure of cloud turbulence

 

Leitung:

Dieter Etling

Bearbeitung:

Theres Riechelmann

Laufzeit:

2012-2014

Förderung durch:

DFG SPP Metström

Kurzbeschreibung:

The entrainment of clear air and its mixing with cloudy air impacts the dynamics and the structure of a cloud over the whole range of spatial and temporal scales. In spite of its ultimate importance for the droplet dynamics with impact on precipitation and cloud albedo - and thus also on the atmospheric mesoscale dynamics - our understanding is rudimentary.

 

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HD(CP)2: High definition clouds and precipitation for advancing climate prediction – Project M2: Benchmarking

 

Leitung:

Siegfried Raasch

Bearbeitung:

Rieke Heinze

Laufzeit:

2012-2015

Förderung durch:

BMBF

Kurzbeschreibung:

Wolken- und Niederschlagsprozesse sind immer noch nicht ausreichend verstanden. Dies führt zu großen Unsicherheiten in Klimaprojektionen. Das Deutschland-weite Forschungsvorhaben HD(CP)2 beschäftigt sich mit der Darstellung von Wolken und Niederschlag durch die Entwicklung eines sehr hoch-aufgelösten Klimamodells. Innerhalb des Teilprojekts M2 wird das LES-Model PALM verwendet, um LES-Daten für die Evaluierung und das Benchmarking des HD(CP)2-Modells zu erzeugen. Es werden verschiedene idealisierte und semi-idealisierte Szenarien simuliert, um die Unsicherheiten die mit der Darstellung von Landoberflächen, der subskaligen Modellierung und dem Einfluss der nächtlichen Grenzschicht auf die Entwicklung des Tagesgangs verbunden sind, zu untersuchen.

 

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Intelligent Modeling the Impact of Unpredictable Adverse Weather on ATM Performance

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Manuela Sauer

Laufzeit:

01/09/2011 - 30/09/2015

Förderung durch:

Eurocontrol (SESAR-WPE ComplexWorld)

Kurzbeschreibung:

Innerhalb dieses PhD Projektes wurde das Verständnis der Interaktion zweier complexer Systeme - Flugverkehr und gefährdendes Wetter - verbessert. Die Entwicklung des Ausweichalgorithmus DIVMET wurde vertieft, um in Fallstudien die Interaktion der Systeme zu untersuchen und Auswirkungen zu identifizieren. Der Einfluss gefährdenden Wetters, wie Gewitter es sind, wurde anhand von Simulationen in verschiedenen Kenntnisständen das den Flug begleitenden und zur Ausweichmanövern drängenden Wetters abgebildet. Strategien zur Adaption der Unsicherheit von Wettervorhersagen und insbesondere Nowcasts von Gewittern wurden entwickelt.

 

 

High-resolution LES studies of the turbulent structure of the lower atmospheric boundary layer over heterogeneous terrain and implications for the interpretation of scintillometer data

 

Leitung:

Siegfried Raasch

Bearbeitung:

Björn Maronga

Laufzeit:

2010-2012 (phase one), 2012-2014 (phase two)

Förderung durch:

DFG

Kurzbeschreibung:

The turbulent exchange of heat and water vapour are essential land surface - atmosphere interaction processes in the local, regional and global energy and water cycles. Scintillometry can be considered as the only technique presently available for the quasi-operational experimental determination of area-averaged turbulent fluxes needed to validate the fluxes simulated by regional atmospheric models ord derived from satellite images at a horizontal scale of a few kilometers. The scintillometer principle is based on the quantitative evaluation of intensity fluctuations of electromagnetic radiation propagating across the turbulent atmosphere over distances up to several kilometres.

 

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Representation of the convective atmospheric boundary layer during cold-air outbreaks in regional models: a joined study based on observations, Large Eddy Simulation and mesoscale modelling

 

Leitung:

Siegfried Raasch

Bearbeitung:

Christof Lüpkes, Siegfried Raasch, Micha Gryschka, Jens Fricke

Laufzeit:

2010-2015

Förderung durch:

DFG

Kurzbeschreibung:

The interaction between atmospheric processes in polar and mid-latitude regions is strongly affected by cold-air outbreaks (CAOs). During such events cold air originating from (sea)ice covered regions is transported over large distances towards lower latitudes, while over open water a shallow but strongly heated convective boundary layer develops. Although the present generation of weather and regional climate models captures CAOs qualitatively well, there are deficiencies caused by uncertainties in the representation of the multi scale physical CAO processes in models. Open questions concern e.g., the generation of rolls under conditions of strong thermal forcing over the open and partially ice covered ocean and under non- stationary synoptic conditions. Other questions are related to the parameterization of the strong turbulent fluxes, especially in coarse grid resolution. Hence this project aims to study the physical processes in polar CAOs with roll vortices using large eddy simulation (LES) and mesoscale modelling. Furthermore, parameterizations of turbulence in CAOs will be improved with a focus on the most modern low order closures and their applicability to regional scale models. To this end current and new closures are used in a mesoscale model with different resolutions and its results are compared with observations and with the results of LES.

 

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Analysis of the relative importance of terrain heterogeneity, heterogeneous land use and heterogeneous forcing on the ABL structure using LES

 

Leitung:

Siegfried Raasch

Bearbeitung:

Matthias Sühring

Laufzeit:

2010-2014

Förderung durch:

DFG

Kurzbeschreibung:

The atmospheric boundary layer (ABL) is directly affected by the underlying surface. Inhomogeneous meteorological surface forcing can yield to circulations affecting the turbulent exchange in the entire ABL. This research project studies the effect of heterogeneous meteorological forcing on the structure of the ABL using LES. Simulations based on the LITFASS-2003 dataset will be carried out to investigate exchange processes and the state of turbulent mixing under different atmospheric conditions. Furthermore, the measurement strategies of the LITFASS-2003 experiment will be investigated and possible improvements for the measurement strategies will be derived.

 

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High-resolution LES studies of the turbulent exchange processes between forest and atmosphere under heterogeneous conditions

 

Leitung:

Siegfried Raasch

Bearbeitung:

Farah Kanani-Sühring

Laufzeit:

2010-2014

Förderung durch:

DFG

Kurzbeschreibung:

Forests play an important role in the global ecosystem exchange. How well we can estimate this exchange as an input for climate models strongly depends on the quality of the flux measurements. Since the land surface becomes more and more heterogeneous due to land use, wind damage or pests, forests are often not continuous. Therefore, the landscape is characterized by sharp changes in surface conditions, like e.g. at forest edges. Because forest edges are well known to have a complex effect on the forest-atmosphere exchange, experimenters place their flux measurements close to forest edges to study these effects. But due to the lacking detailed knowledge of the near-edge exchange processes, the measurements are difficult to interpret. Using PALM, we investigate in detail the scalar transport and the spatial variability of concentration and flux distributions within and above a forest downstream of a clearing-to-forest transition under different meteorological and plant physical conditions. Further interest is directed towards the investigation of transport processes at the lee-sided forest edge (forest-to-clearing transition). Results of these studies shall contribute to create a firm basic knowledge of the forest-atmosphere exchange in regions affected by a forest edge. This in turn is an essential prerequisite for being able to interpret scalar flux and concentration measurements in such regimes.

 

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Investigation of entrainment processes at the top of the atmospheric boundary layer using innovative experimental and numerical techniques

 

Leitung:

Siegfried Raasch

Bearbeitung:

Florian Herbort

Laufzeit:

2010-2012

Förderung durch:

NTH Bottom-Up

Kurzbeschreibung:

The atmospheric boundary layer (ABL) represents the part of the atmosphere, which is directly affected by the processes taking place at the Earth's surface. The depth of the ABL depends on the meteorological situation and reaches 500 to 2000 meters in mid latitudes. Thereover a thermal inversion usually exists of 10 to 100 meters thickness. This inversion hinders vertical transport from the ABL into the free atmosphere across. The interchange of energy and matter is provided by entrainment which is the input of air from the inversion into the ABL primarily caused by convective processes leading to a deepening of the ABL. Due to the large grid space of numerical weather and climate models, entrainment processes have to be parameterized. An improvement of the knowledge of the entrainment processes benefits numerical weather prediction and climate models, that are currently often equipped with poor parameterizations of the above mentioned processes. However the entrainment region is directly connected to the drive of the large-scale weather development. On the one hand experimental methods using meteorological instruments on mini aircrafts and on the other hand numerical simulations using PALM are expected to enlarge the knowledge of entrainment processes.

 

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LES of atmospheric boundary layer turbulence - contributions to the improvement of turbulence parameterisation schemes by means of systematic studies of higher order statistical moments and their budgets

 

Leitung:

Siegfried Raasch

Bearbeitung:

Rieke Heinze

Laufzeit:

2009-2012

Förderung durch:

DWD Extramurale Forschung

Kurzbeschreibung:

Even in high resolution weather and climate models the turbulence of the atmospheric boundary layer can not be resolved because the underlying processes are on scales much smaller than the grid resolution used in these models. Accordingly, the whole turbulence spectrum has to be parameterised. A lot of these parameterisations are based on the budgets of the second order statistical moments. The budgets are required for example for the estimation of the relative importance of the respective budget terms as well as for the general applicability of the used parameterisation schemes. In this project all the budgets of the second order moments shall be determined with aid of LES. The focus is on the study of convective boundary layers with clouds to examine the influence of various cloud regimes on the budgets. Therefore, it is planned to simulate a boundary layer with trade-wind cumulus as well as with stratocumulus and under a situation of a cold air outbreak. Further, the influence of inhomogeneities on the various budget equations will be studied.

 

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Scalar transfer process of the urban canopy

 

Leitung:

Siegfried Raasch

Bearbeitung:

Marcus Oliver Letzel

Laufzeit:

2009

Kurzbeschreibung:

Coursework for Master in "Architectural Engineering" at Kyushu University (research collaboration between Aya Hagishima at Kyushu University, Fukuoka, Japan; Urban and Architectural Environment Laboratory, Interdisciplinary Graduate School of Engineering Sciences (IGSES) and Marcus Oliver Letzel)

 

 

FLYSAFE

 

Leitung:

Prof. Dr. T. Hauf

Bearbeitung:

Dr. Michael Theusner, Dr. Peer Röhner, Katharina Koppe

Laufzeit:

2004 - 2009

Förderung durch:

Eurocontrol

Kurzbeschreibung:

Im FLYSAFE Projekt wurden den Flugverkehr beeinflussende Wetterphänomene identifiziert, detailliert analysiert und erste Schritte zur Mitigation dieser Effekte eingeleitet. Konzepte für notwendige, das Luftfahrtsystem unterstützende Tools wurden im Rahmen dieses Projektes zwischen den 36 teilnehmenden Institutionen des FLYSAFE Consortiums entwickelt.

 

 

Investigations on in-cloud turbulence effects on drop growth and precipitation formation

 

Leitung:

Siegfried Raasch

Bearbeitung:

Theres Riechelmann

Laufzeit:

2008-2014

Förderung durch:

DFG SPP Metström

Kurzbeschreibung:

The project addresses an intensively discussed problem of cloud microphysics, namely the effect of an in-cloud turbulent flow field on droplet growth and precipitation formation. The questions to be answered are: Which influence has the local cloud turbulence on the collision efficiency of small cloud droplets and thus on the coagulation function where the latter is an important ingredient for numerical simulations of precipitation development? What is the impact of turbulence on cloud microphysics compared to the traditional approach disregarding this effect? Referring to the study PALM is used to perform high-resolution large-eddy simulations of three-dimensional convective clouds in which a large number of water droplets (Lagrangian droplets) is simulated explicitly. The focus is on the spatial and temporal evolution of the droplet spectrum, the spatial distribution of the droplet concentration and the water droplet tracks within turbulent clouds. Animations of three-dimensional convective clouds can be found in our gallery.

 

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Numerische Simulation zur Wechselwirkung der Atmosphäre mit der Tragflügelumströmung

 

Leitung:

Siegfried Raasch

Bearbeitung:

Christoph Knigge

Laufzeit:

2008-2015

Förderung durch:

DFG

Kurzbeschreibung:

Simulationstechniken zur Abschätzung der sicheren Flugbereichsgrenzen im Langsamflug sind während der Flugzeugentwicklung von großer Bedeutung. Die DFG Forschergruppe FOR 1066, an der verschiedene Institute deutscher Universitäten und des Deutschen Zentrums für Luft- und Raumfahrt (DLR) beteiligt sind, entwickelt eine neue Simulationsmethodik zur Untersuchung des Überziehverhaltens von Tragflügeln. Insbesondere wird dabei der Einfluss der atmosphärischen Turbulenz bzw. von Böen berücksichtigt. Im Teilprojekt A2 wird das Modell PALM verwendet um realistische meteorologische Szenarien zu simulieren. Die generierten Daten initialisieren den DLR TAU Code zur Untersuchung der Wechselwirkung eines Tragflügels mit der turbulenten atmosphärischen Strömung. Zusätzlich werden in dem Teilprojekt mehrdimensionale diskrete Böenmodelle entwickelt um die Manövrierbarkeit von Flugzeugen an den Flugbereichsgrenzen zu untersuchen.

 

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Study of the turbulence intensity in a wind turbine wake with the aid of LES

 

Leitung:

Siegfried Raasch

Bearbeitung:

Johannes Lotz

Laufzeit:

2008

Kurzbeschreibung:

Turbulence may be very large in wind turbine wakes and especially if turbines are clustered in arrays significant interference effects take place. To avoid damage and bad efficiency it is advantageous to get to know the characteristics of this turbulence intensity. In this project the turbine is placed in an environment with turbulence properties similar to the ones of the atmosphere. The wind turbine is modelled by drag forces applied at the grid points corresponding to the projected disk area.

 

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Coupled atmosphere-ice-ocean turbulence resolving model

 

Leitung:

Siegfried Raasch, Igor Esau

Bearbeitung:

Marcus Oliver Letzel, Björn Maronga, Micha Gryschka

Laufzeit:

2007-2014

Kurzbeschreibung:

The project objective is to develop a coupled lower atmosphere-ice-upper ocean turbulence resolving model able to simulate interactions between atmospheric and ocean turbulence in presence of sea ice freezing, melting and transporting mechanisms.

 

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Turbulence structure in the urban roughness sublayer: LES reference studies and comparison with data from wind tunnel, scale model and field measurements

 

Leitung:

Siegfried Raasch

Bearbeitung:

Marcus Oliver Letzel

Laufzeit:

2007-2009

Förderung durch:

DFG SPP MetStröm

Kurzbeschreibung:

The multi-scale turbulence within and above urban areas poses an open question concerning generation and interaction of coherent structures that is essential for urban turbulence parameterizations in meteorological models: does it resemble vegetation canopy turbulence or surface layer turbulence or does it form an own turbulence type? This large-eddy simulation (LES) reference study investigates the turbulence structure of the urban roughness sublayer for different canonical cities with consideration of thermal effects. The reasons for the choice of LES for this study are: a) LES is the only numerical method to explicitly resolve the multi-scale turbulence within and above real-size obstacles without significant influence of parameterizations and b) LES offers more applications and is able to deliver more extensive data than the wind tunnel. The project aims at an unprecedented combination of domain size and resolution. Quality assurance includes comparison with data from wind tunnel, scale model and field measurements.

 

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The physics of turbulence over antarctic leads and polynyas and its parameterization: A joint study using observations, LES and a micro-/mesoscale model

 

Leitung:

Siegfried Raasch

Bearbeitung:

Björn Witha

Laufzeit:

2007-2009

Förderung durch:

DFG SPP Antarktis

Kurzbeschreibung:

In regions with large sea ice concentrations there are always open water areas called leads or polynyas depending on their shape. The length of leads varies between a few kilometres and tens of kilometres and their width ranges from several meters to kilometres. The strong sensible and latent heat fluxes above such areas can trigger convection which significantly modifies the atmospheric boundary layer (ABL). For this reason leads are supposed to have a large influence on the energy exchange between the polar ocean and the atmosphere. The goals of this project are to get a better understanding of the still not well understood effects of leads and polynyas on the boundary layer turbulence and to clarify their importance for the energy budget and structure of the ABL. Results of the LES model PALM and new helicopter based observations of turbulence over leads will be used to (further) develop parameterizations of the lead effect which can be used in models with different grid sizes ranging from microscale non eddy resolving models to weather forecast and regional climate models. The parameterizations are tested in the nonhydrostatic model METRAS. An animation of a flow over an ice lead can be found in our gallery.

 

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Validierung des Strahlungstransfermodell SOLIS

Bild zum Projekt Validierung des Strahlungstransfermodell SOLIS

Bearbeitung:

Riyad Mubarak, Gunther Seckmeyer

Laufzeit:

01/2007 - 12/2007

Förderung durch:

Deutsche Wetterdienst (DWD)

Kurzbeschreibung:

Ziel dieses Projekts war es mit Hilfe von Messungen der spektralen Bestrahlungsstärke (300 nm – 2500 nm) auf dem Dach des IMUKs das Strahlungstransfermodell SOLIS zu validieren.

 

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Large-eddy simulation of the surface boundary layer for wind energy applications

 

Leitung:

Thomas Neumann, Siegfried Raasch

Bearbeitung:

Beatriz Cañadillas

Laufzeit:

2006-2009

Förderung durch:

EU Marie Curie Programm

Kurzbeschreibung:

In wind energy, mean flow characteristics are the primary focus in many cases, although turbulence effects are important when considering turbine design specifications as well as turbine operation and maintenance costs. Understanding these effects and their origins are the aim of this project. The first part will be to check whether the LES simulations verify fairly against observations, making use of off-shore wind data at FINO1 meteorological mast located in the North See. This comparison will help to understand better the observations within the PBL as well as to assess the applicability of LES in wind energy, in those cases where the standard measurements (cup anemometers) of the wind flow are not enough. The second part will be to get a better understanding of the inflow wind variation over the turbine rotor.

 

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Urban dispersion modelling using parallel large-eddy simulation

 

Leitung:

Siegfried Raasch

Bearbeitung:

Jin Zhang

Laufzeit:

2006-2007

Förderung durch:

funded jointly by our group and by Manabu Kanda at the Department of International Development Engineering

Kurzbeschreibung:

This project is a cooperation with the Kanda Laboratory at the Department of International Development Engineering, Tokyo Institute of Technology, Japan, that focuses on the urban climate and enviroment. In an urban area, the flow, turbulence structure, and pollutant-transport characteristics are affected by the urban building geometry. To resolve urban climatic problems, it is important to investigate the mechanisms of their impacts. To conduct LES with urban geometry and climate conditions two key problems need to be resolved: Large calculation domain and building resolution. The urban version of PALM has both of these abilities. The objective of this LES study is to investigate the flow, turbulence structure, and pollutant transport in varities of urban geometry by employing PALM and the embeded Lagrangian Dispersion Model. The first step is to test the possibilty for using Lagrangian Models in PALM which requests a large amount of system resources as millions of particles are released during the simulations. Then some analyses are going to be conducted to investigate the differences between PALM and former LES street canyon studies which used different numerical methods. The second step is going to be analysis between different urban geometries. Several different urban geometries like square array, staggered array are going to be set in different thermal codintions. Finally these results are going to be compared with those from outdoor model experiments COSMO, which is being conducted by members of Kanda Laboratory in Tokyo.

 

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Footprint model development and validation for homogeneous and inhomogeneous terrain using high resolution large-eddy simulation

 

Leitung:

Siegfried Raasch

Bearbeitung:

Gerald Steinfeld

Laufzeit:

2005-2008

Förderung durch:

DFG

Kurzbeschreibung:

This project is a cooperation with the working group of Thomas Foken, Department of Micrometeorology, University of Bayreuth, Germany. It brings together expertise on measurements, footprint modelling and LES modelling in the near-surface atmospheric boundary-layer. The objective is to develop footprint algorithms that are applicable in horizontally heterogeneous atmospheric boundary-layer flows within a wide range of thermal stratifications. Footprint models determine the spatial context of a measurement by defining a transfer function between sources or sinks of the signal and the sensor position. The resulting source area is an important quality assessment tool, e.g. to assess the influence of disturbing terrain elements on the data. Although micrometeorological research focuses increasingly on complex terrain, footprint models developed to date are restricted to idealised cases of horizontally homogeneous flow conditions, and thus cannot consider the impact of inhomogeneity. The project focuses on the development and validation of methods to compute footprints above thermally and aerodynamically inhomogeneous surfaces. In a first step, high-resolution LES, allowing the explicit resolution of the turbulent transport processes, will be carried out by the project members of the PALM group. In a second step, the project members of our cooperation partner will modify both forward and backward Lagrangian stochastic (LS) algorithms to account for horizontally heterogeneous flow conditions. The LES datasets will be used both to derive the flow statistics required to run the LS models and to evaluate their produced source areas. The reliability of the footprint results of both LES model and LS algorithms will finally be validated with data from field experiments.

 

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LES of a cumulus cloud with explicit Lagrangian droplets

 

Leitung:

Siegfried Raasch

Bearbeitung:

In-Soo Kang

Laufzeit:

2005-2007

Kurzbeschreibung:

An LES model of a cumulus cloud is developed in which the motion of a large number of water droplets is explicitly simulated. The model provides various important information of the cloud process such as entrainment and internal mixing, the distribution of droplets within a cloud, and the evolution of particle size spectrum. A substantial part of this Korean PhD project is carried out visiting the PALM group in Hannover.

 

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LES study of the spatial representativeness and imbalances of scalar fluxes determined by the temporal EC method

 

Leitung:

Siegfried Raasch

Bearbeitung:

Gerald Steinfeld

Laufzeit:

2004-2005

Förderung durch:

DAAD

Kurzbeschreibung:

Several of the former projects of the PALM group have dealt with the so-called energy imbalance problem so far. However, during the last years, several papers, dealing with the analysis of field experiments, have reported on the fact that not only an imbalance of energy fluxes could be observed, but that a similar rate of underestimation of carbon dioxide fluxes could also be stated. A detailed understanding of how mass is partitioned at the earth's surface is of particular importance for the derivation of appropriate parameterisation schemes in global climate models. Global climate models will even gain in importance in the context of the monitoring of the agreements on carbon dioxide emission, that are defined in the Kyoto protocol, as they offer the only possibility to get area-wide data on carbon-dioxide fluxes. In this study, which is a joint project together with Atsushi Inagaki from the working group of Manabu Kanda, Department of International Development Engineering, Tokyo Institute of Technology, Japan, a systematic investigation of the question, whether the phenomenon of an underestimation of the true representative fluxes in connection with the application of the temporal EC method is restricted to temperature fluxes or whether it has also to be stated for fluxes of a passive scalar, such as carbon dioxide. The simulation of EC measurements of scalar fluxes will be carried out for homogeneously as well as heterogeneously heated/cooled boundary layers and under a variation of the magnitude and direction of the geostrophic wind, of the thermal stratification and of the parameters describing the near-surface heat flux inhomogeneity. Moreover, the impact of an inhomogeneous near-surface scalar flux on a potential underestimation of the representative flux will be examined in detail.

 

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Characterization of the specific solar radiation conditions in polar regions with respect to climate change and ozone depletion

Bild zum Projekt Characterization of the specific solar radiation conditions in polar regions with respect to climate change and ozone depletion

Bearbeitung:

Sigrid Wuttke, Gunther Seckmeyer

Laufzeit:

2002-2004

Förderung durch:

DFG

Kurzbeschreibung:

Während einer Kampagne auf der Neumayer-Station in der Antarktis während des Süd-Sommers 2003/2004 wurden Messungen der spektralen Albedo, der spektralen Strahldichte und der Leuchtdichte durchgeführt.

 

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Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT-O3)

Bild zum Projekt Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT-O3)

Bearbeitung:

Irina Smolskaia, Gunther Seckmeyer

Laufzeit:

2004 - 2009

Förderung durch:

Europäische Kommission

Kurzbeschreibung:

Ziel des Projekts war es Vorhersagen zur Entwicklung des gekoppelten Chemie-Klima-Systems der Atmosphäre mit Schwerpunkt auf Ozonänderungen in der unteren Stratosphäre und den damit verbundenen klimatischen Einfluss zu liefern.

 

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Investigation of physical mechanisms leading to cell broadening and organization of convection in cold air outbreaks utilizing high resolution LES model simulations

 

Leitung:

Siegfried Raasch

Bearbeitung:

Heiko Jansen

Laufzeit:

2003-2006

Förderung durch:

DFG

Kurzbeschreibung:

This project is concerned with the organization of convection structures triggered by so called cold air outbreaks (COA), in which cold air from the continent or sea ice areas drifts above relatively warm water. The large gradient in temperature leads to convection, which at first manifests itself by the creation of horizontal rolls. With increasing distance from the coast, the structure at some point changes to hexagonal cells. The corresponding cloud clusters are readily visible in satellite pictures. Although research for some decades in this field, numerous open questions remain. For example, the aspect ratio of the cells, defined as the ratio of their diameter to their height, cannot be simulated in models as it is observed. In reality, this ratio attains values between 10 and 30, while in models or in laboratory experiments, values of 3-4 are seen. Furthermore, it is unclear, why sometimes open cells (with downwind areas in the inner part of the cell) and sometimes closed cells (upwinds in the inner part) are observed. The origin of the cloud streets close to the coast is not known, either. Such questions are addressed using the high resolution, massively parallel LES model PALM.

 

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Development of a large-eddy simulation model for the ocean mixed layer and its application to the process of mass transport in the ocean

 

Leitung:

Siegfried Raasch

Bearbeitung:

Siegfried Raasch

Laufzeit:

2003-2005

Förderung durch:

DFG and KOSEF

Kurzbeschreibung:

In this project, we will extend the PALM code to be able to simulate and reproduce the realistic turbulence structure of the ocean mixed layer (OML). It will also be expanded to simulate the dispersion of concentration and the motion of suspended particles. The model will be applied to clarify the mass transport process in the OML, such as the transport of dissolved gases and the sedimentation of suspended particles with an aim to understand the biogeochemical processes of the ocean. It will also be used to evaluate and improveone-dimensional ocean mixed layer models, which will be applied to an ocean general circulation model.

 

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Further LES-study on the energy imbalance problem with eddy covariance fluxes

 

Leitung:

Siegfried Raasch

Bearbeitung:

Gerald Steinfeld

Laufzeit:

2003-2004

Förderung durch:

DAAD grant and Academic Frontiers Student Exchange Promotion Program In 2003-2004

Kurzbeschreibung:

This is a joint project together with Manabu Kanda at the Department of International Development Engineering, Tokyo Institute of Technology, Japan. It includes a one-year research stay in Japan. This study is based on the former co-operation between Kanda and Marcus Letzel - LES Study of the Energy Imbalance Problem with Eddy Covariance Fluxes. In their study they carried out simulations of a convective boundary layer with the following prerequisites: horizontal homogenous surface heat flux (or a mere 1D horizontal variation) vanishing horizontal mean value of the vertical velocity. In the scope of this project it should be investigated what the effects are on the observed energy imbalance if at least one of these "ideal case" prerequisites does not apply. For example, the presence of inhomogenities is characteristic of the Earth´s surface. Therefore it is planned to carry out simulations with a prescribed one-dimensional sinusoidal surface heat flux (parameter studies with e.g. varying maximum amplitude and wavelength) in order to study the effects of the thermally induced meso-scale circulation on the observed energy imbalance characteristics. These studies imply a slightly different definition of energy imbalance compared with the former studies of Kanda et al. Furthermore the influence of a synoptic-scale downward movement - as it is observed in high pressure systems - on the energy imbalance should be investigated by a LES-study. The goal is to examine the results of some theoretical studies. Finally, some fundamental comparisons between the heat fluxes calculated by the temporal eddy covariance method and calculated the spatial eddy covariance method will be carried out.

 

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LES-study on the energy imbalance problem for heterogeneous surfaces

 

Leitung:

Siegfried Raasch

Bearbeitung:

Atsushi Inagaki

Laufzeit:

2003-2004

Kurzbeschreibung:

This is a joint project with Siegfried Raasch and Manabu Kanda at the Department of International Development Engineering, Tokyo Institute of Technology, Japan. It includes an 11-month research stay in Germany. This study examines the energy imbalance problem using Large Eddy Simulation model. We represent typical a day-time convective boundary layer with no synoptic-scale vertical motion. By means of this, Kanda et al. (2002) have already revealed the basic characteristics of energy imbalance aroused by a horizontally homogeneous surface. Based on this result, this study investigates the case of heterogeneous surface condition. The heterogenity is given as 1-dimensional sinusoidal surface heating as in Letzel & Raasch (2002). In this study, the influence of the complexity or magnitude of heteroginity heating are important and we examine it in detail. (See also the study by Kanda & Letzel.)

 

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High resolution LES of turbulent flow in the vicinity of buildings including thermal effects

 

Leitung:

Siegfried Raasch

Bearbeitung:

Marcus Oliver Letzel

Laufzeit:

2003-2006

Förderung durch:

DFG

Kurzbeschreibung:

The goal of this project is to look into the detailed structure of building-induced turbulence and how it is affected by thermal stratification. The parallel cluster architecture of the new Hochleistungsrechenzentrum Nord ( HLRN) now principally allows to simultaneously run two coupled or nested model sets on different computer clusters ('grid computing'). This new feature will be applied to PALM within the scope of this project with the aim to extend the scale range of turbulent flow around buildings that is resolvable by LES models by at least an order of magnitude compared with usual models. This yields new prospects for model validation and the development and testing of turbulence parameterizations, especially under convective conditions (unstable thermal stratification). Physical analysis will commence with flow around a single cubic obstacle, followed by street canyon flow, for neutral and convective boundary layers. The results will be used to e.g. validate PALM against laboratory (wind tunnel) experiments, analyze differences between high resolution LES and former LES with lower resolution, and evaluate turbulence parameterizations employed in lower resolution models. Modelling aspects of this project involve PALM code implementations such as turbulent inflow and noncyclic boundary conditions, horizontal grid stretching or grid nesting (see also the studies by Gryschka and Steinfeld), a dynamic subgrid scale model, thermal building wall boundary conditions, and suitable visualization methods. An animation of a flow around a single cube can be found in our gallery.

 

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LES of suspended particle dispersion in the ocean mixed layer

 

Leitung:

Siegfried Raasch

Bearbeitung:

Marcus Herold

Laufzeit:

2003

Förderung durch:

DAAD within the programme "Hochschulpartnerschaften mit Ostasien" (HOST) and KOSEF

Kurzbeschreibung:

This project examines the influence of particle inertia on mean concentration profiles and settling velocity in the ocean mixed layer (OML). Recently, PALM has been used to examine the effects of wave breaking and langmuir circlulation in the OML (Noh, Min & Raasch, 2004). In the present study, the code is extended to simulate particle motion in a Lagrangian framework. Particles are released close to the surface to investigate their three-dimensional motion and the effects of wave breaking and Langmuir circulation. Therefore, the simulation setup is the same as in Noh, Min & Raasch (2004). A one-way coupling of fluid and particles is implemented, assuming a sufficiantly low volume fraction and size of the particles. In this case, Stokes drag can be assumed and the particle acceleration is determined by the difference of fluid and particles and the particle time scale tau_p only. tau_p and the terminal particle settling velocity w_s are used as independent parameters to characterize groups of particles. First analyses show that vertical mixing of particles with slow settling velocities (e.g. ws / u* = 0.01, 0.1) is dominated by the fluid motion and the significant influence of the Langmuir circulation rather than particle inertia. In this case, the mean profiles of particle concentration differ only slightly from the profiles of passive tracers. The particles become well mixed in the OML instead of sinking through the lower boundary. The results are similar for a whole range of Stokes numbers (St = taup / tauf= 0.01, 0.1, 0.3, 1.0) However, particles with ws / u* = 1.0 are more independent of the fluid motion. Some particles are rapidly mixed downward by the Langmuir circulation, but the greater part reaches the terminal settling velocity ws. Profiles show a distinct concentration maximum which broadens and descends with time.

 

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Investigation of coherent structures in the surface layer with the help of highly-resolved LES data

 

Leitung:

Siegfried Raasch

Bearbeitung:

Sonja Weinbrecht

Laufzeit:

2003

Kurzbeschreibung:

Within the scope of the comparison of LES-data with data gained by acoustic tomography (see project Acoustic Tomography of the ABL for the Validation of an LES Model) a simulation with a very high resolution had been performed. The simulation results showed streaky structures which occured near the surface and were visible in the temperature field, the vertical wind field and the wind component in the mean wind direction. Such structures have already been observed in several simulation results and experiments, but not yet in such a high resolution. So these data will be further analysed with respect to the turbulent structures in the surface layer to get a better understanding of the turbulent flow near the surface.

 

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Regional evaporation at grid / pixel scale over heterogeneous land surfaces (EVA-GRIPS) - numerical investigation of turbulent structures over heterogeneous land surfaces with a parallelized LES model (EVA-NITS)

 

Leitung:

Siegfried Raasch

Bearbeitung:

Jörg Uhlenbrock

Laufzeit:

2002-2004

Förderung durch:

BMBF

Kurzbeschreibung:

The main goal of the joint proposal EVA-GRIPS is the determination of area-averaged evaporation on the scale of a gridbox of a regional climate model or of pixels of sattelite images over heterogeneous land surfaces. This goal should be achieved through a combination of measurements, numerical modeling and interpretation of satellite data. The special goal in this subproject is the simulation of the atmospheric boundary layer over the Lindenberg area with a very high resolution. The LES-model shall provide independent sets of tubulence data, which will be compared with the measurements of turbulence gathered by experimental groups and which will complete these measurements. Furthermore, these data sets will be used to validate the turbulence-parameterizations of mesocale models.

 

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Vertical transports of energy and trace gases at anchor stations and their spatial and temporal extrapolation under complex natural conditions (VERTIKO) - structure of turbulent fluxes under inhomogeneous surface conditions (STINHO) - simulation of boundary layer turbulence

 

Leitung:

Siegfried Raasch

Bearbeitung:

Michael Schröter

Laufzeit:

2001-2004

Förderung durch:

BMBF

Kurzbeschreibung:

The sub-project STINHO (STruktur des turbulenten Transports über INHOmogener Unterlage) is part of the cooperative project VERTIKO (VERTIkaltransporte von Energie und Spurenstoffen an Ankerstationen unter KOmplexen natürlichen Bedingungen). STINHO consists of 3 parts. The experimental investigations (part 1, Acoustic Research Group at LIM) about an inhomogeneous surface with acoustical and optical methods provide near surface averaged values of parameters which are relevant for part 2. Part 2 - which is handles in this working group - deals with the high resolution simulation of the near-surface boundary layer above inhomogeneous terrain by means of the parallelized large-eddy simulation model PALM. In addition to the near-surface experiment and to provide information on the real boundary layer structure to initialize the LES the airborne turbulent measurement system HELIPOD is used (part 3). The physical description of the energy budget variations unter turbulent flow conditions above homogeneous surfaces is possible using one-dimensional models and/or micrometeorological measuring techniques, which determine the energy exchange processes considering only the height above the underlying surface. Such a procedure does not consider the horizontal energy fluxes, which must exist between different types of the surface in heterogeneous tarrain. Currently accepted parameterizations for the turbulent transport of energy only consider the vertical direction neglecting horizontal energy fluxes. The project achieves a better understanding of the turbulent energy fluxes inside the atmospheric boundary layer, because these fluxes consist of horizontal and vertical components especially visibel in experiments and simulations unter inhomogeneous flow conditions near a heterogeneous surface. Project results will contribute to an improvement of the parameterizations of energy fluxes over heterogeneous terrain for the use in large scale model.

 

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LES study of the energy imbalance problem with eddy covariance fluxes

 

Leitung:

Siegfried Raasch

Bearbeitung:

Manabu Kanda; Marcus Oliver Letzel

Laufzeit:

2001-2002

Kurzbeschreibung:

This is a joint project together with Manabu Kanda at the Department of International Development Engineering, Tokyo Institute of Technology, Japan. It includes a one-year research stay in Japan. The spatial representativeness of heat fluxes obtained with the eddy covariance method from point measurements and the mechanism of the so-called energy imbalance problem are investigated through numerical experiments using PALM. LES experiments are performed for a convective boundary layer over a flat surface as a best case scenario completely free of sensor errors and the uncertainties associated with field mearurements. The results of this study are as follows; (1) Point measurements systematically underestimate the heat flux (negative imbalances), which is attributed to local advection effects. (2) Imbalance bias and variance decrease with increasing wind speed. (3) Longer time averaging gives smaller bias but larger variance of imbalance. (4) Higher elevation gives larger bias and variance of imbalance. (5) Turbulent Organized Structures are shown to account for these statistical features of imbalance. (6) High-pass filters may reduce the variance of imbalance if properly designed. (7) Surface inhomogeneities can significanlty effect the imbalance. (See also the studies by Schröter, Inagaki and Steinfeld.)

 

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Application of Tele-Immersion in Wide Area Networks

 

Leitung:

Siegfried Raasch, Stephan Olbrich (RRZN), Hans-Christian Hege (ZIB)

Bearbeitung:

Siegfried Raasch

Laufzeit:

2001-2002

Förderung durch:

BMBF

Kurzbeschreibung:

The goal of this project is to realize a tele-immersive, multi-modal virtual reality environment, which provides three-dimensional, collaborative presentation, exploration and discussion capabilities on the basis of 3D representations. Extensions for steering numerical simulations shall lead to a "Virtual Laboratory" scenario, which could be applied in the context of high performance computing, serving for several purposes, such as better understanding of complex, dynamic relationships, getting important design parameters of expensive or dangerous installations, or developing advanced educational technologies. Several partners are involved in this project in charge of the "Regionales Rechenzentrum für Niedersachsen" ( RRZN) and the "Konrad-Zuse Zentrum für Informationstechnik" in Berlin ( ZIB). Our special goal is the stereoscopic visualization of huge time-dependent data on a local workstation, which have been created on a remote massively parallel computer (in our case by the LES model PALM). In principle, state-of-the-art front-end graphic workstations and software are unable to handle such huge data sets (single scalar arrays may be as big as several Gigabyte). Our idea is to apply the postprocessing of the raw data into geometric 3D scenes (polygons, lines, or points) as part of the MPI-based parallel simulation. Only the geometric scenes are then transferred in a specially designed binary format to the local graphic workstation and visualized with the WEB-browser plugin DocShow-VR, developed at the RRZN.

 

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Development of a High Performance Parallel Large Eddy Simulation Model and its Application to the Atmospheric Boundary Layer

 

Leitung:

Yign Noh; Siegfried Raasch

Laufzeit:

2000-2002

Förderung durch:

DFG and KOSEF

Kurzbeschreibung:

This is a joint project together with the Department of Atmospheric Sciences at Yonsei University, South Korea. Its goal is the optimization of PALM for the efficient use on state-of-the-art parallel architectures (i.e. shared memory clusters like the IBM-Regatta) and its application to various problems of atmospheric and oceanic boundary layers (e.g. improvement of mixed layer parameterizations). The project permits the exchange of scientists for two months per year.

 

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European Database for UV Climatology and Evaluation (EDUCE)

Bild zum Projekt European Database for UV Climatology and Evaluation (EDUCE)

Bearbeitung:

Tahera Ali, Gunther Seckmeyer

Laufzeit:

2000 - 2003

Förderung durch:

Europäische Kommission

Kurzbeschreibung:

Ziel des EDUCE-Projekts war die Untersuchung des europäischen UV-Klimas. Das IMUK hat als eine von 20 wissenschaftlichen Instituten aus 12 europäischen Ländern dazu beigetragen eine europäische Datenbank für UV-Messungen aufzubauen.

 

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First attempts of the validation of an LES model by acoustic tomography data

 

Leitung:

Siegfried Raasch

Bearbeitung:

Sonja Weinbrecht; Marcus Oliver Letzel

Laufzeit:

1999-2003

Förderung durch:

DFG

Kurzbeschreibung:

The aim of this project was to investigate turbulent convective structures near the surface by means of LES and acoustic tomography. So this was a cooperative project between the Institute of Meteorology, University of Leipzig and the Institute of Meteorology and Climatology, University of Hannover. By acoustic tomography ( Acoustic Research Group at LIM) high resolution and volume-averaged data can be measured and compared directly with the high resolution and volume-averaged simulation data (our group). All simulations were run with our LES-model PALM. By comparing the simulation results with the measured data we tried to evaluate the LES-model PALM and wanted to test different subgrid-scale parametrization models implemented in the LES-model compare their results. In preparation of such a quantitative comparison first the sensitivity of the LES-model against the boundary conditions and the initial parameters was observed. We found that the simulations results are highly sensitive towards the surface roughness length (if the surface temperature is prescribed). With the help of the sensitivity study results it is now possible to name the measurement accuracies of the boundary conditions and initial parameters which are needed to perform a quantitative comparison. Furthermore the influence of measurement errors of these parameters on the simulation results can now be quantified. A first comparison with acoustic tomography data showed some differences which can be explained with the help of the sensitivity study and with inaccurate measurements of acoustic travel time and acoustic transmitter-receiver distances. To check the method of the acoustic tomography acoustic travel time measurements have been performed with the LES-model. So the simulated data could be compared with the simulated acoustic travel times which were analysed using the algorithms of the acoustic tomography method. With the help of this comparison some problems with the algorithm to separate the mean wind influence from the sonic speed were detected. These problems will be solved by using reciprocal acoustic travel time measurement methods. (See also the follow-up study by Weinbrecht.)

 

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Investigation of the evolution of organized convection during a cold air outbreak using a parallelized large-eddy simulation model

 

Leitung:

Siegfried Raasch

Bearbeitung:

Michael Schröter

Laufzeit:

1998-2001

Förderung durch:

DFG

Kurzbeschreibung:

During so called cold-air outbreaks cold and stably stratified air-masses are advected across relatively warm surfaces. The heating from below causes the development of a convective boundary layer in which a typical sequence of convection pattern can be observed: As soon as the cold air hits the warm surface, roll-like secondary flow pattern develop, appearing in satellite images as so called cloud-streets. Further downstream the roll pattern gradually changes to a hexagonal cellular convection pattern. While in classical laboratory experiments of Rayleigh-Benard convection the aspect ratios of the convection patterns are about 1:3, values up to 1:30 are typically observed during cold-air outbreaks. The aspect ratio is defined as the ratio between the wavelength or diameter of the pattern to their height. With boundary layer heights of 2 km, diameters of the convection pattern of about 60 km and more could be observed. Within the last two decades the phenomenon of cold-air outbreaks was increasingly studied using three-dimensional numerical models. These simulations suggested that adiabatic heat sources like latent heat release due to condensation are a condition precedent to observe large aspect ratios. Unfortunately, these simulations could not be carried out satisfactorily so far due to insufficient computer resources. To study the evolution of convective structures during cold-air outbreak situations the computational domains have to be large enough to contain the large cellular pattern, and they must have a spatial resolution fine enough in order to resolve smaller scales (single up- and downdrafts). Hence, the results of previous studies are afflicted with different uncertainties: Either the model domain was too small in order to allow a undisturbed cell growth or a too coarse resolution (up to 2 km) had to be used in order to be able to cover larger areas. Consequently it was unpossible to decide whether interactions between smaller and larger scales may affect the cell structure. For the first time today's massively parallel computers provide the possibility to perform simulations using large model domains and a fine grid-spacing simultaneously. Hence, the aim of this study was to apply a large-eddy simulation model (LES) that is specially designed for the use on massively parallel computers to cold-air outbreak situations to identify the physical processes which lead to the large aspect ratios observed during cold-air outbreak situations. This study removed the uncertainties of the earlier studies. Actually, strong evidence is given that diabatic heat sources are responsible for the existence of large aspect ratios. In contrast to earlier simulations it could be shown that cellular pattern also occur in the dynamic field variables. A further result of this study is that the numerically simulated role-like structures described so far in the literature were obviously due to a insufficient model resolution or unsuitable sub-grid parameterization. As a possible reason that convection rolls failed to appear in earlier simulations and in the simulations performed here, the non-consideration of the shear-increasing impact of spatially inclined boundary layer inversions was detected. (See also the study by Gryschka.)

 

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LES of the atmospheric boundary layer over water surfaces partially covered with sea-ice

 

Leitung:

Siegfried Raasch

Bearbeitung:

Guido Harbusch

Laufzeit:

1996-2000

Förderung durch:

DFG SPP Antarktis

Kurzbeschreibung:

This project used PALM simulations to investigate how the turbulent structure of the atmospheric boundary layer is influenced by small scale surface inhomogeneities with typical diameters of about the boundary layer height zi. The inhomogeneities were designed as discontinuous, chessboard-like variations of the surface heat flux that was prescribed as the bottom boundary condition. A large heat flux amplitude was chosen to create conditions typical of the Arctic or Antarctic marginal ice zone during cold air outbreaks. One main aim of this study was to determine the effects of such large heat flux variations and to describe the influence exerted by variations of incident flow speed and direction. Particular interest was paid to the inhomogeneity-induced secondary circulations that were analyzed using phase averages. Compared with continuous inhomogeneities of the same size, discontinous inhomogeneities principally cause similar, but much stronger effects. In contrast to a homogeneously heated boundary layer the horizontally averaged second and third moments are considerably affected by the inhomogeneities. Significant effects, however, only occur when the diameter of the inhomogeneities is at least as large as the height of the boundary layer. Then, especially the vertical energy transport increases, and the effects of the inhomogeneities start to show up even in the vertical temperature and heat flux profiles that don't exhibit the shapes typical of a homogeneous CBL any longer. The vertical gradient of potential temperature is slightly positive within the entire mixing layer, and the heat flux profile departs from its usual linear shape. These changes, though, are not due to the discontinuous form of the inhomogeneities but mainly due to the large heat flux amplitude. The structure of the secondary circulations, however, is very sensitive to the wavelength and form of the inhomogeneities as well as the heat flux amplitude, wind speed and direction. The main controlling parameter is the near-surface temperature distribution and hence the horizontal pressure gradient perpendicular to the mean flow direction. The secondary circulations vary from direct circulations with updraughts over the centers of the heated areas to rather indirect circulations with updraughts on both sides of the centers and downdraughts just above them. Roll-like circulation pattern occur for a background flow of more than 2.5 m/s. Previous studies often showed that even a moderate background flow of 5 m/s completely eliminates all potential effects of the surface inhomogeneities. In contrast, the present study demonstrated that the influence due to the increasing background flow strongly depend on the orientation of backgraound flow relative to the surface inhomogeneity. On the one hand, even under an incident flow of 7.5 m/s secondary circulations remain strong provided that the wind direction is parallel to the diagonal direction of the chess-board pattern. On the other hand, the effects of the inhomogeneities are considerably weakened even for background velocities as slow as 2.5 m/s provided that the wind vector of the backgroundflow is perpendicular to the side lines of the chess-board squares. (See also the studies by Letzel and Herold.)

 

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Masterarbeiten / Diplomarbeiten

Large-Eddy Simulation of Wind Flows in the Vicinity of Buildings – A Comparison of LES and RANS Results

 

Leitung:

Siegfried Raasch

Bearbeitung:

Katrin Scharf

Laufzeit:

2015-2016

Kurzbeschreibung:

This study aims at showing the advantages of LES over RANS for urban applications. The investigation is motivated by two main focuses which summarize as follows: 1. Although many comparisons of LES and RANS simulations for an idealized building configuration exist today, only very few are carried out for an actual city area. Yet, for engineers and professionals that are asked to give an opinion on planned building configurations, a comparison of LES and RANS for an actual area is of big interest. For this reason this study compares results of a RANS simulation of an urban area to LES results emphasizing the differences in mean wind speed in pedestrian level close to the buildings. 2. To date, not many studies analyzed the turbulent character of wind flows on pedestrian level, however people are not only influenced by mean wind speeds. What is really having an impact on people is the actual wind speed with its variation in space and time. Therefore, information on the turbulent nature of the flow is taken from the LES. A comprehensive analysis of changes in wind speed and direction is carried out as part of this research including a detailed area-wide gust analysis.

 

 

Entwicklung, Implementierung und Validierung eines Algorithmus für das Spalten und Zusammenfassen von Partikeln in einem Langrangeschen-Wolkenphysik-Modell

 

Leitung:

Siegfried Raasch

Bearbeitung:

Johannes Schwenkel

Kurzbeschreibung:

In Lagrangeschen-Wolkenphysik-Modellen (engl. Lagrangian Cloud Models, LCMs ) werden Wolkentropfen explizit als Partikel simuliert. In realen Wolken ändert sich die Anzahlkonzentration von Wolken- zu Regentropfen um mehrere Größenordnungen. Allerdings wird – aufgrund begrenzter Computer-Ressourcen- das Konzept der Super-Tropfen verwendet, wobei ein Super-Tropfen eine große Anzahl realer Tropfen mit identischen Eigenschaften darstellt. Dies führt dazu, dass in typischen Anwendungen Supertropfen verwendet werden, die einige hundert Millionen realer Tropfen darstellen. Mit diesen Ansatz können derzeit große Tropfen im LCM nicht korrekt statistisch repräsentiert werden. Im Rahmen dieses Projekts soll ein nach Unterstrasser und Sölch (2014) entwickelter Algorithmus modifiziert und implementiert werden, der ab einen bestimmten Radius Super-Tropfen aufteilt und somit die Anzahl der dargestellten Tropfen pro Partikel reduziert. Mithilfe dieser Methode soll ein verbesserte Darstellung des Tropfenwachstums erfolgen. Um den Rechenaufwand der erhöhten Super-Tropfen-Anzahl zu verringern soll des Weiteren ein Algorithmus getestet werden, der kleine und gut repäsentierte Super-Tropfen zusammenfasst.

 

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Flugsimulationen in mit LES generierten bodennahen turbulenten Windfeldern

 

Leitung:

Siegfried Raasch

Bearbeitung:

Sebastian Giersch

Laufzeit:

2016-2017

Kurzbeschreibung:

Flugsimulatoren müssen in der Lage sein, Piloten auf möglichst realistische Art und Weise auf den Flugbetrieb und das Flugverhalten vorzubereiten. Das mittlere Windfeld mit seinen turbulenten Schwankungen ist hierbei ein wichtiger Parameter, der berücksichtigt werden muss. In Simulatoren wird dies über die Angabe von mittleren Windgeschwindigkeiten und -richtungen in unterschiedlichen Höhen realisiert. Zusätzlich dazu wird die Turbulenz über statistische Modelle beschrieben. Piloten bestätigen jedoch, dass die Simulation insbesondere der bodennahen Turbulenz ausgelöst durch die umliegende Orographie, Vegetation oder Bebauung nicht der Realität entspricht. Eine Möglichkeit zur Verbesserung dieses Zustandes ist die Verwendung möglichst realer Windsituationen, welche auch die Besonderheiten einzelner Flughäfen berücksichtigen. So sollen mit dem LES Modell PALM turbulente Windfelder generiert werden, um diese anschließend in den frei verfügbaren Flugsimulator FlightGear einzuspeisen. Ziel dieser Methodik ist es, die Auswirkungen auf das Flugverhalten einer landenden Passagiermaschine gegenüber denen unter Verwendung des bisher in FlightGear implementierten Ansatzes eines Dryden Turbulenzspektrums zu überprüfen.

 

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Grobstruktursimulationen der städtischen konvektiven Grenzschicht mit thermisch aktiven Gebäudewänden

 

Leitung:

Siegfried Raasch

Bearbeitung:

Katja Lohmüller

Laufzeit:

2015-2016

Kurzbeschreibung:

In diesem Projekt soll unter konvektiven atmosphärischen Bedingungen die städtische Grenzschicht am Beispiel von idealisierten Städtekonfigurationen und der Stadt Macau untersucht werden. Als Motivation für diese Studie diente ein in der Vergangenheit durchgeführtes Projekt, in dem die Verhältnisse in Macau bei neutraler Schichtung im Vordergrund standen. Für das aktuelle Projekt wird der Schwerpunkt auf der Veränderung der Belüftung in der urbanen Grenzschicht durch die Realisierung thermisch aktiver Gebäudewände nach der Methode von Park et al. (2012) liegen. Der Beitrag von Wärmeemissionen durch Gebäude ist derzeit in PALM nur sehr idealisiert implementiert. Wärmeströme an Gebäudeflächen können nur fest und zeitlich konstant vorgegeben werden. Tatsächlich bestimmt sich in der Natur dieser Wärmestrom dynamisch durch die Temperaturdifferenz zwischen Gebäudewand und anliegender Luft sowie durch den Betrag der wandparallelen Windgeschwindigkeit. In Zukunft wird es durch die Implementierung möglich sein, die zeitlich und räumlich variable Verteilung der Wärmeströme an den vertikalen Gebäudewänden aus den Wandtemperaturen mittels Monin-Obukhov-Theorie zu prognostizieren. Mit entsprechenden Simulationen soll demonstriert werden, wie groß der Einfluss zeitlich und räumlich variabler Wärmestromverteilungen auf das Windfeld innerhalb einer städtischen Grenzschicht ist und wie sich dadurch die Belüftungsverhältnisse ändern.

 

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Simulation und Analyse des Flugverkehrs im Fall eines Squall Line-Ereignisses über Österreich

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Patrick Hupe

Laufzeit:

01/12/2013 - 31/03/2015

 

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Thermischer Komfort in der Stadt – Simulationen mit einem Multi-Agenten Modell

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Anna Lena Bittner

Laufzeit:

01.06.2015-31.05.2016

Kurzbeschreibung:

Mehr als die Hälfte der Weltbevölkerung lebt heute in städtischem Gebiet. Hinzu kommt, dass ein signifikanter Anteil der Bewegungen innerhalb der Stadt zu Fuß erfolgt. Der Fußgänger ist dabei, im Gegensatz zu Bewegungen im Auto oder in der Bahn, direkt dem Wetter ausgesetzt. Ein Ziel von Städteplaner ist es deshalb Städte so zu gestalten, dass sie einen höchstmöglichen thermischen Komfort für Fußgänger bieten. Im Rahmen der Masterarbeit soll deshalb der thermische Komfort von Fußgängern innerhalb der Stadt analysiert werden. Dazu wird zunächst ein Multi-Agent Modell entwickelt mit dem die Bewegung von Fußgängern innerhalb von Städten simuliert werden kann. Anschließend soll der thermische Komfort von jedem Fußgängers durch die Physiologische Äquivalent Temperatur (PET) beschrieben werden.

 

 

Implementierung der anleastischen Approximation in das LES Model PALM und ihre Anwendung auf hochreichende Konvektion

 

Leitung:

Siegfried Raasch

Bearbeitung:

Helge Knoop

Laufzeit:

2015 - 2016 (1 Jahr)

Kurzbeschreibung:

Der am IMUK entwickelte PALM-Code basiert bisher auf einem Boussinesq-approximierten Gleichungssystem mit konstanter Dichte. Er darf daher nur für flache Konvektion eingesetzt werden. Zukünftige Anwendungen, insbesondere auch Simulationen realer meteorologischer Situationen mit hochreichender Konvektion, erfordern die Berücksichtigung der Dichteänderung mit der Höhe. Die so genannte anelastische Approximation und das sich ergebende Gleichungssystem berücksichtigen diese Dichteänderung mit der Höhe. Ihre Implementierung in den PALM-Code ist daher eine Möglichkeit diesen zukünftig auch für die turbulenzauflösende Untersuchung hochreichender Konvektion zu verwenden. Das Ziel dieser Masterarbeit soll nunmehr sein, die anelastische Approximation im PALM-Code zu realisieren. Der entsprechend modifizierte Code soll dann für einen Fall mit hochreichender Konvektion getestet und mit Simulationsergebnissen aus der Literatur verglichen werden.

 

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Kopplung von DIVMET mit dem Flugsimulationsmodell NAVSIM

 

Leitung:

Prof. Dr. Thomas Hauf, Manuela Sauer

Bearbeitung:

Alina Fiehn

Kurzbeschreibung:

Das am Institut entwickelte Programm DIVMET wurde mit dem globalen Flugsimulationsmodell NAVSIM verbunden, indem ein funktionierendes Kommunikationsprotokoll für die beiden Programme entwickelt wurde. Damit können mehrere Flugzeuge gleichzeitig in NAVSIM überwacht und umgeleitet werden.

 

 

Untersuchungen der Hintergrundströmung und des Kamineffekts in ausgewählten unterirdischen Stadtbahnsystemen

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Agnes Straaten

Laufzeit:

01.05.2014 -

Kurzbeschreibung:

Im Rahmen dieser Masterarbeit soll der Einfluss der Wetterlage auf die Hintergrundströmung und den Kamineffekt im Stadtbahnsystem von Hannover untersucht werden. Dabei sollen Messungen an verschiedenen Haltestellen durchgeführt und anschließend mit vorliegenden Messdaten aus dem Dortmunder Stadtbahnsystem verglichen werden. Diese Arbeit findet in Kooperation mit der Arbeitsgruppe Höhlen- und U-Bahnklimatologie der Ruhr-Universität Bochum, Mario Franz von der Uni Bonn und der üstra Hannoversche Verkehrsbetriebe AG statt.

 

 

Bestimmung synthetischer Windklimatologien

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Julia Iov

Laufzeit:

01.01.2014 -

Kurzbeschreibung:

Die Masterarbeit beschäftigt sich mit der Bestimmung synthetischer Windklimatologien. Hierzu wird ein Verfahren angewendet, das aus gemessenen Bodenwinddaten und Windfeldsimulationen Windklimatologien an benachbarten Flächen berechnet.

 

 

Interaktion zwischen dem Innen- und Außenraumklima bei Wohngebäuden

 

Leitung:

Prof. Dr. Günter Groß

Bearbeitung:

B.Sc. Katharina Isensee

Laufzeit:

01.01.2014 -

Kurzbeschreibung:

In dieser Masterarbeit soll die Verknüpfung zwischen dem Innen- und Außenraumklima von Wohngebäuden betrachtet werden. Hierzu werden Messungen innerhalb und außerhalb eines Gebäudes durchgeführt und überprüft, wie sich das Außenklima auf das innere Raumklima auswirkt. Insbesondere sollen dabei die Messungen in Tropennächte betrachtet werden.

 

 

Activation of aerosols studied with an LCM

 

Bearbeitung:

Fabian Hoffmann

Laufzeit:

2012-2013

Kurzbeschreibung:

The activation of aerosols is neglected or simplified in many current cloud physical modeling approaches. By simulating so-called super-droplets, which are Lagrangian particles representing a certain number of real droplets of the same features (e.g., radius, aerosol mass), the LCM (Lagrangian Cloud Model) offers a very accurate parametrization of the activation process considering solute and curvature effects.

 

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Shadow effects of shallow convective clouds on the atmospheric boundary layer including feedbacks on the cloud field

 

Bearbeitung:

Tobias Gronemeier

Laufzeit:

2012-2014

Kurzbeschreibung:

In a former master thesis at the PALM group it was shown that cloud shadows do affect the atmospheric boundary layer, but the used cloud definition was heavily idealised so that clouds were only defined by upwind velocity. In this master thesis the cloud definition will be more realistic due to a simulated liquid boundary layer and a cloud definition based on the liquid water content. To investigate the feedbacks of cloud shadows on the cloud field statistics of the clouds calculated after a method of Dawe and Austin (2012) will be used.

 

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The influence of surface heterogeneities on the formation frequency and intensity of dust devils

 

Bearbeitung:

Maren Weismüller

Laufzeit:

2012

Kurzbeschreibung:

Dust devils have been simulated by large-eddy simulation models with a resolution of up to 1m. However, the intensity of those simulated dust devils does not reach the intensity of observed dust devils. Since one possible reason is that previous studies only used homogeneously heated surfaces, this study will investigate the effect of thermal surface heterogeneities on the formation of dust devils.

 

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The influence of clouds on secondary circulation triggered by surface heterogeneities

 

Bearbeitung:

Sonja Drüke

Laufzeit:

2010

Kurzbeschreibung:

Since the study of Shen and Leclerc (1995) and others later on it is known that surface heterogeneities trigger secondary circulations. In this master thesis the influence of clouds on these secondary circulations shall be investigated. LES will be carried out for the convective boundary layer over an inhomogenious surface, considering the shadow of clouds.

 

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Coriolis instability in coupled atmosphere-ocean LES

 

Bearbeitung:

Jens Fricke

Laufzeit:

2010-2011

Kurzbeschreibung:

The Coriolis force does not change the total energy of a flow. However it does affect its hydrodynamic stability with respect to small perturbation and therefore modifies the energy budget indirectly by transferring energy between the three velocity components. In the past, these instability mechanisms (e.g. inflection point instability) have been studied theoretically and numerically, both, in the atmospheric boundary layer (driven by a body pressure force), and the ocean mixed layer (driven by the surface stress). The studies identified very large effects of the Coriolis force on the flow structure. In this thesis, the effect of the Coriolis force will be studied for the first time for the coupled atmosphere-ocean boundary layer under neutral / freshwater conditions, using coupled PALM simulations for atmosphere and ocean.

 

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Footprint modelling via LES for the LITFASS-2003 measuring campaign

 

Bearbeitung:

Farah Kanani

Laufzeit:

2009-2010

Kurzbeschreibung:

In heterogeneous terrain, the measurement of turbulent fluxes involves several difficulties. The flux that is measured at a certain sensor point is not produced directly beneath the sensor, but is rather produced in areas upwind of the sensor. Therefor, the areas of influence on a flux measurement have to be determined to diagnose the representativeness of a measurement. The area of influence is called "footprint" of a measurement. In this thesis footprints shall be determined for the measuring sites of the LITFASS-2003 measuring campaign using the parallelized LES-model PALM with an embedded Lagrangian particle model. Particles are released into the flow, moving by advection and dissipation, and are registered once intersecting with a given measuring height. With the knowledge of the particles' origin the area of influence on the measurement can be determined. Modelling footprints in heterogeneous terrain requires a large number of particles to ensure for sufficient footprint statistics. The main objective of this thesis is to determine wether or not the available computational resources meet this requirement for modelling footprints in heterogeneous terrain.

 

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Development of a higher order advection scheme and its implementation in PALM

 

Bearbeitung:

Matthias Sühring

Laufzeit:

2009-2010

Kurzbeschreibung:

The numerical solution of the advection equation holds qualitatively strong dependencies on the wavenumber. Even scales in the range of the grid length show large errors in amplitude and phase speed. Furthermore, lower order discretization of the advection term implicates non-physical oscillations at strong gradients. So Stevens at al. (1999) assume that the resolved entrainment flux at the top of the boundary layer depend on the quality of the advection scheme. The 5th order finite difference scheme of Wicker and Skamarock (2002) possesses a well trade-off between numerical properties and computational costs. This thesis deals with the development of a higher order finite difference scheme based on the scheme of Wicker and Skamarock (2002) and its implementation in PALM. Also an optimization for large numbers of processors shall be accompanied. Furthermore, simulations of the convective boundary layer are carried out to show the enhanced numerical properties of this higher order advection scheme.

 

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Influence of small scale heterogeneities on the near surface turbulence structure

 

Bearbeitung:

Linda Voß

Laufzeit:

2009-2010

Kurzbeschreibung:

Inhomogeneities due to differences in the surface structure may trigger secondary circulations in the convective boundary layer. Turbulence measurements with the eddy-covariance method are not able to detect these fluxes. Heterogeneities with diameters of a few hundred meters are therefore suspected to be partly responsible for the non-closure of the energy balance in a height of 2 m (height of energy balance stations). Within this thesis high resolution LES simulations for such small scale heterogeneities shall be performed and analyzed with respect to possible stationary near surface circulations.

 

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Investigations of self organized and forced roll convection during a cold air outbreak with LES

 

Bearbeitung:

Jens Kampmeyer

Laufzeit:

2009-2010

Kurzbeschreibung:

This Thesis deals with two types of roll convection. At first, the laminar scenario conditions from Gryschka M. et.al (2005) are used to generate self organized roll convection via turbulent inflow according to Lund T. et al. (1998). In the second part, results of Gryschka M. et.al (2008) are used for investigate the influence of ice inhomogeneities to so called forced rolls. Therefore the subjection of explicit ice gabs-variation to roll-reaching in flow direction will be studied.

 

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LES study on the effect of secondary circulations on the boundary layer turbulence during LITFASS-2003

 

Bearbeitung:

Björn Maronga

Laufzeit:

2009-2010

Kurzbeschreibung:

Former large-eddy simulation (LES) studies of Uhlenbrock (2006) investigated the turbulent structure of the atmospheric boundary layer over heterogeneous land surfaces during the LITFASS-2003 measurement campaign. The results showed the development of thermal induced mesoscale circulations (such as roll convections) and its impact on fluxes of different scales in the boundary layer. Within the scope of this thesis some of these simulations shall be repeated with technical improvements of the parallelized LES-model PALM. Furthermore, correlations between the structure of the secondary circulations and the land surface heteorogeneities shall be identified, especially in case of roll convections, to give better explanations about the structure of these circulations. In addition to this purpose, corresponding PALM simulations considering topography and possibly a vegetation model could be compared to the "flat" simulations.

 

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LES of Kármán vortex streets in the atmospheric boundary layer

 

Bearbeitung:

Rieke Heinze

Laufzeit:

2008-2009

Kurzbeschreibung:

Atmospheric vortex streets consist of mesoscale eddies with vertical axis in the wake of large islands. They resemble classical Kármán vortex streets which occur in laboratory experiments behind a cylinder. The islands after which vortex streets appear typically have diameters about 20km and mountain tops about 2-4km. Usually, vortex streets can be found in the stratocumulus capped mixed layer over the ocean when there is a strong elevated inversion well below the island top. Since then, the air is forced to go mainly around the mountain and the stratocumulus clouds act as tracer. The object of the project is the investigation of atmospheric vortex streets with aid of LES. The topography consists of a single idealized mountain. Characteristic parameters like shedding rate or propagation velocity shall be determined. At first, the dry version of PALM is used to generate Kármán like vortex streets. Then, it is planned to simulate a stratocumulus capped boundary layer with vortex street like cloud pattern. An animation of a Kármán vortex street can be found in our movie gallery.

 

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LES study on the applicability of the Taylor hypothesis in the urban boundary layer

 

Bearbeitung:

Yvonne Breitenbach

Laufzeit:

31/03/2008-30/06/2009

Kurzbeschreibung:

Tower measurements only give direct information on the vertical and temporal structure of the urban atmosphere; horizontal information may be deduced using Taylor’s Hypothesis of frozen turbulence (Taylor, 1938). In vegetation canopy flow, however, the local mean velocity is an insufficient estimator for the convection velocity, and near the canopy Taylor’s Hypothesis is not applicable (Shaw et al., 1995). The stiff urban canopy poses the same questions: is Taylor’s Hypothesis valid, and if so, which is the best estimator for the convection velocity in the urban roughness sublayer? To answer these questions, PALM directly evaluates and compares single-point and two-point statistics.

 

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LES of quarters in Hong Kong

 

Bearbeitung:

Carolin Weinreis

Laufzeit:

2007-2008

Kurzbeschreibung:

This project is a cooperation with the Chinese University of Hong Kong. The objective of this study is to investigate the ventilation on pedestrian level in two selected quarters of Hong Kong (Tsim Sha Tsui and Mong Kok in Kowloon). Hong Kong as one of the most densely built-up and populated cities in the world has a strong interest to improve their urban climate by increasing the natural wind flow within the city. Very tall buildings, certain architectural structures or street positions have the capability to enhance the wind velocity. PALM is used to simulate the wind flow and to explicitly resolve the turbulent structures around obstacles. For direct comparison simulations with different wind directions and topographies are generated. Our purpose of this study is to provide guidelines for urban designers. How can they plan future projects in Hong Kong to improve the ventilation?

 

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Large-eddy simulation of flow and pollutant dispersion in street canyons

 

Bearbeitung:

Martina Krane

Laufzeit:

2006-2007

Kurzbeschreibung:

The objective of the project is the examination of turbulent wind flow and pollutant dispersion in street canyons. A street canyon is a narrow street flanked by tall buildings. The street canyon domain is a region where large amounts of pollutants are released near the ground from motor vehicle exhausts and near the roof level from domestic (or industrial) chimneys. The analysis and forecasting of a single source contribution to the total pollution does require a fairly precise knowledge of the dispersion process inside the street canyon. The flow inside a street canyon is characterized by manifold recirculation regions overlapping with complex wake structures and the fluid motion governing the dispersion process is highly turbulent. To understand the flow and dispersion characteristics inside street canyons a lot of studies have already been made. An often used model to analyse flow and pollutant dispersion in street canyons is the k-ε model, which parametrizes the turbulence of the flow completely. Sini et al. (1996: Pollutant dispersion and thermal effects in urban street canyons; Atmos. Env. Vol. 30, No. 15, 2659-2677) have shown that for wind directions approximately perpendicular to the street canyon three different flow regimes depending on the value of the aspect ratio (street width/ canyon height) are formed, the isolated roughness flow, the wake interference flow and the skimming flow. The number and arrangement of the vortex inside the street canyon affect the vertical exchange and so the pollutant dispersion inside the street canyon significantly. Aim of this project is to use the LES-model PALM, which resolves the turbulence of the flow, to analyse if Sini et al. describe the vortex inside the street canyon correctly and if the turbulence parametrization in k-ε models can describe the turbulent exchange in street canyons quantitatively and qualitatively correct. Besides this the flow pattern and the pollutant dispersion inside street canyons shall be examined for three different aspect ratios.

 

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LES of dust devils

 

Bearbeitung:

Theres Riechelmann

Laufzeit:

2006-2007

Kurzbeschreibung:

The objective of this project is to analyze dust devils in the atmospheric boundary layer using LES, particularly regarding to its thermodynamical structure, formation mechanism and dependency of dust devil characteristics on atmospheric conditions. Dust devils are particle loaded vertical convective vortices in the convective boundary layer. They are characterized by high rotating wind speeds, significant electrostatic fields, and by reduced pressure and enhanced temperature at their centers. Convective vortices can be found on Earth and Mars and are formed as a result of vertical instability in the atmosphere (e.g. due to ground heating by strong insolation), wherever a superadiabatic atmospheric lapse rate, a source of vorticity and a supply of sand, dust or debris are present. Although there has been more than one century of investigations about dust devils, the knowledge of dust devils is still limited. Former investigations were focused, beside the structure and characteristics of dust devils, at impacts on convection, arid zone erosion and sediment transport. Furthermore theoretical vortex models were developed and compared with observations and laboratory simulations of dust devils. Since a couple of years there also has been LES studies of dust devils. Kanak et al. (2005) likely produced the first LES of dust devil-scale vortices on Earth that had physical characteristics in reasonable agreement with observations. But they also located some limitations and open questions during their work. Within this project it is now tried to analyze and solve some of these and former questions. For those purposes we planned a simulation which resolves the relevant large-scale turbulent structures of the atmospheric boundary layer and the small-scale dust devil itself simultaneously for the first time. An animation of a dust devil can be found in our gallery.

 

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Monte-Carlo-simulation of radiation transfer through an inhomogeneous cloud layer

 

Bearbeitung:

Ulrich Hamann

Laufzeit:

2003-2004

Kurzbeschreibung:

A Monte Carlo simulation for radiation models the path of numerous photon packages through the atmosphere. This approach is necessary, if the atmosphere is not horizontally homogeneous, but contains variable formed clouds. The cloud fields used in this work are provided by the LES model of the PALM group. The simulation will include the transfer of wavelength in the infrared, the visible and the UV spectrum. The goal of the simulation are the heating rate at the surface derived from the irradiance as well as the heating inside the clouds. As this method demands a high computational effort, the program will be parallelized.

 

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Large-eddy simulation of cold air outbreaks with a stationary domain

 

Bearbeitung:

Micha Gryschka

Laufzeit:

2002-2003

Kurzbeschreibung:

With recent simulations (Schröter) the PALM group could show that all LES based computations of roll vortices during cold air outbreaks are due to modell insufficiencies, i.e. unphysical conditions. One possible reason could be that a slanted inversion is needed for the generation of roll vortices and thereby a shear flow at the top of the boundary layer. To examine this, it is necessary to simulate the ABL with a stationary model domain. Therefore noncylic lateral boundaries must be implemented into the PALM code, which is part of the work of this thesis.

 

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Coupling of LES models of different spatial resolution performing on massively parallel computers

 

Bearbeitung:

Gerald Steinfeld

Laufzeit:

2002-2003

Kurzbeschreibung:

Large-eddy-simulations (LES) resolve the large, energy containing elements of a turbulent flow explicitly, while those elements that are smaller than a specific scale, which depends on the chosen spatial filter, have to be parameterized. As the maximum eddy size decreases with declining distance to a given physical boundary, even the dominant flow pattern are subgrid scale and therefore not resolved explicitly any more in the vicinity of the boundary. The presently known parameterization schemes are not able to describe the turbulence near a physical boundary in a fully satisfactory way. Another possibility to consider the turbulence structure near a physical boundary is to run - in addition to the original LES-model - a nested fine grid model in those areas of the coarse-grid domain which are situated near to the physical boundaries. Two different kinds of nested grid models are known in literature. On the one hand there exists the concept of one-way-interaction (OWI). Here, information is only transmitted from the coarse to the fine grid, as the boundary conditions for the fine grid model are derived from the coarse grid model. On the other hand, the additional consideration of fine grid data for the evaluation of the coarse-grid data leads to the concept of two-way-interaction (TWI). Within the scope of this thesis the possibility to run OWI- and TWI-nesting was added to the parallelized LES-model PALM. It was found out that in the case of using a OWI the non-physical boundaries of the fine grid become an artificial source of production of TKE. A simulation with the same resolution in the coarse and the nested grid showed that this behaviour appears to be independent of the concrete method applied to derive boundary conditions from coarse grid data for the nested fine grid. In a similar simulation using the TWI-scheme a different development between coarse and fine grid fields did not occur, which means that the coarse grid data always contains the necessary information to provide appropriate boundary conditions for the nested grid. Further, "real" nesting simulations, applying the TWI scheme led to an improvement compared with the corresponding OWI-scheme results (e.g. no maximum of the variances of u and v resp. TKE could be observed at the last grid point below the fine grid boundary), but the obtained results were still not totally convincing so that further investigations on these problems exceeding the scope of this thesis would be necessary before nesting can be used as a standard tool in PALM.

 

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The influence of two-dimensional surface heterogeneities on the structure and development of the atmospheric boundary layer

 

Bearbeitung:

Marcus Herold

Laufzeit:

2001-2002

Kurzbeschreibung:

Heterogeneities of the surface vertical heat flux may induce mesoscale circulations. This has been confirmed by recent observations and numerical studies. In the studies of this diploma thesis a parallelized large-eddy simulation model has been used to examine the influences of two-dimensional continuous and discontinuous inhomogeneities arranged in a chessboard-type pattern. Previous numerical studies revealed strong influences of a number of parameters on the strength and type of the secondary circulations. The most important are perturbation wavelength, perturbation amplitude, shape of the inhomogeneity, wind speed and wind direction. Inhomogeneities with a horizontal extent of the boundary layer height or larger were found to have the largest effects on the mean thermal and dynamical properties of the boundary layer, such as vertical temperature profiles and horizontal diffusion. Thus, the purpose of this study was to examine the effects of heterogeneities at scales of tens of kilometers, the scales of which the strongest effects had been reported. The simulation results revealed some significant differences to previous studies on one-dimensional heterogeneities of the same size and two-dimensional perturbations of the size of the boundary layer height (and smaller). For example, the development of secondary circulations was found to be strongly dependent on the shape of the inhomogenity. The temporal oscillations found in studies on one-dimensional inhomogeneities could not be observed in runs with discontinuous perturbations of this study. The two-dimensionality also triggers more complex circulation patterns caused by near-surface convergence lines over the areas of high heat flux values and corresponding divergences at boundary layer height. Due to the nature of the velocity field in the circulation, increased horizontal diffusion and velocity variances can be observed in the lower and upper part of the boundary layer, while their values in the middle remain almost unchanged compared to cases with uniform surface heating. (See also the studies by Schröter and Uhlenbrock.)

 

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Development of a Multigrid-Scheme in order to Solve Elliptic Differential Equations on Massivly Parallel Computers and its Implementation in the LES Model PALM

 

Bearbeitung:

Jörg Uhlenbrock

Laufzeit:

2000-2001

Kurzbeschreibung:

In the near future, computations with the LES-model PALM should also be done with an irregular (bottom) boundary condition (e.g. due to buildings). Thereafter, the modified model will be used for investigations of the tubulent flow structure in a simplified urban area. To solve the Boussinesq-approximated Navier-Stokes equations, the application of a so-called "pressure solver" is necessary after each timestep in the model, resulting in a windfield with no divergence. With regard to our model, the pressure solver is just a Poisson equation. In the actual model version the solution of the poisson equation is realized by a non-iterative procedure, based on Fast-Fourier Tansformation (FFT). But this method cannot be applied to an irregular boundary condition. For such a boundary, iterative procedures are more suitable, e.g. the Gauss-Seidel method. Furthermore, the iterative methods can easily be paralellized. Unfortunately, the convergence of these methods on large computational grids, as used by our model runs, is very slow. Therefore, the FFT has been the only practical method to solve the Poisson equation up to date. A realistic alternative is provided by the so-called multigrid method, which accelerates the convergence of iterative methods. The solution of the Poisson equation then becomes independent of grid size. The target of this thesis is to develop a parallelized version of such a multigrid method for solving the Poisson equation, and its implementation in PALM.

 

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Large-eddy simulations of thermally-induced circulations in the convective boundary layer

 

Bearbeitung:

Marcus Oliver Letzel

Laufzeit:

2000-2001

Kurzbeschreibung:

Mesoscale circulations induced thermally by differential boundary layer heating due to surface inhomogeneities on scales of 10 km and more can significantly affect the average profiles and the structure of the atmospheric boundary layer. The inhomogeneities studied were one-dimensional and were represented in the model by sinusoidal surface heatflux variations. The results of this numerical study suggest that mesoscale circulations may considerably impede the typical quasi-steady convective boundary layer development. The circulations are affected - often nonlinearly - by a number of parameters: Mean surface heatflux, perturbation wavelength and amplitude, subsidence rate, background wind magnitude and direction as well as the geographical latitude. The simulated mesoscale circulations were periodic, but asymmetric, which clearly shows up in form of higher wavenumber peaks in the power spectra of velocity and temperature. Vertical profiles and time series demonstrate that the onset of the mesoscale circulation triggers off a temporal boundary layer oscillation, whose period is proportional to the time that the inhomogeneous surface heatflux information takes to be advected from its minimum to its maximum and whose amplitude decreases with time due to friction. (See also the follow-up study by Herold and the studies by Schröter and Uhlenbrock.)

 

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Bachelorarbeiten

Simulation des Ausweichens von Flugzeugen vor Wetter und Verkehr

 

Leitung:

Prof. Dr. Thomas Hauf, Manuela Sauer

Bearbeitung:

Kezia Lange

Laufzeit:

01/04/2014 - 31/10/2014

Kurzbeschreibung:

Innerhalb dieser Bachelorarbeit soll das vorhandene Modell DIVMET, welches das Ausweichen von Flugzeugen vor gefährlichem Wetter simuliert, so erweitert werden, dass damit auch das Ausweichen vor einem anderen Flugzeug simuliert wird.

 

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Die Atmosphäre über der antarktischen Weddell-See - Stabilität und interne Grenzschichtstruktur im Winter

 

Leitung:

Prof. Dr. Thomas Hauf

Bearbeitung:

Annika Bollmann

Laufzeit:

01/11/2013 - 31/05/2014

Kurzbeschreibung:

Bachelorarbeit am Institut für Flugführung der TU Braunschweig. Die Grenzschichtstruktur und Stabilität der Atmosphäre über zugefrorenem Meereis zeichnet sich im Winter typischerweise durch eine Bodeninversion oder eine dünne, mechanisch gemischte Grenzschicht von etwa 30 - 350 m Dicke aus. Informationen über die Atmosphäre über dem Meereis der Polargebiete im Winter basieren überwiegend auf Messungen in der Arktis. Ziel der Bachelorarbeit ist es, einen Datensatz von Grenzschichtmessungen auszuwerten, die während der Winterfahrt der Polarstern ANT-XXIX/6 (8. Juni 2013, Kapstadt, bis 12. August 2013, Punta Arenas) in der Antarktis mit dem unbemannten meteorologischen Flugzeug MMAV aufgenommen wurden. Dabei steht im Vordergrund die Auswertung der Temperaturdaten.

 

 

Aufstellung eines Modellgleichungssystems auf Basis der anelastischen Approximation und Bestimmung der Auswirkungen auf die PALM-Modellnumerik

 

Leitung:

Siegfried Raasch

Bearbeitung:

Sebastian Giersch

Laufzeit:

2014

Kurzbeschreibung:

Hochreichende Konvektion und die damit verbundene Wolkenbildung hat einen großen Einfluss sowohl auf das aktuelle Wettergeschehen als auch auf das Klima der Erde. Zum besseren Verständnis dieses Phänomens und dessen Wechselwirkungen mit der Atmosphäre sind Large-Eddy Simulationen notwendig, da diese im Gegensatz zu den Wettervorhersagemodellen den Einfluss der Turbulenz auf hochreichende konvektive Bewegungen explizit berücksichtigen. Eine Möglichkeit numerische Simulationen von hochreichender Konvektion zu realisieren ist die anelastische Approximation, welche im Gegensatz zu einem inkompressiblen Gleichungssystem Dichteänderungen mit der Höhe zulässt. Das am IMUK in Hannover entwickelte Large-Eddy Simulationsmodell PALM basiert zur Zeit auf der inkompressiblen Form der Kontinuitätsgleichung. Deshalb sind Berechnungen von hochreichenden konvektiven Bewegungen nicht zulässig. Um diese zukünftig zu ermöglichen, wird im Folgenden die anelastische Approximation eingeführt. So wurde ein anelastisches Gleichungssystem hergeleitet, mit dem auch hochreichende Konvektion berechnet werden kann. Außerdem wurden zwei im Modell implementierte numerische Verfahren an die anelastische Approximation angepasst. Betrachtet wurde dabei die Bestimmungsgleichung für den Stördruck und das Advektionsverfahren von Wicker und Skamarock (2002). Weiterhin wurden verschiedene numerische Simulationen durchgeführt, um die an das anelastische Gleichungssystem angepassten numerischen Verfahren zu validieren. Es konnte bestätigt werden, dass die aus den beiden Approximationen abgeleiteten Gleichungen viele Gemeinsamkeiten haben, und dass das inkompressible Gleichungssystem aus dem Anelastischen unter der Annahme einer konstanten Dichte hergeleitet werden kann. Auch die numerischen Simulationen verdeutlichen die Gemeinsamkeiten. So wurden die auf einer inkompressiblen Strömung basierenden numerischen Verfahren durch einzelne Modifikationen erfolgreich an die anelastische Approximation angepasst. Daher lassen die Ergebnisse den Schluss zu, dass sich ein numerisches Simulationsmodell auf Basis einer inkompressiblen Strömung ohne großen Aufwand auf das anelastische Gleichungssystem umstellen lässt. Somit wird es in naher Zukunft möglich sein, mit PALM hochreichende konvektive Vorgänge zu analysieren.

 

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Bestimmung der Entrainment- und Encroachmentraten am Oberrand der konvektiven Grenzschicht mit Hilfe von Lagrangeschen Partikeln.

 

Leitung:

Siegfried Raasch

Bearbeitung:

Helge Knoop

Laufzeit:

2013 (5 Monate)

Kurzbeschreibung:

Entrainment und Encroachment Prozesse haben einen großen Einfluss auf das Verhalten einer wolkenlosen konvektiven Grenzschicht. Entrainment beschreibt den Mischvorgang am Oberrand dieser Grenzschicht und Encroachment das thermodynamische Wachstum dieser Schicht. Die zugrunde liegenden physikalischen Gesetzmäßigkeiten dieser Prozesse sind bis heute nicht gut verstanden. Eine zuverlässige und realistische Entrainment Parametrierung ist jedoch von großer Bedeutung für numerische Wettervorhersagemodelle und atmosphärische Ausbreitungsrechnungen. In dieser Studie wird ein neues Entrainment Bestimmungsverfahren entwickelt, das auf Lagrangeschen Partikeln basiert. Es ermöglicht die Bestimmung dreidimensional aufgelöster Entrainment Raten in Grobstruktursimulationen.

 

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Vereisungswarnung mit ADWICE in den bodennahen Modellschichten

 

Leitung:

Thomas Hauf

 

 

Vereisungswarnung mit ADWICE in den bodennahen Modellschichten

 

Leitung:

Thomas Hauf

Bearbeitung:

Johannes Schwenkel

Kurzbeschreibung:

Mithilfe des Modells ADWICE können Gebiete identifiziert werden, in denen ein Vereisungspotential vorliegt. Insbesondere für den Flugverkehr sind solche Warnungen von großer Bedeutung um die Gefahr von Vereisung einzuschätzen. Im Rahmen dieses Projekts soll untersucht werden, ob ADWICE geeignet ist um Vereisungswarnungen für Windkraftanlagen zu erstellen.

 

 

Nutzung von AMDAR-Daten zur Unterstützung der Vereisungsdiagnose von ADWICE über Europa

 

Leitung:

Thomas Hauf

Laufzeit:

2013

 

 

LES of a simple block canopy and sensitivity studies with the vegetation model in PALM

 

Bearbeitung:

Hannes Schulz

Laufzeit:

2012

Kurzbeschreibung:

Large-Eddy Simulation of a neutrally stratified, pressure-driven turbulent flow above and within a 15 m high idealized vegetation canopy. The influence of the vegetation elements was described in PALM by the embedded vegetation model after Watanabe (2004). Mean profiles and turbulence statistics were validated against results from other LES studies and experimental data. Coherent turbulent structures developing according to the mixing-layer analogy (Raupach et al., 1996) were analyzed by calculating turbulence length scales from mean vertical velocity gradients and two-point correlations at the canopy top. A sensitivity study was performed to investigate effects of the numerical grid resolution on the model’s results, particularly with regard to vertical transport and the spacing between coherent eddies forming from inflection point instabilities at the canopy top.

 

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Developement and test of a positive definite method for the Wicker-Skamarock-advection scheme

 

Bearbeitung:

Lennart Böske

Laufzeit:

2011-2012

Kurzbeschreibung:

The 5th order advection scheme of Wicker and Skamarock (2002) as it is implemented in PALM - hereafter WS5 - is neither positive definite nor monotonic. In case of sharp gradients of a transported scalar, discretisation errors of WS5 can lead to non-physical negative values or unwanted amplification of initial extrema. In order to achieve positive definiteness or monotonicity so-called flux limiters can be used to correct the solutions given by WS5 (Skamarock, 2006; Wang et al., 2009). In this thesis two low-order flux limiters (positive definite and monotonic) for WS5 are investigated using 1D-simulations, focusing on shape preserving qualities of the modified scheme. With regard to a potential future implementation in PALM the increase in computing time caused by the flux limiters is evaluated.

 

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Numerical impacts on the strength of simulated dust devils

 

Bearbeitung:

Fabian Hoffmann

Laufzeit:

2011-2011

Kurzbeschreibung:

The strength of simulated dust devils is smaller than observed values. Particularly, the pressure deviation from ambient is more than one order of magnitude too small. The impact of numerical methods is investigated on the strength, frequency and structure of dust devils in a high resolution large-eddy simulation. Analysed numerical methods are the calculation of perturbation pressure and different advection schemes (Piacsek and Williams (1970) and Wicker and Skamarock (2002)).

 

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