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Book ; Online ; Thesis: High resolution numerical simulations of dust devils in the convective boundary layer – effects of detailed process representation on Vortex development and dust release

Giersch, Sebastian [Verfasser] / Raasch, Siegfried [Akademischer Betreuer]

2024

Author's details	Sebastian Giersch ; Betreuer: Siegfried Raasch
Keywords	Geowissenschaften ; Earth Sciences
Subject code	sg550
Language	English
Publisher	Gottfried Wilhelm Leibniz Universität
Publishing place	Hannover
Document type	Book ; Online ; Thesis
Database	Digital theses on the web

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Book ; Online ; Thesis: Development of high resolution large-eddy simulation tools for assessment of turbulent flux measurements

Abdul Huq, Sadiq Umar [Verfasser] / Mauder, Matthias [Akademischer Betreuer] / Eiff, Olivier [Akademischer Betreuer] / Raasch, Siegfried [Akademischer Betreuer]

2024

Author's details	Sadiq Umar Abdul Huq ; Matthias Mauder, Olivier Eiff, Siegfried Raasch
Keywords	Geowissenschaften ; Earth Sciences
Subject code	sg550
Language	English
Publisher	KIT-Bibliothek
Publishing place	Karlsruhe
Document type	Book ; Online ; Thesis
Database	Digital theses on the web

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Book ; Online: Scan strategies for wind profiling with Doppler lidar – an large-eddy simulation (LES)-based evaluation

Rahlves, Charlotte / Beyrich, Frank / Raasch, Siegfried

eISSN: 1867-8548

2022

Abstract: Doppler-lidar scan techniques for wind profiling rely on the assumption of a horizontally homogeneous wind field and stationarity for the duration of the scan. As this condition is mostly violated in reality, detailed knowledge of the resulting ... ...

Abstract	Doppler-lidar scan techniques for wind profiling rely on the assumption of a horizontally homogeneous wind field and stationarity for the duration of the scan. As this condition is mostly violated in reality, detailed knowledge of the resulting measurement error is required. The objective of this study is to quantify and compare the expected error associated with Doppler-lidar wind profiling for different scan strategies and meteorological conditions by performing virtual Doppler-lidar measurements implemented in a large-eddy simulation (LES) model. Various factors influencing the lidar retrieval error are analyzed through comparison of the wind measured by the virtual lidar with the “true” value generated by the LES. These factors include averaging interval length, zenith angle configuration, scan technique and instrument orientation (cardinal direction). For the first time, ensemble simulations are used to determine the statistically expected uncertainty of the lidar error. The analysis reveals a root-mean-square deviation (RMSD) of less than 1 m s −1 for 10 min averages of wind speed measurements in a moderately convective boundary layer, while RMSD exceeds 2 m s −1 in strongly convective conditions. Unlike instrument orientation with respect to the main flow and scanning scheme, the zenith angle configuration proved to have significant effect on the retrieval error. Horizontal wind speed error is reduced when a larger zenith angle configuration is used but is increased for measurements of vertical wind. Furthermore, we find that extending the averaging interval length of lidar measurements reduces the error. In addition, a longer duration of a full scan cycle and hence a smaller number of scans per averaging interval increases the error. Results suggest that the scan strategy has a measurable impact on the lidar retrieval error and that instrument configuration should be chosen depending on the quantity of interest and the flow conditions in which the measurement is performed.
Subject code	333
Language	English
Publishing date	2022-05-09
Publishing country	de
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Article: An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin–Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions

Maronga, Björn / Knigge, Christoph / Raasch, Siegfried

Boundary-layer meteorology. 2020 Feb., v. 174, no. 2

2020

Abstract: Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first grid level above the surface. We ... ...

Abstract	Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves grid convergence of these quantities. The LES data indicate improved grid convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved grid convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
Keywords	Monin-Obukhov similarity theory ; heat transfer ; mixing ; sensible heat flux ; shear stress ; simulation models ; temperature profiles ; wind speed
Language	English
Dates of publication	2020-02
Size	p. 297-325.
Publishing place	Springer Netherlands
Document type	Article
ZDB-ID	1477639-X
ISSN	1573-1472 ; 0006-8314
ISSN (online)	1573-1472
ISSN	0006-8314
DOI	10.1007/s10546-019-00485-w
Database	NAL-Catalogue (AGRICOLA)

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Book ; Online: Scan strategies for wind profiling with Doppler lidar – An LES-based evaluation

Rahlves, Charlotte / Beyrich, Frank / Raasch, Siegfried

eISSN: 1867-8548

2021

Abstract: Lidar scan techniques for wind profiling rely on the assumption of a horizontally homogeneous wind field and stationarity for the duration of the scan. As this condition is mostly violated in reality, detailed knowledge of the resulting measurement error ...

Abstract	Lidar scan techniques for wind profiling rely on the assumption of a horizontally homogeneous wind field and stationarity for the duration of the scan. As this condition is mostly violated in reality, detailed knowledge of the resulting measurement error is required. The objective of this study is to quantify and compare the expected error associated with Doppler-lidar wind profiling for different scan strategies and meteorological conditions by performing virtual measurements implemented in a large-eddy simulation (LES) model. Various factors influencing the lidar retrieval error are analyzed through comparison of the wind measured by the virtual lidar with the ‘true’ value generated by the LES. These factors include averaging interval length, zenith angle configuration, scan technique and instrument orientation. For the first time, ensemble simulations are used to determine the statistically expected uncertainty of the lidar error. The analysis reveals a root-mean-square deviation (RMSD) of less than 1 m s −1 for 10 min averages of wind speed measurements in a moderately convective boundary layer, while RMSD exceeds 2 m s −1 in strongly convective conditions. Unlike instrument orientation and scanning scheme, the zenith angle configuration proved to have significant effect on the retrieval error. Horizontal wind speed error is reduced when a larger zenith angle configuration is used, but is increased for measurements of vertical wind. Results suggest that the scan strategy has a relevant effect on the lidar retrieval error and that instrument configuration should be chosen depending on the quantity of interest and the flow conditions in which the measurement is performed.
Subject code	333
Language	English
Publishing date	2021-12-13
Publishing country	de
Document type	Book ; Online
Database	BASE - Bielefeld Academic Search Engine (life sciences selection)

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Book: Verbundvorhaben M2 - Benchmarking - Leistungsanalyse: Teilvorhaben 2

Raasch, Siegfried

Abstract: Ziel dieses Vorhabens ist die Entwicklung und Generierung einer Reihe von Benchmark-Faellen, die von der HD(CP)2-Gemeinschaft genutzt werden koennen. Darueber hinaus soll erkundet werden, wie die Unsicherheiten bei der Repraesentation von ... ...

Institution	Leibniz Universitaet Hannover, Institut fuer Meteorologie und Klimatologie, Herrenhaeuserstr. 2, 30419, Hannover, DE
Abstract	Ziel dieses Vorhabens ist die Entwicklung und Generierung einer Reihe von Benchmark-Faellen, die von der HD(CP)2-Gemeinschaft genutzt werden koennen. Darueber hinaus soll erkundet werden, wie die Unsicherheiten bei der Repraesentation von Landoberflaechen, der Wolkenmikrophysik und des Aerosols die Interpretation der geplanten HD(CP)2-Simulationen beeinflussen koennen. Die Benchmark-Faelle werden aus einer Reihe von koordinierten Simulationen bestehen, bei denen zwei etablierte Large-Eddy Simulations (LES)-Modelle zum Einsatz kommen. Diese Simulationen dienen als Referenz zum Testen des HD(CP)2 Modells und als Datenquelle zur Evaluierung von in anderen Teilprojekten eingesetzten Modellen. Am IMuK wird ein Teil des HD(CP)2 Benchmarking Projektes bearbeitet. Zusammen mit den Projektpartnern vom MPI-M in Hamburg wird das Setup fuer eine Prototyp-Simulation aufgestellt. Dieses Setup soll den Tagesgang einer konvektiven Grenzschicht abbilden und auf Messungen basieren. Ein wesentlicher Aspekt dieses Vorhabens ist es, die sich fuer das HD(CP)2-Modell ergebenden Unsicherheiten bezueglich der Parametrisierung von Landoberflaechenheterogenitaeten zu ermitteln.
Keywords	Aerosol ; Simulation ; Evaluation ; Prototyp ; Grenzschicht ; Wolke ; Klimasystem ; Ausbreitungsvorgang ; Atmosphaere ; Konvektion ; Zusammenarbeit ; Simulationsrechnung ; Octylphenol
Language	German
Document type	Book
Remark	Projektbeginn: 01.10.2012 Projektende: 30.09.2015 Foerdernummer: 01LK1203B
Database	Environmental research database (UFORDAT) of the German Federal Environment Agency (UBA)

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Article: Heterogeneity-Induced Heat-Flux Patterns in the Convective Boundary Layer: Can they be Detected from Observations and is There a Blending Height?—A Large-Eddy Simulation Study for the LITFASS-2003 Experiment

Sühring, Matthias / Raasch, Siegfried

Boundary-layer meteorology. 2013 Aug., v. 148, no. 2

2013

Abstract: An understanding of how the convective boundary layer (CBL) is mixed under heterogeneous surface forcing is crucial for the interpretation of area-averaged turbulence measurements. To determine the height and degree to which a complex heterogeneous ... ...

Abstract	An understanding of how the convective boundary layer (CBL) is mixed under heterogeneous surface forcing is crucial for the interpretation of area-averaged turbulence measurements. To determine the height and degree to which a complex heterogeneous surface affects the CBL, large-eddy simulations (LES) for two days of the LITFASS-2003 experiment representing two different wind regimes were undertaken. Spatially-lagged correlation analysis revealed the turbulent heat fluxes to be dependent on the prescribed surface flux pattern throughout the entire CBL including the entrainment layer. These findings prompted the question of whether signals induced by surface heterogeneity can be measured by airborne systems. To examine this question, an ensemble of virtual flights was conducted using LES, according to Helipod flight measurements made during LITFASS-2003. The resulting ensemble-averaged heat fluxes indicated a clear dependence on the underlying surface up to the top of the CBL. However, a large scatter between the flux measurements in different ensemble runs was observed, which was the result of insufficient sampling of the largest turbulent eddies. The random and systematic errors based on the integral length scale did not indicate such a large scatter. For the given flight leg lengths, at least 10–15 statistically independent flight measurements were necessary to give a significant estimate of heterogeneity-induced signals in the CBL. The need for ensemble averaging suggests that the observed blending of heterogeneity-induced signals in the CBL can be partly attributed to insufficient averaging.
Keywords	heat transfer ; meteorology ; mixing ; turbulent flow ; wind
Language	English
Dates of publication	2013-08
Size	p. 309-331.
Publishing place	Springer-Verlag
Document type	Article
ZDB-ID	1477639-x
ISSN	1573-1472 ; 0006-8314
ISSN (online)	1573-1472
ISSN	0006-8314
DOI	10.1007/s10546-013-9822-1
Database	NAL-Catalogue (AGRICOLA)

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Article: Spatial Variability of Scalar Concentrations and Fluxes Downstream of a Clearing-to-Forest Transition: A Large-Eddy Simulation Study

Kanani-Sühring, Farah / Raasch, Siegfried

Boundary-layer meteorology. 2015 Apr., v. 155, no. 1

2015

Abstract: The aim of this large-eddy simulation study is to improve the yet scarce understanding of the scalar (e.g. [Formula: see text]) transport in forest-edge flows. In order to contribute to a basic knowledge on the scalar transport, we focus on idealized ... ...

Abstract	The aim of this large-eddy simulation study is to improve the yet scarce understanding of the scalar (e.g. [Formula: see text]) transport in forest-edge flows. In order to contribute to a basic knowledge on the scalar transport, we focus on idealized neutral flows across a clearing-to-forest transition, with a passive scalar released from a horizontally homogeneous source at the clearing and at the forest floor. Corresponding to previous studies, we found pronounced peaks in scalar concentration and flux downstream of the forest edge, where the flux peak significantly exceeded the given surface flux. We investigated for the first time those transport mechanisms that steer the scalar accumulation, by analyzing the terms in the scalar transport equation. The analysis reveals that the accumulation is accomplished by the streamwise convergence of the mean and turbulent transport. In order to investigate how the concentration and flux enhancement behaves under different conditions, we performed a series of simulations with varying forest density and wind speed. We demonstrate a strong dependence of the peak location and magnitude on forest density. With increasing density, concentration and flux peaks were found closer to the forest edge and peak values increased significantly. Decreasing the wind speed caused an increase of the concentration peak, while the flux peak remained unaffected. For an adequate interpretation and design of micrometeorological measurements near forest edges, it is necessary to identify the regions (fetches) with enhanced concentrations and fluxes. We therefore analyzed different approaches for a proper fetch estimation.
Keywords	edge effects ; equations ; forest litter ; forests ; wind speed
Language	English
Dates of publication	2015-04
Size	p. 1-27.
Publishing place	Springer-Verlag
Document type	Article
ZDB-ID	1477639-x
ISSN	1573-1472 ; 0006-8314
ISSN (online)	1573-1472
ISSN	0006-8314
DOI	10.1007/s10546-014-9986-3
Database	NAL-Catalogue (AGRICOLA)

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

Raasch, Siegfried

Abstract: 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 ... ...

Institution	Leibniz Universitaet Hannover, Institut fuer Meteorologie und Klimatologie, Herrenhaeuserstr. 2, 30419, Hannover, DE
Abstract	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. LES results 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 investigations will be based on previous collaboration between the Pi s at IMUK and AWI, where a simplified version of the LES model was used and a microscale non eddy resolving model was developed for simple lead shapes
Keywords	Region ; Meereis ; Konzentrat ; Bewaesserung ; Vorkaufsrecht ; Dosierung ; Messgeraet ; Latentwaerme ; Konvektion ; Grenzschicht ; Energie ; Meer ; Atmosphaere ; Brunnen ; Wirkung ; Turbulenz ; Gebaeude ; Hubschrauber ; Blei ; Schlichtemittel ; Bemessung ; Wetterprognose ; Regionales Klimamodell ; Physik ; Studie ; Antarktis
Language	English
Document type	Book
Remark	project start: 01/01/2006 project end: grant ID: RA 617/14
Database	Environmental research database (UFORDAT) of the German Federal Environment Agency (UBA)

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Article: On the Effect of Surface Heat-Flux Heterogeneities on the Mixed-Layer-Top Entrainment

Sühring, Matthias / Maronga, Björn / Herbort, Florian / Raasch, Siegfried

Boundary-layer meteorology. 2014 June, v. 151, no. 3

2014

Abstract: We used a set of large-eddy simulations to investigate the effect of one-dimensional stripe-like surface heat-flux heterogeneities on mixed-layer top entrainment. The profiles of sensible heat flux and the temporal evolution of the boundary-layer depth ... ...

Abstract	We used a set of large-eddy simulations to investigate the effect of one-dimensional stripe-like surface heat-flux heterogeneities on mixed-layer top entrainment. The profiles of sensible heat flux and the temporal evolution of the boundary-layer depth revealed decreased entrainment for small heat-flux amplitudes and increased entrainment for large heat-flux amplitudes, compared to the homogeneously-heated mixed layer. For large heat-flux amplitudes the largest entrainment was observed for patch sizes in the order of the boundary-layer depth, while for significantly smaller or larger patch sizes entrainment was similar as in the homogeneous case. In order to understand the underlying physics of this impact, a new approach was developed to infer local information on entrainment by means of the local flux divergence. We found an entrainment maximum over the centre of the stronger heated surface patch, where thermal energy is accumulated by the secondary circulation (SC) that was induced by the surface heterogeneity. Furthermore, we observed an entrainment maximum over the less heated patch as well, which we suppose is to be linked to the SC-induced horizontal flow convergence at the top of the convective boundary layer (CBL). For small heat-flux amplitudes a counteracting effect dominates that decreases entrainment, which we suppose is the horizontal advection of cold air in the lower, and warm air in the upper, CBL by the SC, stabilizing the CBL and thus weakening thermal convection. Moreover, we found that a mean wind can reduce the heterogeneity-induced impact on entrainment. If the flow is aligned perpendicular to the border between the differentially-heated patches, the SC and thus its impact on entrainment vanishes due to increased horizontal mixing, even for moderate wind speeds. However, if the flow is directed parallel to the border between the differentially-heated patches, the SC and thus its impact on entrainment persists.
Keywords	air ; cold ; energy ; heat transfer ; mixing ; wind speed
Language	English
Dates of publication	2014-06
Size	p. 531-556.
Publishing place	Springer-Verlag
Document type	Article
ZDB-ID	1477639-x
ISSN	1573-1472 ; 0006-8314
ISSN (online)	1573-1472
ISSN	0006-8314
DOI	10.1007/s10546-014-9913-7
Database	NAL-Catalogue (AGRICOLA)

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