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Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

Brötz, Björn and Eigenmann, Rafael and Dörnbrack, Andreas and Foken, Thomas and Wirth, Volkmar (2014) Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions. Boundary-Layer Meteorology, pp. 1-19. Springer. DOI: 10.1007/s10546-014-9921-7 ISSN 0006-8314

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Official URL: http://dx.doi.org/10.1007/s10546-014-9921-7

Abstract

We investigate the evolution of the early-morning boundary layer in a low-mountain valley in south-western Germany during COPS (convective and orographically induced precipitation study) in summer 2007. The term low-mountain refers to a mountainous region with a relief of gentle slopes and with an absolute altitude that remains under a specified height (usually 1,500 m a.s.l.). A subset of 23 fair weather days from the campaign was selected to study the transition of the boundary-layer flow in the early morning. The typical valley atmosphere in the morning hours was characterized by a stable temperature stratification and a pronounced valley wind system. During the reversal period—called the low wind period—of the valley wind system (duration of 1–2 h), the horizontal flow was very weak and the conditions for free convection were fulfilled close to the ground. Ground-based sodar observations of the vertical wind show enhanced values of upward motion, and the corresponding statistical properties differ from those observed under windless convective conditions over flat terrain. Large-eddy simulations of the boundary-layer transition in the valley were conducted, and statistical properties of the simulated flow agree with the observed quantities. Spatially coherent turbulence structures are present in the temporal as well as in the ensemble mean analysis. Thus, the complex orography induces coherent convective structures at predictable, specific locations during the early-morning low wind situations. These coherent updrafts, found in both the sodar observations and the simulation, lead to a flux counter to the gradient of the stably stratified valley atmosphere and reach up to the heights of the surrounding ridges. Furthermore, the energy balance in the surface layer during the low wind periods is closed. However, it becomes unclosed after the onset of the valley wind. The partition into the sensible and the latent heat fluxes indicates that missing flux components of sensible heat are the main reason for the unclosed energy balance in the considered situations. This result supports previously published investigations on the energy balance closure.

Item URL in elib:https://elib.dlr.de/88837/
Document Type:Article
Title:Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Brötz, BjörnDLR, IPA and Univ. Mainz, IPAUNSPECIFIED
Eigenmann, RafaelUniv. Bayreuth, BayreuthUNSPECIFIED
Dörnbrack, AndreasDLR, IPAUNSPECIFIED
Foken, ThomasUniv. Bayreuth, BayreuthUNSPECIFIED
Wirth, VolkmarUniv. Mainz, IPAUNSPECIFIED
Date:2014
Journal or Publication Title:Boundary-Layer Meteorology
Refereed publication:Yes
Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
DOI :10.1007/s10546-014-9921-7
Page Range:pp. 1-19
Publisher:Springer
ISSN:0006-8314
Status:Published
Keywords:Coherent structures, Convection in a valley, Energy balance closure, Large-eddy simulation
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Aeronautics
HGF - Program Themes:air traffic management and operations
DLR - Research area:Aeronautics
DLR - Program:L AO - Air Traffic Management and Operation
DLR - Research theme (Project):L - Climate, Weather and Environment
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics
Institute of Atmospheric Physics > Atmospheric Trace Species
Institute of Atmospheric Physics > Transport Meteorology
Deposited By: Brötz, Björn
Deposited On:22 Apr 2014 16:59
Last Modified:31 Jul 2019 19:46

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