The Impact of Large-Scale Winds on Boundary Layer Structure, Thermally Driven Flows, and Exchange Processes over Mountainous Terrain

Weinkaemmerer, Jan; Ďurán, Ivan Bašták; Schmidli, Jürg

doi:10.1175/jas-d-21-0195.1

Cited by 5 publications

(27 citation statements)

References 32 publications

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“…The drying of the valley center is a result of the subsiding branch of the cross-valley circulation (Weinkaemmerer et al, 2022). A comparison of Figures 4h and 6d shows that the moisture transport by turbulence is nearly negligible at upper levels.…”

Section: Turbulent Moisture Fluxmentioning

confidence: 91%

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Boundary‐layer plumes over mountainous terrain in idealized large‐eddy simulations

Weinkaemmerer

Göbel

Serafin

et al. 2023

Quart J Royal Meteoro Soc

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Coherent plume structures in the convective boundary layer over non‐flat terrain are investigated using large‐eddy simulation. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary‐layer plumes objectively. Conditional sampling allows to quantify the contribution of plume structures to the vertical transport of heat and moisture. A first set of simulations analyses the flow over an idealized valley, where the terrain elevation only varies along one horizontal coordinate axis. In this case, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope‐wind circulation, although it is more important for heat than for moisture transport. A second set of simulations considers flow over terrain with a complex texture, drawn from an actual digital elevation model. In this case, conditional sampling is carried out by using a simple domain‐decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large‐scale winds are present in the free atmosphere. Large‐scale, upper‐level winds tend to reduce the vertical moisture transport by the slope winds.This article is protected by copyright. All rights reserved.

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Section: Turbulent Moisture Fluxmentioning

confidence: 91%

“…By means of spatial and temporal averaging, the flow is decomposed as shown in Weinkaemmerer et al (2022).…”

Section: Flow Decompositionmentioning

confidence: 99%

Boundary‐layer plumes over mountainous terrain in idealized large‐eddy simulations

Weinkaemmerer

Göbel

Serafin

et al. 2023

Quart J Royal Meteoro Soc

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“…The symmetry of the quasi-two-dimensional setup used in this study simplifies the decomposition of the flow (Weinkaemmerer et al, 2022). First, the turbulent fluctuations are separated from the mean flow by a Reynolds decomposition.…”

Section: Flow Averaging and Decompositionmentioning

confidence: 99%

“…In both zonal and meridional directions, periodic boundary conditions are imposed, imitating an infinitely long valley. The symmetry of the quasi‐two‐dimensional setup used in this study simplifies the decomposition of the flow (Weinkaemmerer et al, 2022).…”

Section: Budget Analysismentioning

confidence: 99%

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Stratus over rolling terrain: Large‐eddy simulation reference and sensitivity to grid spacing and numerics

Weinkaemmerer

Ďurán

Westerhuis

et al. 2022

Quart J Royal Meteoro Soc

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The formation of low stratus cloud over idealized hills is investigated using numerical model simulations. The main driver for the cloud formation is radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, the advection terms in the prognostic equations for heat and moisture produce vertical mixing across the upper cloud edge, leading to a loss of cloud water content. This behavior is depicted via a budget analysis. More precisely, this spurious mixing is caused by the diffusive error of the advection scheme in regions where the sloping surfaces of the terrain‐following vertical coordinate intersect the cloud top. This study shows that the intensity of the (spurious) numerical diffusion depends strongly on the horizontal resolution, the order of the advection schemes, and the choice of scalar advection scheme. A large‐eddy simulation with 4‐normalm$$ \mathrm{m} $$ horizontal resolution serves as a reference. For horizontal resolutions of a few hundred meters and simulations carried out with a model setup as used in numerical weather prediction, a strong reduction of the simulated liquid‐water path is observed. In order to keep the (spurious) numerical diffusion at coarser resolutions small, at least a fifth‐order advection scheme should be used. In the present case, a weighted essentially nonoscillatory scalar advection scheme turns out to increase the numerical diffusion along a sharp cloud edge compared with an upwind scheme. Furthermore, the choice of vertical coordinate has a strong impact on the simulated liquid‐water path over orography. With a modified definition of the sigma coordinate, it is possible to produce cloud water where the classical sigma coordinate does not allow any cloud formation.

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Large-eddy simulations of the mountain boundary layer : daytime exchange processes and nocturnal fog formation

Weinkaemmerer

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In this dissertation, different aspects of turbulent transport and thermally driven flows over complex terrain are investigated. Two publications concentrate on the vertical heat and moisture exchange in the convective boundary layer over mountainous terrain. To study this, Large-Eddy Simulation (LES) is used. Both turbulent and advective transport mechanisms are evaluated over the simple orography of a quasi-two-dimensional, periodic valley with prescribed surface fluxes. Here, terrain elevation varies along only one of the horizontal coordinate axes. Even a relatively shallow orography, possibly unresolved in existing numerical weather prediction models, modifies the domain-averaged moisture and temperature profiles. For the analysis, the flow is decomposed into a local turbulent part, a local mean circulation, and a large-scale part. An analysis of the turbulent kinetic energy and turbulent heat and moisture flux budgets shows that the thermal circulation significantly contributes to the vertical transport. It is found that thermal upslope winds are important for the moisture transport from the valley to the mountain tops. In total, moisture export out of the valley is mostly accomplished by the mean circulation. On the temperature distribution, which is horizontally relatively homogeneous, the thermal circulation has a weaker impact. If an upper-level wind is present, it interacts with the thermal circulation. This weakens the vertical transport of moisture and thus reduces its export out of the valley. The heat transport is less affected by the upper-level wind because of its weaker dependence on the thermal circulation. These findings were corroborated in a more realistic experiment simulating the full diurnal cycle using radiation forcing and an interactive land surface model. Based on these results, coherent turbulent structures in the convective boundary layer over non-flat terrain are studied in further detail. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary-layer plumes objectively. Conditional sampling allows to quantify the contribution of plume structures to the vertical transport of heat and moisture. In case of the idealized valley, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope-wind circulation, although it is more important for heat than for moisture transport. A set of less idealized simulations considers the flow over three-dimensional terrain. In this case, conditional sampling is carried out by using a simple domain-decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large-scale winds are present in the free atmosphere. The tools for flow decomposition and budget analysis are also applied in another idealized case with a quasi-two-dimensional valley featuring the stable boundary layer. Here, the formation of a low stratus cloud is investigated. The main driver for the cloud formation is radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, the advection terms in the prognostic equations for heat and moisture produce vertical mixing across the upper cloud edge leading to a loss of cloud water content. However, this behavior is not due to any kind of thermally-driven circulation. Instead, this spurious mixing is caused by the diffusive error of the advection scheme in regions where the sloping surfaces of the terrain-following vertical coordinate intersect the cloud top. It is shown that the intensity of the (spurious) numerical diffusion strongly depends on the horizontal resolution, the order of advection, and the choice of the scalar advection scheme. A LES with 4 m horizontal resolution serves as a reference. For horizontal resolutions of a few hundred meters, carried out with a model setup as it is used in Numerical Weather Prediction, a strong reduction of the simulated liquid-water path is observed. In order to keep the (spurious) numerical diffusion at coarser resolutions small, at least a fifth-order advection scheme should be used. In the present case, a WENO scalar advection scheme turns out to increase the numerical diffusion along a sharp cloud edge compared to an upwind scheme. Furthermore, the choice of the vertical coordinate has a strong impact on the simulated liquid-water path over orography. With a modified definition of the terrain-following sigma coordinate, it is possible to produce cloud water where the classical sigma coordinate does not allow any cloud formation.

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The Impact of Large-Scale Winds on Boundary Layer Structure, Thermally Driven Flows, and Exchange Processes over Mountainous Terrain

Cited by 5 publications

References 32 publications

Boundary‐layer plumes over mountainous terrain in idealized large‐eddy simulations

Boundary‐layer plumes over mountainous terrain in idealized large‐eddy simulations

Stratus over rolling terrain: Large‐eddy simulation reference and sensitivity to grid spacing and numerics

Large-eddy simulations of the mountain boundary layer : daytime exchange processes and nocturnal fog formation

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