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  • Author or Editor: Johannes S. Wagner x
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Johannes S. Wagner
,
Alexander Gohm
, and
Mathias W. Rotach

Abstract

The role of horizontal model grid resolution on the development of the daytime boundary layer over mountainous terrain is studied. A simple idealized valley topography with a cross-valley width of 20 km, a valley depth of 1.5 km, and a constant surface heat flux forcing is used to generate upslope flows in a warming valley boundary layer. The goal of this study is to investigate differences in the boundary layer structure of the valley when its topography is either fully resolved, smoothed, or not resolved by the numerical model. This is done by performing both large-eddy (LES) and kilometer-scale simulations with horizontal mesh sizes of 50, 1000, 2000, 4000, 5000, and 10 000 m. In LES mode a valley inversion layer develops, which separates two vertically stacked circulation cells in an upper and lower boundary layer. These structures weaken with decreasing horizontal model grid resolution and change to a convective boundary layer over an elevated plain when the valley is no longer resolved. Mean profiles of the LES run, which are obtained by horizontal averaging over the valley show a three-layer thermal structure and a secondary heat flux maximum at ridge height. Strong smoothing of the valley topography prevents the development of a valley inversion layer with stacked circulation cells and leads to higher valley temperatures due to smaller valley volumes. Additional LES and “1 km” runs over corresponding smoothed valleys reveal that differences occur mainly because of unresolved topography and not because of unresolved turbulence processes. Furthermore, the deactivation of horizontal diffusion improved simulations with 1- and 2-km horizontal resolution.

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Tanja C. Portele
,
Andreas Dörnbrack
,
Johannes S. Wagner
,
Sonja Gisinger
,
Benedikt Ehard
,
Pierre-Dominique Pautet
, and
Markus Rapp

Abstract

The impact of transient tropospheric forcing on the deep vertical mountain-wave propagation is investigated by a unique combination of in situ and remote sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere and lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile, and the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes () and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m−1 and 1500 m, respectively. Their maxima were phase shifted to the maximum low-level forcing by ≈8 h. Small-scale waves ( km) were continuously forced, and their flux values depended on wave attenuation by breaking and reflection in the UTLS region. Only maximum flow over the envelope of the mountain range favored the excitation of longer waves that propagated deeply into the mesosphere. Their long propagation time caused a retarded enhancement of observed mesospheric gravity wave activity about 12–15 h after their observation in the UTLS. For the UTLS, we further compared observed and simulated with fluxes of 2D quasi-steady runs. UTLS momentum fluxes seem to be reproducible by individual quasi-steady 2D runs, except for the flux enhancement during the early decelerating forcing phase.

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