Despite many field studies for a range of BBL or SBL roughness under stably stratification, quantitative explanation for all processes causing drag to the flow is lacking. The behaviour of atmospheric stratified flows is still not represented or described well enough. One of the small-scale processes that hampers the parametrizations during stable conditions is generation of topographically induced gravity waves. Until now the quantitative knowledge about the topographically induced gravity wave drag (τ_wave) to the total momentum budget is rather limited.
Ideal conditions to develop theory, improve our understanding and calculation of energy and momentum budget are hard to find in the nature, both in the atmosphere and ocean. The CNRM-GAME Toulouse stratified water flume allowed us to set up and maintain stable density gradients and to measure fluxes in controlled stratified conditions. By applying moderate topography, with a respect to the vertical scale of the SBL, in the bottom of the towing tank we were able to study the τ_wave.
The experiment main objective was to quantify and describe the influence of the τ_wave divergence on the boundary-layer dynamics in general. The momentum and energy budget parametrization were studied with a view to account for τ_wave. We were able to distinguish between turbulence and wave-based fluctuations by means of spectral analysis, to separate wave and turbulence induced momentum fluxes. We present vertical profiles of flow speed, stream lines, turbulent fluxes, and wave drag for a variety of flow speeds and density.
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