13 Exploring the role of wave drag in the stable stratified oceanic and atmospheric bottom boundary layer in a stratified flume

Monday, 9 June 2014
Palm Court (Queens Hotel)
Michal Adam Kleczek, Wageningen University, Wageningen, Netherlands; and G. J. Steeneveld and A. Paci

The stably stratified atmospheric (SBL) and the oceanic bottom (BBL) boundary layer play a vital role in the current understanding of the climate and the climate change. SBL and BBL are especially important for many environmental issues such as air quality, fog forecasting, wind energy engineering, marine ecology and biology, as well as for the ocean modelling.

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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