Validation of a Multiple-IBL approach of wind stress field over lakes with patchy emergent vegetation

Krámer, Tamás and Napoli, Enrico and Józsa, János and Torma, Péter (2014) Validation of a Multiple-IBL approach of wind stress field over lakes with patchy emergent vegetation. AQUATIC SCIENCES. ISSN 1015-1621 (Submitted)

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Abstract

An algebraic, equilibrium wind stress model is presented and validated to improve the forcing of hydrodynamic lake models in the presence of contorted shorelines, islands or patchy vegetation cover. This diagnostic model describes the roughness-induced wind changes over the free-surface by resolving the spatial development of multiple nested Internal Boundary Layers (IBLs) at the bottom of the Atmospheric Boundary Layer under neutrally stratified conditions. As a result, it is used to extrapolate the wind stress field from a single onshore or offshore wind observation, thus requiring minimal data. Furthermore, it is a much cheaper method for approximating momentum transfer at the air-water interface than numerical wind models. The multiple-IBL wind stress model is validated using field observations and 3D numerical simulations in a fetch-limited bay of Lake Neusiedl, Hungary. Modelled wind speeds are found to fit time-averaged measurements along a fetch, and also confirm that the multiple-IBL approach is more accurate than one that describes the lowest IBL only. Finally, we show that the circulation of the bay is highly sensitive to the variability of the wind stress field. Modelled currents, especially in the wake of the shoreline, tend to have the wrong orientation if uniform wind action is assumed, but correct orientation is obtained by taking into account the systematic variability of the wind field linked to roughness changes. The presented multiple-IBL model thus proves to be a simple and reliable tool to predict wind stress distribution due to roughness-induced IBLs.

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