Similarity Scaling Over a Steep Alpine Slope
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In this study, we investigate the validity of similarity scaling over a steep mountain slope (30–41\(^\circ \)). The results are based on eddy-covariance data collected during the Slope Experiment near La Fouly (SELF-2010); a field campaign conducted in a narrow valley of the Swiss Alps during summer 2010. The turbulent fluxes of heat and momentum are found to vary significantly with height in the first few metres above the inclined surface. These variations exceed by an order of magnitude the well-accepted maximum 10 % required for the applicability of Monin–Obukhov similarity theory in the surface layer. This could be due to a surface layer that is too thin to be detected or to the presence of advective fluxes. It is shown that local scaling can be a useful tool in these cases when surface-layer theory breaks down. Under convective conditions and after removing the effects of self-correlation, the normalized standard deviations of slope-normal wind velocity, temperature and humidity scale relatively well with \(z/\varLambda \), where \(z\) is the measurement height and \(\varLambda (z)\) the local Obukhov length. However, the horizontal velocity fluctuations are not correlated with \(z/\varLambda \) under all stability regimes. The non-dimensional gradients of wind velocity and temperature are also investigated. For those, the local scaling appears inappropriate, particularly at night when shallow drainage flows prevail and lead to negative wind-speed gradients close to the surface.
KeywordsDrainage flow Downslope flow Flux divergence Flux–gradient relationships Flux–variance relationships Local similarity Mountain winds Surface layer
The authors are grateful to all the collaborators at the Laboratory of Environmental Fluid Mechanics at EPFL who helped with the field campaign, and in particular to Hendrik Huwald. The authors would also like to thank Alain Rousseau from the Institut National de la Recherche Scientifique. This work was funded by the Swiss National Foundation under grant 200021-120238 and by the Office of Naval Research Program Award # N00014-11-1-0709, Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program.
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