engleski

The angle of the near-surface wind-turning in weakly stable boundary layers

The angle between the near-surface and geostrophic wind vector, α0, is discussed in the light of existing and improved Ekman theory and, on the other hand, recently obtained numerical results corroborated by some experimental data, both obtained by other researchers. The latter results for weakly stably stratified boundary layers, also based on large-eddy simulation (LES) data, give an angle that is about α0 ≈ 35°. If the Ekman theory is applied slightly above the horizontal surface z > 0, for almost any gradually varying eddy diffusivity K(z), which is more realistic than K = const used at z = 0 in the classic theory, a closer analytic value to α0 can be provided (32° to 37°) than that in the classic Ekman theory (45°). Alternatively, and without deploying the refined Ekman surface layer theory already suggested here, one may a priori use the previously confirmed result, α0 ≈ 35°, together with any smooth K(z) in order to find the corresponding surface layer depth. These results, bridging the gap between the existing theory toward fine numerical and limited experimental data, may aid further analyses ofweakly stably stratified boundary layers.The information about the angle α0 should be considered in NWP, air-pollution, wind-energy and climate models ; otherwise, many important boundary-layer features will remain modelled inadequately.

eddy diffusivity; Ekman layer; meandering; surface layer; WKB theory

nije evidentirano