Abstract
The angle between the nearโsurface and geostrophic wind vector, ฮฑ~0~, 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 largeโeddy simulation (LES) data, give an angle that is about ฮฑ~0~ โ๏ธ 35ยฐ. 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 ฮฑ~0~ can be provided (32ยฐ to 37ยฐ) than that in the classic Ekman theory (45ยฐ). Alternatively, and without deploying the refined Ekman surface layer theory already suggested here, one may a priori use the previously confirmed result, ฮฑ~0~ โ๏ธ 35ยฐ, 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 of weakly stably stratified boundary layers. The information about the angle ฮฑ~0~ should be considered in NWP, airโpollution, windโenergy and climate models; otherwise, many important boundaryโlayer features will remain modelled inadequately. Copyright ยฉ 2011 Royal Meteorological Society