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

Abstract

The angle between the near-surface and geostrophic wind vector, α 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 large-eddy simulation (LES) data, give an angle that is about α 0 ≈ 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 α 0 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, α 0 ≈ 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 of weakly stably stratified boundary layers. The information about the angle α 0 should be considered in NWP, air-pollution, wind-energy and climate models; otherwise, many important boundary-layer features will remain modelled inadequately.

Turbulence Modeling for the Stable Atmospheric Boundary Layer and Implications for Wind Energy

“…3. As pointed out by Grisogono and Enger [18], the angle between the near-surface and geostrophic wind vector can be an important parameter for mesoscale prediction models that do not resolve the atmospheric boundary layer. The simulated surface angle under moderate stability is around 35 • , which agrees well with previous numerical studies of Beare et al [6].…”

Stably Stratified Atmospheric Boundary Layers

“…Nappo 2012, Abarbanel et al (1984), and Grisogono (1994) examined instability due to inflection points that may also contribute to turbulence at large Richardson numbers, even though such instability may be constrained by stratification (Grisogono 2011). Gage (1971) theoretically found that for a simple shear flow, inflection point instability increases the critical Richardson number, which in a more complex very stable boundary layer might augment turbulence for all Richardson numbers.…”

Wind turning in the atmospheric boundary layer over land

“…The zeroth-order Ekman solution with a constant eddy diffusivity gives an angle of 45 • between the surface wind and the geostrophic wind. Less simplified theories exist which give lower values are closer to the observed ones (Grisogono, 2011). However, observational studies are rather limited in that, to our knowledge, each study generally uses measurements from only one or a few observational locations and most studies are made in the midlatitudes.…”