Baroclinicity and directional shear explain departures from the logarithmic wind profile

Abstract

AbstractSimilarity and scaling arguments underlying the existence of a logarithmic wind profile in the atmospheric surface layer (ASL) rest on the restrictive assumptions of negligible Coriolis effects (no wind turning in the ASL) and vertically‐uniform pressure gradients (barotropic atmospheric boundary layer, ABL). This paper alleviates these asymptotic arguments to provide a realistic representation of the ASL where the common occurrence of baroclinicity (height‐dependent pressure gradients) and wind turning, traditionally treated as outer‐layer attributes, take part in modulating the ASL. The approximation of a constant‐stress ASL is first replaced by a refined model for the Reynolds stress derived from the mean momentum equations to incorporate the cross‐isobaric angle (directional shear) and a dimensionless baroclinicity parameter (geostrophic shear). A model for the wind profile is then obtained from first‐order closure principles, correcting the log‐law with an additive term that is linear in height and accounts for the combined effects of wind turning and baroclinicity. Both the stress and wind models agree well with a suite of large‐eddy simulations in the barotropic and baroclinic ABL. The findings provide a methodology for the extrapolation of near‐surface winds to some 200 m in the near‐neutral ABL for wind energy applications, the validation of the surface cross‐isobaric angle in weather and climate models, and the interpretation of wind turning in field measurements and numerical experiments of the Ekman boundary layer.