The Contribution of Organized Roll Vortices to the Surface Wind Vector in Baroclinic Conditions

Abstract

Observations show that, for a given geostrophic forcing, baroclinity acting on the planetary boundary layer produces a nearly sinusoidal modification of the near-surface wind. Compared to barotropic conditions the speed is enhanced in the direction of the thermal wind and the cross-isobar angle increases (decreases) in cold (warm) advection. These modifications are asymmetric with respect to the thermal wind orientation. Two-layer similarity models that match a stratification-dependent surface layer to a stratification and baroclinity dependent Ekman layer simulate aspects of this asymmetric baroclinic modification if the cold advection conditions are more unstably stratified than the warm advection conditions. The authors demonstrate that roll vortices in a baroclinic planetary boundary layer produce an asymmetric surface wind modification in neutral stratification that can work in concert with the coupling between stratification and baroclinity to enhance the net effect of baroclinity on the surface wind. It is further demonstrated that the roll modification effect can be as much as or even more that the pure thermal wind effect, although both are secondary to the pure frictional effect. This baroclinic roll modification works to increase the low-level poleward mass transport and the near-surface westerly momentum in the midlatitudes.