The role of boundary layer processes in mid-latitude cyclones

It is usually assumed, often implicitly, that frictional effects of the atmospheric boundary layer affect mid-latitude cyclones through Ekman pumping. But, at least in its standard form, this mechanism is derived for only barotropic vortices. Potential vorticity (PV) provides a natural framework to understand the role of air-sea interaction and boundary layer processes, such as friction and heat fluxes, on mid-latitude cyclones, which typically develop through baroclinic processes. We show that there are two mechanisms whereby air-sea interaction and boundary layer processes generate PV: one corresponds to the usual Ekman process and the other is a new, baroclinic, process. The baroclinic frictional process is certainly important and seem to often be the dominant process. In extreme conditions heat fluxes may also be important to the cyclone develoment. These findings have implications for identifying the aspects of air sea interaction that are important to development of mid-latitude cyclones.