A diagnostic study of the evolution of an intense North American anticyclone during winter 1989

Abstract

The complete height tendency equation is used as the diagnostic tool. Results show that diabatic processes are relatively unimportant and that the principal forcing mechanisms in the anticyclogenesis are vorticity advection and differential thermal advection. The cold low-level air over Alaska enhances the anticyclogenesis by promoting a positive contribution from the differential thermal advection. The cold low-level temperatures preclude strong cold-air advection at low levels in the anticyclone; strong upper-level cold-air advection thus results in a large positive contribution from the differential thermal advection. The vertical advection of static stability opposes the other forcings, acting to slow the development. It is determined that the forcing mechanisms are substantially different for the intense North American anticyclone when compared with the cold-core anticyclone driven by radiative cooling, even though the thermodynamic structures of the two anticyclones are similar. -from Authors