AN EXAMINATION OF PRESSURE TENDENCY MECHANISMS IN AN IDEALIZED SIMULATION OF EXTRATROPICAL CYCLOGENESIS

A numerical simulation of idealized Type-A extratropical cyclogenesis is used to examine pressure tendency mechanisms following a surface lo w center through its evolution. it is found that horizontal density ad vection, which maximizes near a developing tropopause undulation (pote ntial vorticity anomaly). is the primary mechanism by which the densit y and hydrostatic pressure is reduced in the column above the developi ng surface low center. The density and pressure tendencies associated with horizontal velocity divergence and vertical mass divergence are a n order of magnitude larger than the horizontal advective tendencies b ut are of opposite sign and lend to offset each other. The residual te ndencies that result from combining the horizontal velocity divergence and vertical mass divergence are associated with positive density and pressure tendencies that generally oppose but do not counteract the n egative tendencies produced by upper-level horizontal advection until late in the evolution of the cyclone. However, this residual divergenc e is the primary mechanism for the generation of the upstream low dens ity warm pool over the low portion of the tropopause undulation that i s later advected over the surface low. In general, these results are c onsistent with historical as well as more recent studies that suggest the importance of tropopause-level temperature advection in the promot ion of the observed height tendency patterns accompanying the superpos ition of tropopause- and groundbased potential vorticity anomalies dur ing Type B cyclogenesis. From a hydrostatic tendency perspective, it i s concluded that baroclinic instability is a process whereby low densi ty, warm air especially above the tropopause that has formed owing to divergence and vertical motion effects is advected downstream and over low-level baroclinic zones and nascent cyclones. In this framework, T ype B is distinguished from Type A cyclogenesis by the existence of an initial reservoir of low density, warm, air above a tropopause undula tion prior to surface cyclogenesis.