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 low center through its evolution. It is found that horizontal density advection, which maximizes near a developing tropopause undulation (potential vorticity anomaly), is the primary mechanism by which the density and hydrostatic pressure is reduced in the column above the developing surface low center. The density and pressure tendencies associated with horizontal velocity divergence and vertical mass divergence are an order of magnitude larger than the horizontal advective tendencies but are of opposite sign and tend to offset each other. The residual tendencies 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 negative tendencies produced by upper-level horizontal advection until late in the evolution of the cyclone. However, this residual divergence is the primary mechanism for the generation of the upstream low density warm pool over the low portion of the tropopause undulation that is later advected over the surface low. In general, these results are consistent with historical as well as more recent studies that suggest the importance of tropopause-level temperature advection in the promotion of the observed height tendency patterns accompanying the superposition of tropopause- and ground-based potential vorticity anomalies during Type B cyclogenesis. From a hydrostatic tendency perspective, it is concluded that baroclinic instability is a process whereby low density, 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, Type B is distinguished from Type A cyclogenesis by the existence of an initial reservoir of low density, warm air above a tropopause undulation prior to surface cyclogenesis.