A potential-vorticity diagnosis of the importance of initial structure and condensation heating in observed extratropical cyclogenesis

Low-level cyclogenesis is initiated by a large-amplitude tropopause perturbation that develops over western North America. Using potential-vorticity (PV) inversion diagnostics, we show how the near-surface winds associated with this upper disturbance create a localized, warm, thermal anomaly within a surface baroclinic zone. The distribution of precipitation and the diabatic generation of a positive low-level PV feature near the cylone center are also controlled by the tropopause perturbation. Development culminates in a superposition of positive anomalies of tropopause PV, moisture-induced PV, and surface potential temperature θ, with contributions to the total low-level circulation being about 30%, 20%, and 50%, respectively. This case is compared with a different cyclogenesis event (4-5 February 1988), characterized by an initially small-amplitude upper-level wave and relatively fixed structure during growth. -from Author