A Potential-Vorticity Diagnosis of the importance of initial Structure and Condensational Heating in Observed Extratropical Cyclogenesis

Abstract

The dynamics of a cyclone development over the midwestern United States on 15 December 1987 are investigated with a focus on the relationship between cyclone structure and condensational heating. 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 cyclone center are also controlled by the tropopause perturbation. Development culminates in a superposition of positive anomalies of tropopause PV, moisture-induc6d PV, and surface potential temperature ?, 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. The vertical structure in the February 1988 case allowed the ascent induced by the tropopause and surface anomalies to reinforce. The nearly fixed structure and long development period led to a diabatically produced PV perturbation that was twice as intense as the low-level PV in the December cyclone. While comparable precipitation and PV generation rates were present in the December case, structural transience limited the intensity of the moisture-induced PV perturbation.