A CASE OF RAPID CONTINENTAL MESOSCALE CYCLOGENESIS .2. MODEL AND OBSERVATIONAL DIAGNOSIS

The rapid surface cyclogenesis of March 1984 is examined from an obser vational and modeling perspective, in terms of both potential vorticit y (PV) and traditional quasigeostrophic reasoning, during its evolutio n from a mesoscale cyclone to a state in which it is identifiable as a large-scale extratropical cyclone. The first stage of the cyclonic de velopment is characterized by a surface warm anomaly forming as a cons equence of surface heat fluxes. Subsequently, a lower-tropospheric PV maximum develops in association with a mesoscale pattern of rainfall i n excess of 10 mm h(-1). The numerical forecasts replicated the evolut ion of both features, though more slowly than actually occurred. This organized rainfall occurs in response to a vigorous midtropospheric cy clonic vorticity maximum. Lower-tropospheric PV generation is found to be the unique feature of the rapid mesoscale cyclogenesis that is dir ectly related to condensation heating, with both horizontal and vertic al gradients of heating contributing. The former component of PV gener ation occurs only during the first hours of incipient cyclogenesis and is uniquely related to the mesoscale precipitation pattern in a regio n of strong baroclinity and vertical wind shear. The second stage of d evelopment occurs when high-PV stratospheric air arrives over the cycl one center, and induces further rapid spinup. The resulting rapid spin up is dependent not only on the existence of this reservoir of high-PV air, but also on its interaction with the lower-tropospheric PV maxim um that was produced by condensation heating. The rapid small-scale cy clogenesis may be explained by the following sequence of events. Stron g surface heating produces a surrogate surface PV anomaly. The associa ted planetary boundary layer heating and moistening leads to moist con vection that occurs in the midst of a strong lower-tropospheric barocl inic zone. Such convection and its consequent latent heating in the mi dst of strong vertical wind shear is responsible for the generation of a lower-tropospheric PV maximum and the incipient mesoscale cyclogene sis. The interaction of this mesoscale PV anomaly with a strong upper- level trough, or a strong PV anomaly that extends from the stratospher e down to 600 mb, produces the second phase of rapid cyclogenesis in w hich the surface cyclone is transformed into a large-scale extratropic al cyclone. The rapid cyclogenesis depends crucially on the existence of the upper trough, the amplitude of boundary layer heating, the stre ngth of condensation, and the interaction of these processes.