A Case of Rapid Continental Mesoscale Cyclogenesis. Part II: Model and Observational Diagnosis

Abstract

The rapid surface cyclogenesis of March 1984 is examined from an observational and modeling perspective, in terms of both potential vorticity (PV) and traditional quasigeostraphic reasoning, during its evolution from a mesoscale cyclone to a state in which it is identifiable as a large-scale extratropical cyclone. The first stage of the cyclonic development is characterized by a surface warm anomaly forming as a consequence of surface heat fluxes. Subsequently, a lower-tropospheric PV maximum develops in association with a mesoscale pattern of rainfall in excess of 10 mm h?1. The numerical forecasts replicated the evolution of both features, though more slowly than actually occurred. This organized rainfall occurs in response to a vigorous midtropospheric cyclonic vorticity maximum. Lower-tropospheric PV generation is found to be the unique feature of the rapid mesoscale cyclogenesis that is directly related to condensation heating, with both horizontal and vertical gradients of heating contributing. The former component of PV generation occurs only during the first hours of incipient cyclogenesis and is uniquely related to mesoscale precipitation pattern in a region of strong baroclinity and vertical wind shear. The second stage of development occurs when high-PV stratospheric air arrives over the cyclone center, and induces further rapid spinup. The resulting rapid spinup is dependent not only on the existence of this reservoir of high-PV air, but also on its interaction with the lower-tropospheric PV maximum that was produced by condensation heating. The rapid small-scale cyclogenesis may be explained by the following sequence of events. Strong surface heating produces a surrogate surface PV anomaly. The associated planetary boundary layer heating and moistening leads to moist convection that occurs in the midst of a strong lower-tropospheric baroclinic zone. Such convection and its consequent latent heating in the midst of strong vertical wind shear is responsible for the generation of a lower-tropospheric PV maximum and the incipient mesoscale cyclogenesis. The interaction of this mesoscale PV anomaly with a strong upper-level trough, or a strong PV anomaly that extends from the stratosphere down to 600 mb, products the second phase of rapid cyclogenesis in which the surface cyclone is transformed into a large-scale extratropical cyclone. The rapid cyclogenesis depends crucially on the existence of the upper trough, the amplitude of boundary layer heating, the strength of condensation, and the interaction of these processes.