On the Evolution of theQE IIStorm. II: Dynamic and Thermodynamic Structure

The existence of convection and the hurricane-like structure in the explosively-developing cyclone studied in Part I motivates us to assess the importance heating had on this cyclogenesis. To accomplish this, a method to evaluate the three-dimensional thermodynamic and dynamic structure of the atmosphere is proposed, so that we may evaluate potential vorticity changes in the vicinity of this cyclone. Results indicate a 24 h lower tropospheric generation of from five to thirteen times the value observed at 1200 GMT 9 September 1978. An evaluation of physical effects on thickness change following the surface center shows a large mean tropospheric temperature rise to be due to bulk cumulus heating effects, which could be important in the extraordinary potential vorticity generation concurrent with this cyclone's explosive development. These vertically integrated values of heating motivate us to solve the quasi-geostrophic omega and vorticity equations forced by an idealized heating function with specified horizontal scale, level of maximum heating and total heating. Resulting theoretical omega profiles and height falls during the 24 h period of explosive development for the observed integrated values of heating, vorticity-stability parameter, and over a wide range of levels of maximum heating readily account for the observed explosive cyclogenesis. It is hypothesized that the relatively weak baroclinic forcing operative in this case helped to organize the convective bulk heating effects on a scale comparable to the cyclone itself in an atmosphere which is gravitationally stable for large-scale motions and gravitationally unstable for the convective scale. This CISK-like mechanism, evidently operative in this case, is further hypothesized to be important in other explosively-developing extratropical cyclones, just as it is generally regarded to be crucial in tropical cyclone development.