The dynamics of an explosively developing cyclone simulated by a general circulation model

The synoptic structure and dynamics of an explosively developing winter extratropical cyclone simulated by NCAR's CCM2 general circulation model is examined and compared with cyclones that have developed explosively in nature. The primary diagnostic tool utilized in this analysis is the Zwack-Okossi equation. Synoptic results show that in addition to yielding an unusually large cyclone, the development initially occurred in the absence of a prominent upper-level trough. Rather, the surface disturbance was accompanied by an unusually strong upstream jet streak. Despite these departures from normally observed synoptic structure, the dynamics of the cyclone development and decay conformed to the dynamics found in observational cases. In particular, the cyclone developed in response to cyclonic vorticity and warm air advection, both of which maximized in the upper troposphere, and latent heat release, which were sufficient to overcome the cyclolytic effects of adiabatic cooling, friction, and sensible heating. Decay ensued when a sufficiently deep layer of cold air was advected into the center of the cyclone. Thus, even though CCM2 simulations may sometimes misrepresent eddy structures, these same simulations are capable of representing the dynamics of the growth of such structures in an essentially correct fashion.