A Composite Diagnosis of Synoptic-Scale Extratropical Cyclone Development over the United States

Abstract

This paper presents a composite diagnosis of synoptic-scale forcing mechanisms associated with extratropical cyclone evolution. Drawn from 12 cyclone cases that occurred over the continental United States during the cool season months, the diagnosis provides a “climatology” of development mechanisms for difference categories of cyclone evolution ranging from cyclone weakening through three stages of cyclone intensification. Computational results were obtained using an “extended” form of the Zwack–Okossi equation applied to routine upper-air and surface data analyzed on a 230 km × 230 km grid. Results show that cyclonic vorticity advection, which maximizes in the upper troposphere, was the primary contributor to cyclone development regardless of the stage of development. A second consistent contributor to development was latent heat release. Horizontal temperature advection, often acknowledged as a development mechanism, was found to contribute to development only during more intense stages. During weakening and weaker development stages, temperature advection opposed development, as the warm-air advection invariably found at upper levels was dominated by cold air advection in the lower half of the sphere. In the more intense stages, development was moderated by dry-adiabatic cooling associated with the ascending vertical motions.