Abstract
The primary goal of this study is to compare the principal adiabatic and diabatic mechanisms responsible for the behavior of an extratropical cyclone system. To accomplish this goal, the height tendency is solved from two forms of the height tendency equation. One is the classical quasi-geostrophic (QG) form; the other is a modified form identified as the ‘extended’ height tendency equation. The latter retains the essential components of the QG form but replaces the geostrophic wind where it appears in the equation by the observed wind, adds the effects of diabatic heating (latent heat release), and allows three dimensional varying static stability. In this study, the behavior of an intense cyclone is poorly described by the QG form but is realistically represented by the extended form.
Using this latter diagnostic tool, the evolution of a 9–11 January 1975 cyclone event is analyzed with the following major results: 1) a comparison of the terms included in the extended height tendency equation indicates that the vorticity advection has a primary influence on the movement and propagation of the wave system, particularly in the upper troposphere; 2) the differential thermal advection plays a secondary, but important, role in the wave development and is comparable to the vorticity advection in the lower troposphere; 3) in general, the influence of latent heating is less than the other mechanisms, although it forces significant height falls over a limited region at lower levels at the time of maximum precipitation; and 4) the vertical advection of static stability makes a significant contribution and, in general, opposes the other forcings, thus acting to slow the wave propagation and development.
