Height Tendency Diagnostics Using a Generalized Omega Equation, the Vorticity Equation, and a Nonlinear Balance Equation

Abstract

Height tendency dynamics are studied with a system consisting of a generalized omega equation, the vorticity equation, and a nonlinear balance equation. By using the first two equations, vorticity tendency is first partitioned into components associated with vorticity advection, thermal advection, friction, diabatic heating, and an ageostrophic tendency term. The nonlinear balance equation is then employed to interpret the vorticity tendency components in terms of height tendencies. The height tendencies due to vorticity advection and friction can be divided into parts associated with the direct forcing and the vertical motions induced by this forcing. This division illustrates the efficiency of vertical motions in smoothing out the vertical gradients in the forcing field. The system is solved over a global domain, but the main emphasis is on an analysis of the ?Presidents? Day cyclone? of February 1979. Although the calculations do not fully capture the observed decrease in the deepening rate of this cyclone from 19 to 21 February, they suggest a change in its dynamics. On 19 February thermal advection and diabatic heating due to latent heat release are both found to make a large contribution to intensify the system, on 21 February only the latter makes a contribution. Vorticity advection by the nondivergent flow favors the deepening of the low on both 19 and 21 February, but anticyclonic vorticity advection by low-level convergent winds is identified as a damping mechanism comparable in importance to surface friction. It is also found that the formally passive characteristics of the environment like the stability and vorticity distributions modify the calculated height tendencies rather strongly.