HEIGHT TENDENCY DIAGNOSTICS USING A GENERALIZED OMEGA-EQUATION, THE VORTICITY EQUATION, AND A NONLINEAR BALANCE EQUATION

Height tendency dynamics are studied with a system consisting of a gen eralized omega equation, the vorticity equation, and a nonlinear balan ce equation. By using the first two equations, vorticity tendency is f irst partitioned into components associated with vorticity advection, thermal advection, friction, diabatic heating, and an ageostrophic ten dency term. The nonlinear balance equation is then employed to interpr et the vorticity tendency components in terms of height tendencies-The height tendencies due to vorticity advection and friction can be divi ded into parts associated with the direct forcing and the vertical mot ions induced by this forcing. This division illustrates the efficiency of vertical motions in smoothing out the vertical gradients in the fo rcing field. The system is solved over a global domain, but the main e mphasis is on an analysis of the ''Presidents' Day cyclone'' of Februa ry 1979. Although the calculations do not fully capture the observed d ecrease in the deepening rare of this cyclone from 19 to 21 February, they suggest a change in its dynamics. On 19 February thermal advectio n and diabatic heating due to latent heat release are both found to ma ke a large contribution to intensify the system, on 21 February only t he latter makes a contribution. Vorticity advection by the nondivergen t flow favors the deepening of the low on both 19 and 21 February, but anticyclonic vorticity advection by low-level convergent winds is ide ntified as a damping mechanism comparable in importance to surface fri ction. It is also found that the formally passive characteristics of t he environment like the stability and vorticity distributions modify t he calculated height tendencies rather strongly.