Diagnostic Studies of a Two-Dimensional Simulation of Frontogeneiss in a Moist Atmosphere

Abstract

The two-dimensional model simulation of frontogenesis in a moist atmosphere discussed by Hsie and others is used to provide dynamically consistent data for diagnostic calculations to quantify the physical process important in the frontogenesis. The terms in the prognostic equations for the horizontal gradient of potential vorticity, relative vorticity, and static stability are evaluated to indicate the relative importance of confluence, shear, tilting, and frictional and diabatic process in generating these properties of the front. An evaluation of the potential vorticity and equivalent potential vorticity fields and their budgets indicates that symmetric instability is not present in the dry simulation, but occurs in the moist simulation and is associated with the formation of rainbands. The effect of condensation on the energetics of the frontgenesis is studied by evaluation of the kinetic energy budget. The latent heat of condensation generates available potential energy on the mesoscale and enhances the conversion of zonal available potential energy to eddy available potential energy by enhancing the north-south component of flow across the front. It also enhances the conversion of eddy available potential energy to eddy kinetic energy by producing a stronger direct secondary circulation in the moist simulation. The diagnostic Sawyer-Eliassen equation is used to partition the forcing responsible for the generation of the ageostrophic secondary circulation. The process that produce this circulation are geostrophic shearing deformation, vertical exchange of heat and momentum, and latent heat release. The Sawyer-Eliassen diagnostic studies revealed a number of facts concerning the generation of the secondary circulation. The major findings are: 1) The lame-scale features of both the moist and dry ageostrophic circulations are generated by geostrophic deformational forcing; 2) a jet of vertical velocity observed ahead of the surface cold front in the dry simulation is the result of forcing by both deformational and frictional process; 3) the magnitude and structure of the vertical motion field in the moist case are produced primarily by latent heat release; 4) the increase in the horizontal ageostrophic circulation observed in the moist case is due to combined effects of deformation and latent heating; and 5) the turbulent exchange of heat had very little influence on the generation of secondary circulation because of a thermally insulated lower boundary.