A Numerical Study of the Role of Meso Beta and Meso Gamma Scale Vertical Exchange Processes in the Evolution of Upper Tropospheric Frontal Systems
Final rept. 1 Mar 1986-28 Feb 1990
MESOSCALE ENVIRONMENTAL SIMULATIONS AND OPERATIONS INC HAMPTON VA
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During the four year research project, several specific areas of knowledge were enhanced in the process of simulating meso-beta and meso-gamma scale frontogenesis and frontolysis. First, a new theory was developed, based on numerical simulations and mesoscale observations, of ageostrophic along-stream frontogenesis. The six-stage process explains how a stratified flow can trigger a sequence of unbalanced nonlinear adjustments which result in an along-stream frontal zone. Second, the effects of nonhydrostatic and turbulent motions on the fronts formed via the mechanism in ageostrophic along-stream frontogenesis was investigated. It was found that the turbulent covariances generated over length scales of about 5-10 km could drastically modify hydrostatic frontogenesis and produce substantial vertical heat and momentum fluxes particularly in dry adiabatic regions which exhibited significant magnitudes of velocity convergence. Third, it was found that both meso-beta scale hydrostatic and meso- gamma scale nonhydrostatic processes accompanying ageostrophic along-stream frontogenesis could produce significant local increases in relative vorticity equivalent to 1-3 times the Coriolis parameter or larger over time periods of 1- 3 hours and tens of minutes, respectively. The former via the combined tilting and convergence of vorticity during isentropic folding and the latter via tilting and the vertical transport of vorticity during the turbulence caused by ageostrophic frontogenesis. Locally, the potential vorticity was not conserved during the largest nonhydrostatic vertical transport processes resulting in lower tropospheric increases of nearly 300 in 30 minutes during ageostrophic along-stream frontogenesis.