Synoptic and Frontal-Cyclone Scale Instabilities in Baroclinic Jet Flows

Abstract

Baroclinic instabilities of jet flows are investigated by means of numerical simulations of the nonhydrostatic Boussinesq equations on the f plane. Frontal small scales are accurately described thanks to precise numerical methods (mixed spectral?high-order finite differences). First, direct numerical simulations are used to carry out linear instability studies. The authors show the existence for a critical value of Ro/F = 1.5, under which baroclinic instability dominates (Ro and F are the Rossby and the Froude numbers). Cyclogenesis events are then explored for various degrees of baroclinicity of the basic state with emphasis on the preferential amplification of cyclonic vorticity. The asymmetry of the vorticity field is quantified, and deterministic and statistical analyses of the various terms involved in the vorticity equation show simply how vertical stretching mechanisms are responsible for this asymmetry. The late stage of the instability following cutoff of the primary wave occlusion is then studied at high Reynolds number using large-eddy simulations techniques. They reveal the spontaneous development of intense small-scale secondary cyclones at various locations along the cold front. These are shown to result from similar vorticity amplification mechanisms to the primary large-scale cyclones.