SYNOPTIC AND FRONTAL-CYCLONE SCALE INSTABILITIES IN BAROCLINIC JET FLOWS

Baroclinic instabilities of jet flows are investigated by means of num erical simulations of the nonhydrostatic Boussinesq equations on the f plane. Frontal small scales are accurately described thanks to precis e numerical methods (mixed spectral-high-order finite differences). Fi rst. direct numerical simulations are used to carry out linear instabi lity studies. The authors show the existence far a critical value of R o/F = 1.5, under which baroclinic instability dominates (Ro and F are the Rossby and the Froude numbers). Cyclogenesis events are then explo red for various degrees of baroclinicity of, the basic state with emph asis on the preferential amplification of cyclonic vorticity. The asym metry of the vorticity field is quantified, and deterministic and stat istical analyses of the various terms involved in the vorticity equati on 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 usi ng large-eddy simulations techniques. They reveal the spontaneous deve lopment of intense small-scale secondary cyclones at various locations along the cold front. These are shown to result from similar vorticit y amplification mechanisms to the primary large-scale cyclones.