PRIMARY AND SECONDARY INSTABILITIES OF A BAROCLINIC JET

We investigate the instabilities taking place in a baroclinic jet with the aid of large-eddy simulations of the non-hydrostatic Navier-Stoke s equations (Boussinesq approximation). Thanks to accurate numerical m ethods (mixed spectral/high order finite differences), the mechanisms leading to preferential amplification of cyclonic vorticity are clearl y identified. The analysis of the various terms involved in the vertic al vorticity equation shows that vertical stretching mechanisms are re sponsible for this asymmetry. The use of large-eddy simulation techniq ues makes possible the study at high Reynolds number of the late stage of the instability. The development on the cold front of small-scale secondary instabilities is observed, yielding a strong amplification o f cyclonic vorticity.