Baroclinic instability and severe storms

We first review mechanisms leading to primary and secondary instabilities o riginating from a thermal front in a rotating stably stratified atmosphere or ocean. This is done in particular in the light of the work by Garnier et al (1996 C. R. Acad. Sci., Paris B 323 161-8; 1998 J. Atmos. Sci. 55 1316- 35), where direct-numerical simulations (DNS) and large-eddy simulations (L ES) of Navier-Stokes equations within the Boussinesq approximation were car ried out. In these articles, braids of intense cyclonic vertical relative v orticity stretched in the fronts reconnecting the primary vortices were fou nd at the ground and under the lid of the computational domain. We present an animation of primary-vortex and front formation in the DNS, and give an interpretation of the vortex braids using the equations of motion and poten tial-vorticity conservation. Afterwards, the results of Garnier et al using LES, which showed an instabi lity of the braids yielding very intense secondary vortices forming along t he cold front, are compared with satellite observations corresponding to th e severe storm of 26 December 1999. This storm, together with its companion on 28 December, caused casualties and immense damage in France and neighbo uring countries. We discuss possible analogies and differences, as well as some consequences in terms of numerical weather forecasting.