A NUMERICAL INVESTIGATION OF THE IMPACT OF MOIST PROCESSES UPON THE STRUCTURE OF ACTIVE COLD FRONTS:APPLICATION OF A 3D MESOSCALE MODEL IN FRONTAL SITUATIONS

A 3D mesoscale model is applied in a marine cold front case in order to investigate the impact of latent heating on the structure of cold fronts.Results of the moist and dry simulations are compared to stress the effects of moist processes.It is found that both the temperature gradient and the cyclonic vorticity across the frontal zone considerably increase due to the latent heating,especially in the lower troposphere.A thermally direct cross-front ageostrophic circulation.forced by frontogenesis,is found better developed and organized in the moist case than in the dry case.This cross-front direct ageostrophic circulation,particularly its ascending branch,is considerably enhanced by latent heating due to increased frontogenetical forcing and reduced effective static stability in the rising motion region where condensation occurs.One important feature in most observed cold fronts is the presence of an intense rising cell just above their leading edge.This intense rising cell is well simulated in the moist case but is less clear and much weaker in the dry case,indicating the important contribution by the latent heat release to the formation of this intense updraft above the surface cold front.