DEFORMATION FRONTOGENESIS - OBSERVATION AND THEORY

A low-level deformation frontogenesis event that occurred during the S tormscale Operational and Research Meteorology-Fronts Experiment Syste ms Test (STORM-FEST) held program is analyzed in the context of semige ostrophic theory. The observed evolution and vertical structure of the potential temperature and alongfront wind fields are compared to that predicted by both numerical and analytical solutions of the semigeost rophic equations initialized at the onset of the deformation frontogen esis. The model solutions provide relatively accurate predictions of t he surface potential temperature distribution 5 h later, when the fron togenesis ended. The point along the front with the steepest potential temperature gradient is observed to move closer to the point with the highest relative vorticity by an amount that is in rough agreement wi th the model prediction. Vertical profiles of potential temperature fr om soundings show a nearly mixed layer below similar to 400 m that can not be predicted by the inviscid solutions, but there is good agreemen t with inviscid theory above this level. The observed profiles of alon gfront wind are characterized by a low-level jet with maximum speed at the level of the inversion, and the vertical shear below the jet maxi mum is opposite that predicted by the thermal wind equation. The semig eostrophic model does appear to depict this frontogenesis event in the upper layer, while the lower layer is dominated by surface drag and s hear-induced turbulent mixing.