Deformation Frontogenesis and Related Boundary Layer Processes: Observation and Theory

A low-level deformation frontogenesis event that occurred during the STORM-FEST field program is analyzed in the context of semigeostrophic 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 semigeostrophic equations, initialized at the onset of the deformation frontogenesis. The model solutions provide relatively accurate predictions of the surface potential temperature distribution 5 h later when the frontogenesis 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 with the model prediction. Vertical profiles of potential temperature from soundings show a nearly-mixed layer below ~400 meters that cannot be predicted by the inviscid solutions, but there is good agreement with inviscid theory above this level. The observed profiles of alongfront wind are characterized by a low -level jet with maximum speed at the level of the inversion, and the vertical shear below the jet maximum is opposite that predicted by the thermal wind equation. Inertial oscillation is shown to control the evolution of the low -level jet, and the predicted winds above the mixed layer agree with the equilibrium profile about which the oscillation occurs. Estimates of terms in the heat equation using surface data reveal that a cross-front variation in diabatic cooling is responsible for the cessation of frontogenesis. After accounting for the effects of inertial oscillation and nonuniform diabatic cooling, the semigeostrophic model does appear to depict this frontogenesis event in the upper layer. The lowest 400-m of the boundary layer is dominated by surface drag and shear-induced turbulent mixing.