Boundary Layer Structure and Processes in Mid - Ocean Storms.

Measurements taken during the Storm Transfer and Response Experiment (STREX) are used to analyze boundary layer structures and processes in the vicinity of North Pacific storms. Case studies are carried out for the pre -frontal, post-frontal, and frontal sectors of storms. The effects of sub-grid scale processes on the boundary layer and the overlying atmosphere receive special emphasis. The pre-frontal boundary layers are nearly neutrally stratified and the surface heat and moisture fluxes are small. The surface fluxes tend to be downward just ahead of the fronts and are of greater magnitude during stronger storms. Even though the actual entrainment velocities are small, the entrainment fluxes are generally the most important sources of total heat for the pre-frontal boundary layers. Entrainment rates determined from budgets compare well with results calculated from relationships determined in laboratory studies of shear-driven entrainment. Heat and moisture budgets are evaluated in two post-frontal situations. In both cases surface heat and moisture fluxes are the dominant sources of total heating within the boundary layers. The entrainment velocities are larger in post-frontal than pre-frontal regions, but entrainment has only a small and positive net effect on the total heat content of the post-frontal boundary layers. Penetrative convection represents the major sink of boundary layer moisture for the case with a long atmospheric fetch over the ocean. A single strong cold front is analysed. The Sawyer Eliassen secondary circulation equation is used to compare the effects of geostrophic forcing, diabatic heating, and friction on the synoptic-scale ageostrophic flow at the front. Friction is found to be the primary process forcing the low-level updraft at the front. Combined kinematic and thermodynamic analyses show strong relative inflow of warm boundary layer air toward the front from the east and a weaker inflow of cold air from the west. The frontogenetical processes are evaluated and compared with those from previous studies. It is proposed that the intensity of turbulent mixing limits the scale of a front.