Prefrontal and Postfrontal Boundary Layer Processes over the Ocean

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  • 1 Department of atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

Measurements taken during the Storm Transfer and Response Experiment (STREX) are used to analyse boundary layer structures and processes in North Pacific storms. Heat and moisture transfers at the surface and through the top of the boundary layer are evaluated for three cases with warm, southerly flow ahead of cold fronts and two cases of cooler, westerly and northerly flow behind fronts.

The prefrontal boundary layers are nearly neutrally stratified and surface heat and moisture fluxes are small. Surface fluxes tend to be downward just ahead of the fronts and are of greater magnitude during stronger storms. Entrainment fluxes at the top of the prefrontal boundary layers are generally larger than surface fluxes and are the dominant sources of heating for the boundary layers. Entrainment rates determined from budgets compare well with laboratory studies of shear-driven entrainment.

In the postfrontal cases, surface heat and moisture fluxes are the dominant sources of total heating within the boundary layers. Entrainment velocities are larger in postfrontal than prefrontal regions, but entrainment has only a small and positive net effect on the total heat content. In postfrontal transition layers the Richardson numbers are large, and entrainment is forced by turbulence generated by buoyancy in the surface layers and radiative and evaporative cooling from the tops of stratocumulus clouds. Cumulus-scale penetrative convection represents the major sink of boundary layer moisture for one case with a long atmospheric fetch over the ocean.

Abstract

Measurements taken during the Storm Transfer and Response Experiment (STREX) are used to analyse boundary layer structures and processes in North Pacific storms. Heat and moisture transfers at the surface and through the top of the boundary layer are evaluated for three cases with warm, southerly flow ahead of cold fronts and two cases of cooler, westerly and northerly flow behind fronts.

The prefrontal boundary layers are nearly neutrally stratified and surface heat and moisture fluxes are small. Surface fluxes tend to be downward just ahead of the fronts and are of greater magnitude during stronger storms. Entrainment fluxes at the top of the prefrontal boundary layers are generally larger than surface fluxes and are the dominant sources of heating for the boundary layers. Entrainment rates determined from budgets compare well with laboratory studies of shear-driven entrainment.

In the postfrontal cases, surface heat and moisture fluxes are the dominant sources of total heating within the boundary layers. Entrainment velocities are larger in postfrontal than prefrontal regions, but entrainment has only a small and positive net effect on the total heat content. In postfrontal transition layers the Richardson numbers are large, and entrainment is forced by turbulence generated by buoyancy in the surface layers and radiative and evaporative cooling from the tops of stratocumulus clouds. Cumulus-scale penetrative convection represents the major sink of boundary layer moisture for one case with a long atmospheric fetch over the ocean.

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