Abstract
Aircraft observations obtained during the Frontal Air–Sea Interaction Experiment (FASINEX) are used to investigate the structure of the marine atmospheric boundary layer in the vicinity of an ocean front. A quasi-stationary sea surface temperature (SST) discontinuity of 2°C was maintained across the frontal zone throughout the duration of the experiment The primary response of the atmosphere to changes in the SST was observed in the surface-related turbulence fluxes. In the case of warm air flowing over cold water, the boundary layer appears to develop an internal boundary layer (IBL) in response to the sudden change in the sea surface temperature. The organized updrafts and downdrafts within this layer collapse with entrainment–detrainment processes in these cells dominating the turbulence statistics. The IBL grows in response to the wind shear in this layer, although the surface shear stress is much smaller on the colder side of the front than on the warm. The depth of the IBL, and, in the absence of the IBL, the mixed layer are found to scale with the friction velocity and the Coriolis parameter.
The IBL confines the surface-related turbulent mixing and shear-driven processes to the lower layers of the atmosphere. Thus. the shallow boundary layer cloud field appears to be maintained primarily by radiative transfer within the cloud layer. Multiple cloud-capped mixed layers were frequently observed throughout the experiment. They appear to be directly related to the horizontal variation of the SST with deeper boundary layer and higher cloud levels formed over warmer water.
