The Influence of Coastal Shape on Winter Mesoscale Air-Sea Interaction

Abstract

In the case of cold air outbreaks, the combination of the coastal shape and the sea surface temperature (SST) pattern is shown to have a profound effect in establishing a low level mesoscale atmospheric circulation as a result of differential heating due to both variations in overwater path length and the SST. A convergence (or divergence) line then forms along a line exactly downwind of the major bend in the coastline. This is consistent with the structure of the cloud patterns seen in a high resolution Landsat picture of the cloud streets. The major features are also simulated well with a boundary layer model. The dominant convergence line is marked by notably larger clouds. To its cast the convective roll clouds grow downstream in accord with the deepening of the boundary layer. To its west (i.e., coastal side) near the convergence line where the induced pressure field forces a strong westerly component in the boundary layer, the wind shear across the inversion gives rise to Kelvin-Helmholtz waves and billow clouds whose orientation is perpendicular to the shear vector and to the major convergence line. The result is a pattern of cloud streets oriented N–S along the wind direction to the cast of the convergence line, and billow clouds oriented essentially E–W to the west of that line. It is also suggested that the induced mesoscale circulation will feed back on the ocean by intensifying the wind-generated ocean wave growth and altering their orientation.