Airborne scatterometer detection of winds and sea surface roughness changes across the gulf stream front

This article investigates the scatterometer radar cross-section distributions and wind stress spatial variability across a sea surface temperature (SST) front under different wind directions and synoptic scale atmospheric forcings. Shipboard meteorological measurements in concert with airborne Ku-band (14.0 Ghz) rotating scatterometer data show evidence of mesoscale circulation near the front during low ambient winds (4 m/s) blowing from the cold side of the SST front to the warm side. The mesoscale signature is characterized by a 25–35° counterclockwise shift in the wind direction and a maximum of 4.9 dB increase in the backscattering cross section across the front. The mesoscale circulation is reduced when the wind direction reverses, and the ambient wind flow increases to 13.6 m/s. The maximum cross-section change across the front is reduced to 3.6 dB. The azimuthal characteristics of the scatterometer data are compared with the predictions of two scatterometer model functions. According to the models, the anisotropy of the short waves is inversely related to the wind speed, such that the short wave directional spreads tend to broaden as the wind speed increases. The scatterometer data, however, show greater short-wave anisotropy at the higher wind speeds. This measured difference could be caused by current velocity changes or by longwave directional changes across the front.