Observational evidence of a wind direction signal in SSM/I passive microwave data

Recent experiments have demonstrated that a wind direction signal is apparent in passive microwave brightness temperature measurements of the sea surface. Extensive matches of Special Sensor Microwave Imager (SSM/I) brightness temperatures from several Defense Meteorological Satellite Program (DMSP) satellites, with wind direction measurements from both moored buoys and spaceborne scatterometers clearly show directional signals at 19, 37, and 85 GHz for both horizontal and vertical polarization. The wind direction signal obtained from the mooring matches is noisy due to the limited number of matches but varies little from satellite to satellite. The greater number of matches with the scatterometers results in a clearer directional signal and enables detailed studies of the dependence of the directional signal on environmental conditions. The amplitude of the mean directional signal increases with wind speed. The up/downwind asymmetry most prominent at vertical polarization varies mainly with wind speed, while the crosswind differences also vary with sea state and sea surface temperature. The variability about the mean directional signal is equal to or greater than the amplitude of the mean signal and increases with increasing water vapor content. Because of the large variability, large numbers of SSM/I observations under similar environmental conditions must be averaged before the directional signal is apparent. Comparisons between the observed directional signals and existing model predictions will lend insight into the physical mechanisms responsible for the signal