Simulation of passive microwave wind direction signatures over the ocean using an asymmetric-wave geometrical optics model

The geophysical nature of polarized azimuthal microwave brightness temperature signatures over the open ocean and their relation to surface wind-driven anisotropies are investigated. The broadband nature of previously observed wind-direction signatures suggests the predominance of a frequency-independent thermal emission mechanism involving scattering and emission from asymmetric ocean gravity waves and ocean foam. To verify this hypothesis, an asymmetric-wave Monte Carlo ocean surface model and a geometrical optics electromagnetic emission model of the upwelling radiation field were developed. The model incorporates the effects of ocean wave asymmetry, ocean foam, and multiple geometric scattering. Model calculations compare favorably with the results of a previous study that used data from the Special Sensor Microwave Imager (SSM/I) at 19 and 37 GHz and at wind speeds of 7.9 and 12.2 m/s and at 53 degrees incidence. The good agreement between the satellite data and the asymmetric-wave geometrical optics model supports the hypothesis that azimuthal brightness temperature signatures at wind speeds characteristic of moderate-to-heavy wind conditions are to a great extent caused by ocean wave asymmetry and foam.