Remote sensing of the near surface-ocean wind vector under low-wind conditions using scatterometry

The University of Massachusetts collected C- and Ku-band ocean backscatter measurements at 21.0°, 31.0°, 41.5° and 52.0° incidence, using a C-band and a Ku-band conical scanning airborne scatterometer. These high spatial resolution backscatter measurements differ significantly, for low-wind cases, from the predicted responses of the CMOD4 and SASS-II model functions. The nulls in the azimuthal backscatter response tend to decrease at a much faster rate than predicted as the winds decrease below 5 m s^-1, the measured backscatter response does not follow a power-law relationship with wind speed nor with wind stress, and a cutoff wind speed, below which the backscatter drops to immeasurable quantities, is present. These characteristics are predicted by the Donelan and Pierson (1987) model, and comparing the measured results with this model shows good agreement. However, when the spatial resolution of the measurements are degraded to an area equivalent to that of satellite-based measurements, the deep nulls and fast fall off rate are no longer present. Such effects may bias the backscatter measurements from satellite-based platforms towards the higher winds within the footprint, and thus overestimate the wind speed