The combined effect of surface rain and wind on scatterometer observations of surface roughness

Abstract

The wind stress on the sea surface is closely related to the sea surface roughness. When rain impacts affect this roughness, we need to learn the consequences for the air-sea momentum exchange. Our data shows that areas of highest winds correlate closely with the heaviest rainfall. Changes in the sea surface radar cross section from the combined effects of wind and rain, on scales of tens of kilometers, are being studied using the QuikSCAT scatterometer and simultaneous NEXRAD three-dimensional measurements of rain within Hurricane Ike. Buoys, NOAA HRD H*Winds and related data provide the additional wind information. From the remote sensing perspective, these results will show the dependence of the sea surface radar cross section, at Ku-band, as a function of the rainrate, wind speed and relative direction, and polarization. At this microwave frequency the surface backscatter is controlled by the centimeter-scale roughness, but at these high wind speeds the simple models based on Bragg scattering are not useful. In order to study the air-sea interaction that is related to surface fluxes (e.g., momentum, sensible heat, and latent heat) during rain events, extended experimental investigations are needed. Heavy rain in the boundary layer changes the profiles of wind and stratification which alter the surface stress and turbulent heat fluxes. The wind driven rain also creates roughness properties that need to be modeled in order to interpret the Ku-band NRCS at the two polarizations, When high winds also exist (>20 m/s), the interaction is complicated by sea spray.