Rain modification of scatterometer radar cross section due to changes in surface roughness

Scatterometers determine surface wind through observations of radar backscatter from the ocean surface. This backscatter is modified by rain in three ways: (1) backscatter from rain increases the signal, (2) attenuation by rain reduces the signal, and (3) rain drops change the roughness of the surface, usually increasing the signal. The first two processes are highly dependent on the characteristics of rain, which are determined from NEXRAD observations. The modification of the surface roughness is examined herein. This study focuses on strong wind cases, finding that the increase in roughness is dependent on the polarization and frequency of the scatterometer radar. From a remote sensing perspective, the roughness changes are similar to a 5 to 8m/s change in wind speed. We cannot distinguish between small scale increases in wind speed, perhaps related to downbursts, and changes purely due to rain-roughening of the surface. Down drafts would cause substantial increases in surface fluxes, whereas the increase friction from a rougher surface could reduces surface turbulent fluxes of heat and moisture. Implications are further discussed.