Correcting scatterometer ocean measurements for rain effects using radiometer data: Application to SeaWinds on ADEOS-2

Measurements of ocean scattering by the SeaWinds on ADEOS-II can be corrupted by presence of rain. To correct for this, we must first estimate the rain rate and storm height, and use these to estimate the attenuation through and backscatter from the rain. ADEOS-II carries the AMSR radiometer, with rain rate as one product. Either this or an algorithm using the brightness temperature can provide attenuation. Finding storm height requires correlation with some measured parameter; after studying several approaches, we found that the storm height relates well enough with rain rate to use the AMSR rain rate as the input to get height. We show here the results of simulations of performance of the correction algorithm using the imperfect actual correlation between storm height and rain rate. We find that corrections are possible up to a rain-rate threshold that depends on the surface wind speed and direction. Above the threshold, one must discard the data by setting a rain flag. We show threshold curves for allowable errors (due to rain height errors) of 5%, 10%, and 15%. Unacceptable errors are largely due to rain echo that greatly exceeds the surface echo.