Impact of rain cell on scatterometer data: 2. Correction of Seawinds measured backscatter and wind and rain flagging

Abstract

Rain can strongly modify the normalized radar cross section (NRCS) measured by Ku‐band scatterometers and alter the wind vector retrieval. Part 1 of this paper (Tournadre and Quilfen, 2003) presented a theoretical model of interaction between rain and scatterometer signal and used it to quantify the effect of rain on the backscatter and on wind vectors. Their results showed that the scatterometer data are strongly affected by rain, that they are extremely sensitive to the rain distribution within scatterometer resolution cells, and that the normalized radar cross section (NRCS) variability induced by rain could be a good indicator for rain flagging. The model is further tested and validated on a tropical cyclone case using colocated high resolution rain and Seawinds NRCS data. The model is used to compute attenuation and volume scattering from Tropical Rainfall Mapping Mission Precipitation Radar (TRMM PR) rain data. The comparison of the high‐resolution (4 km) NRCS to synthetic NRCS computed from National Hurricane Center (NHC) winds and modeled rain terms shows a good qualitative agreement. The rain terms are used to correct the measured NRCS, to infer corrected winds which are significantly improved compared to NHC winds, especially for high winds. The wind correction using low‐resolution rain data (such as Special Sensor Microwave Imager (SSM/I) ones) is also investigated using rain data averaged over wind scatterometer cells. This can also significantly improve the rain retrieval. A new rain flag based on the NRCS variability within wind vector cells is presented and shown to perform better than the operational one.