Evaluating the effect of rain on HY-2A scatterometer measurements

The first Chinese microwave ocean environment satellite HY-2A, carrying a Ku-band scatterometer (HYSCAT), a multi-band radiometer and a dual-frequency altimeter, was successfully launched in August, 2011. The accuracy of Ku-band scatterometer wind retrieval is degraded by the presence of rain. We must explore the effects of rain on the HYSCAT measurements in order to determine a best rain flag for rain-contaminated wind vector cells and calibrate out the effects of rain as much as possible. In this paper, the effects of rain on HYSCAT retrieved winds are investigated using the collocated Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) rain measurements and numerical winds fields from the National Centers for Environmental Prediction (NCEP). The results demonstrate that the presence of rain will significantly change the normalized radar backscattering cross-section (NRCS) of HYSCAT. The effect of rain on NRCS is primarily increasing; thus, there exists a positive wind speed shift for HYSCAT wind speed distribution compared to the NCEP distribution, using the enlarged NRCS. For the wind direction, the presence of rain makes the azimuth modulation less obvious.