Oceanic Rain Flagging Using Radar Backscatter and Noise Measurements From Oceansat-2 Scatterometer

This paper deals with the method developed for rain flagging over oceans using radar backscatter and noise-equivalent brightness temperature (TB) measured by a Ku-band pencil-beam scatterometer launched onboard the Oceansat-2 (OS2) satellite. The OS2 scatterometer (OSCAT) measures radar backscatter and TB by its two pencil-beam antennas at local incidence angles of ~49° (inner) and ~57° (outer) in horizontal and vertical polarizations, respectively. These measurements are affected by rain and heavy clouds, in addition to the ocean surface winds, thus affecting retrieval of wind vectors. The characteristics of TB at OSCAT specifications have been studied based on radiative transfer simulations. The horizontally polarized TB is observed to be sensitive to both wind speed and rain, whereas the vertically polarized TB (TBV), typically at 57°, is found to be insensitive to ocean surface wind speed but is sensitive to rain, indicating its typical utility for rain flagging. As the retrievals of wind vectors are affected by rain, then so are the associated cost functions and their characteristics under rain-free and rainy situations. A combined use of TBV and retrieval cost functions has been made for rain flagging needed in wind directional-ambiguity removal, as well as tagging the retrieval quality. A data-driven dynamical method has been developed to specifically utilize TBV for supporting rain detection in view of the slow temporal drift found in the OSCAT TB data. Rain flags have been generated from OSCAT 25-km grid L2A (Version 1.4) data for the period from November 2009 to August 2010, and the results of comparison with Tropical Rainfall Measuring Mission Precipitation Radar 2A25 data are presented.