SAR observation on sea surface wind caused by rain cell

Abstract

Rain cells or convective rain, the dominant form of rain in the tropics and subtropics, can be easy detected by Synthetic Aperture Radar (SAR) images with high horizontal resolution. The footprints of rain cells on SAR images are caused by the scattering and attenuation of the rain drops, as well as the downward airflow. In this study, we extracted sea surface wind structure caused by rain cells by using a RADARSAT-2 SAR image with a spatial resolution of 100 m for case study. We extracted the sea surface wind speed from SAR image by using CMOD4 geophysical model function with outside wind directions of NCEP final operational global analysis data and microwave scattermeter onboard Chinese HY-2 satellite, respectively. The root-mean-square errors (RMSE) of these SAR wind speeds, validated against NCEP and HY-2, are 1.48 m/s and 2.14 m/s, respectively. Circular signature patterns with brighter on one side and darker on the opposite side on SAR image were interpreted as the sea surface wind speed (or sea surface roughness) variety caused by downdarft associated with rain cell. The wind speed taken from the transect profile which superposes to the wind ambient vectors and go through the center of the circular footprint of rain cell can be fitted as a cosine or sine curve in high linear correlation with the values of no less than 0.80. The background wind speed, the wind speed cause by rain cell and the diameter of footprint of the rain cell with kilometers or tens of kilometers can be acquired by fitting curve. Eight cases interpreted and analysed in this study all showed the same conclusion.