High Spatial Resolution Observation of Rainfall from Space

As mesoscale models improve, they require input data with higher spatial resolution, including rainfall measurements with spatial resolution < 2km, such as those that can be obtained from synthetic aperture radars (SAR). Such measurements are now available from at least half a dozen satellite-borne X-band SARs. These include the TerraSAR-X (TSX) and its twin, the Tandem-X, and the Cosmo-Sky-Med launched by the German DLR and the Italian ASI respectively. Algorithms to retrieve rainfall over land from TSX measurements that were developed are presented, and the inferred rainfall distribution is compared to rainfall around Hurricane Gustav observed by NOAA operational radars. The impact of antenna beam spreading on satellite-borne sensor measurements of coastal rainfall is also illustrated. Coastal rainfall measured by the Tropical Rainfall Measuring Mission (TRMM) over the Brazilian coast both over land and sea is also compared to rainfall observed by TSX. Because the sea surface cross section varies with wind speed and the wind direction with respect to the sub-satellite track, a surface reference derived from clear regions was considered (as in the case of the TRMM surface reference method). Wind roughened seas have relatively large cross sections so that attenuation by rain of radiation reflected from the sea surface is the dominant mechanism that affects the observed radar cross section. Calm seas have lower radar cross sections so that scattering by the volume of hydrometeors becomes more significant. Several examples of rainfall observed over sea surfaces with various degrees of wind roughening are presented, including the observation of precipitation over the BP oil spill in the Gulf of Mexico