POTENTIAL OF SIMULTANEOUS DUAL-FREQUENCY RADAR ALTIMETER MEASUREMENTSFROM TOPEX POSEIDON FOR RAINFALL ESTIMATION OVER OCEANS/

TOPEX/Poseidon - a joint U.S. (NASA) and French (CNES) mission called Ocean Topography Experiment-carries onboard a dual frequency (5.3 GHz and 13.6 GHz) radar altimeter providing simultaneous measurements of r adar backscatter coefficient over the global oceans since August 1992. In the present work, Lee have used the concept of differential attenu ation of the radar signal due to rain at two widely separated frequenc ies to estimate rainfall. Simultaneously available passive microwave r adiometric measurements from TOPEX/Poseidon itself have also been used to delineate and quantify rain over oceanic regions surrounding India . Based on a reasonably good correlation between rainfall inferred fro m radiometric measurements and the difference delta sigma degrees betw een radar backscatter coefficients at 5 GHz and 13 GHz, significant ra in events are isolated during the course of the 1993 South-West (SW) m onsoon season over the Indian region. Monthly maps of these rain event s from altimeter based analysis clearly bring out the nature of rainfa ll activity associated with SUI monsoon. The results from delta sigma degrees also compare very well with the operational quantitative preci pitation estimates available from INSAT-VHRR analyses. Tile advantages and limitations of radar altimeter data are discussed in terms of tho se of the other current and future rain measurement systems. Synergist ic application of the present technique with visible / IR and microwav e techniques hold promise for more precise rainfall measurements from space.