Potential of simultaneous dual-frequency radar altimeter measurements from 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 radar backscatter coefficient over the global oceans since August 1992. In the present work, we have used the concept of differential attenuation of the radar signal due to rain at two widely separated frequencies to estimate rainfall. Simultaneously available passive microwave radiometric 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 from radiometric measurements and the difference δσ° between radar backscatter coefficients at 5 GHz and 13 GHz, significant rain events are isolated during the course of the 1993 South-West (SW) monsoon season over the Indian region. Monthly maps of these rain events from altimeter based analysis clearly bring out the nature of rainfall activity associated with SW monsoon. The results from δσ° also compare very well with the operational quantitative precipitation estimates available from INSAT-VHRR analyses. The advantages and limitations of radar altimeter data are discussed in terms of those of the other current and future rain measurement systems. Synergistic application of the present technique with visible/IR and microwave techniques hold promise for more precise rainfall measurements from space.