GLOBAL OCEANIC PRECIPITATION - A JOINT VIEW BY TOPEX AND THE TOPEX MICROWAVE RADIOMETER

The TOPEX/POSEIDON mission offers the first opportunity to observe rai n cells over the ocean by a dual-frequency radar altimeter (TOPEX) and simultaneously observe their natural radiative properties by a three- frequency radiometer (TOPEX microwave radiometer (TMR)). This work is a feasibility study aimed at understanding the capability and potentia l of the active/passive TOPEX/TMR system for oceanic rainfall detectio n. On the basis of past experiences in rain flagging, a joint TOPEX/TM R rain probability index is proposed. This index integrates several ad vantages of the two sensors and provides a more reliable rain estimate than the radiometer alone. One year's TOPEX/TMR TMR data are used to test the performance of the index. The resulting rain frequency statis tics show quantitative agreement with those obtained from the Comprehe nsive Ocean-Atmosphere Data Set (COADS) in the Intertropical Convergen ce Zone (ITCZ), while qualitative agreement is found for other regions of the world ocean. A recent finding that the latitudinal frequency o f precipitation over the Southern Ocean increases steadily toward the Antarctic continent is confirmed by our result. Annual and seasonal pr ecipitation maps are derived from the index. Notable features revealed include an overall similarity in rainfall pattern from the Pacific, t he Atlantic, and the Indian Oceans and a general phase reversal betwee n the two hemispheres, as well as a number of regional anomalies in te rms of rain intensity. Comparisons with simultaneous Global Precipitat ion Climatology Project (GPCP) multisatellite precipitation rate and C OADS rain climatology suggest that systematic differences also exist. One example is that the maximum rainfall in the ITCZ of the Indian Oce an appears to be more intensive and concentrated in our result compare d to that of the GPCP. Another example is that the annual precipitatio n produced by TOPEX/TMR is constantly higher than those from GPCP and COADS in the extratropical regions of the northern hemisphere, especia lly in the northwest Pacific Ocean. Analyses of the seasonal variation s of prominent rainy and dry zones in the tropics and subtropics show various behaviors such as systematic migration, expansion and contract ion, merging and breakup, and pure intensity variations, The seasonali ty of regional features is largely influenced by local atmospheric eve nts such as monsoon, storm, or snow activities. The results of this st udy suggest that TOPEX and its follow-on may serve as a complementary sensor to the special sensor microwave/imager in observing global ocea nic precipitation.