A combined passive–active microwave retrieval of quantitative rainfall from Topex/Poseidon radar altimeter and Topex microwave radiometer

Synergistic measurements of both active and passive nadir looking microwave sensors onboard Topex/Poseidon (T/P) satellite are explored for their instantaneous rain estimation capability over tropical oceans. Data of T/P altimeter (the differential radar backscatter at C and Ku band frequencies, i.e. δσ° = σ°C−σ°Ku), and Topex microwave radiometer (TMR) (brightness temperatures at 18, 21 and 37 GHz frequencies) coincident with special sensor microwave/imager (SSM/I) and tropical rainfall measuring mission (TRMM)–microwave radiometer (TMI) have been analysed for this purpose. The rainfall response has been separated from the surface variability and atmospheric water content (in terms of water vapour and liquid water), by analysing the TMR data in view of the radiative response of its channels under different ocean–atmospheric conditions over the Indian oceanic region. Among a large collocated data on different spatial and temporal scales, the most restrictive criteria (< 10 km spatial, < 30 min temporal difference) produce the better statistics (in terms of correlation and rms errors) from the rainfall rate from SSM/I and TMI with the combined radar and radiometric observations from T/P and TMR respectively. The analysis thus shows the added advantage of optimally integrated active and passive microwave measurements for instantaneous rain estimation as compared to our earlier study (Varma et al. ) by passive TMR measurements alone, for both better estimation of rain and corrections to T/P radar backscatter due to its path attenuation. The equation is further used to estimate monthly averaged global rain rate maps for its qualitative and quantitative assessment. Typical rain rate maps from blended radar and radiometer for two contrasting seasons for the months of January and July for the year 2000 (during the north‐east and south‐west monsoon respectively), are compared with similar maps of TRMM–precipitation radar of monthly rainfall accumulations, showing most of the regional and global tropical features delineated.