An over-ocean precipitation retrieval using SSM/I multichannel brightness temperatures

A new over-ocean precipitation retrieval algorithm is presented in this study using multi-channel brightness temperatures (TBs) of Special Sensor Microwave Imager (SSM/I) measurements. The basic idea of this algorithm is to find an optimal 2-dimensional precipitation field which gives radiative transfer model-generated temperatures that fit best with the observed TBs. The algorithm utilizes vertically-polarized TBs in order to minimize the sea-surface wind effects. The radiative transfer model of Liu and Curry (1993) is used for computing field-of-views (FOV)-averaged TBs at 19.35, 37 and 85.5 GHz from precipitation fields with a spatial resolution of 25 km. Statistical characteristics of sub-pixel-scale variation and vertical profile of precipitation from GARP Tropical Experiment (GATE) data sets are employed as precipitating cloud models in the radiative transfer calculation to simulate FOV-averaged brightness temperatures. The agreement between the multichannel radiative calculated TBs and the observed TBs is expressed as a cost function which is a sum of the squares of the statistically-normalized differences between the above 2 TB fields. The optimal precipitation field with 25 km resolution is obtained by resolving the gradient equation of the cost function. Precipitation retrievals using this algorithm are executed for two meteorological cases (Typhoon Flo (T9019) at 21 UTC 17 September 1990 and warm frontal rain at 21 UTC 28 April 1988). The retrieved precipitation is compared with the precipitation data observed by the operational radar network of the Japan Meteorological Agency. The results indicate that the algorithm amplifies mesoscale precipitation patterns within the large-scale rain areas. The results also suggest that this amplification improves the linear correlation between the retrieved precipitation and the precipitation obtained from the radar observation. This improvement is ascribable to the effectiveness of the algorithm in utilizing high-frequency channel measurements to redistribute precipitation within large-scale rain areas. Statistical comparisons are also made between the precipitation retrievals using the algorithm and shipboard radar data during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP). An IOP-averaged ratio of the retrieved precipitation to the radar rain intensity is 0.944 for this period. This suggests that the characteristics of sub-pixel-scale variation and the vertical profile of precipitation used in the algorithm is appropriate since the ratio largely depends on these parameters.