A combined satellite infrared and passive microwave technique for estimation of small-scale rainfall

There are numerous applications in climatology and hydrology where accurate information at scales smaller than the existing monthly/2.5 degrees produc ts would be invaluable. Here, a new microwave/infrared rainfall algorithm i s introduced that combines satellite passive microwave (PMW) and infrared ( IR) data to account for limitations in both data types. Rainfall estimates are produced at the high spatial resolution and temporal frequency of the I R data using rainfall information from the PMW data. An IRTb-rain rate rela tionship, variable in space and time, is derived from coincident observatio ns of IRTb and PMW rain rate (accumulated over a calibration domain) using the probability matching method. The IRTb-rain rate relationship is then ap plied to IR imagery at full temporal resolution. MIRA estimates of rainfall are evaluated over a range of spatial and tempor al scales. Over the global Tropics and subtropics, optimum IR thresholds an d IRTb-rain rate relationships are highly variable, reflecting the complexi ty of dominant cloud microphysical processes. As a result, MIRA shows sensi tivity to these variations, resulting in potentially useful improvements in estimate accuracy at small scales in comparison to the Geostationary Opera tional Environmental Satellite Precipitation Index (GPI) and the PMW-calibr ated Universally Adjusted GPI (UAGPI). Unlike some existing PMW/IR techniqu es, MIRA can successfully capture variability in rain rates at the smallest possible scales. At larger scales MIRA and UAGPI produce very similar impr ovements over the GPI. The results demonstrate the potential for a new high -resolution rainfall climatology from 1987 onward, using International Sate llite Cloud Climatology Project DX and Special Sensor Microwave Imager data . For real-time regional or quasi-global applications, a temporally "rollin g'' calibration window is suggested.