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Derivation of sub-footprint scale /Spl sigma/0 observed by TRMM Precipitation Radar

To derive the sub-footprint scale sigma0 from TRMM/PR observation datasets, a new formulation to calculate the sub-footprint sigma0 is devised. The sub-footprint scale sigma0 would be useful to, (1) relieve the underestimate of the path integrated attenuation (PIA) at off-nadir angle observations over land, and (2) examine the nonuniformity of rainfall. The PIA estimated from sigma0 of the standard product "2A21" tends to be biased toward small at off-nadir angle observations over land. Because 2-6 sub-footprint scale sigma0 exists within one instantaneous field of view (IFOV), sigma0 at the surface echo peak is likely unrepresentative for the IFOV especially for observations over land, depending on the surface conditions and the nonuniformity of rain. The underestimate of PIA can be relieved by averaging the subfootprint scale sigma0 over the IFOV (5 km by 5 km) for each angle bins. The effect of the unbiased PIA, estimated with the sub-footprint scale sigma0, on the rain rate estimate is examined. The result shows about 10% increase of the rain rate at off-nadir angle observations over land.