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.