Evaluation of the Effect of the Orbit Boost of the TRMM Satellite on the PR Rain Estimates

The rainfall amount estimated by PR shows a decrease after the satellite altitude was changed from 350 to 402.5 km in August 2001 to extend its lifetime. The major causes of the changes in rain estimates by the orbit boost are (1) the sensitivity degradation by the increase of satellite altitude, (2) the increase of the range of surface clutter by the increase of the footprint size, and (3) the mismatch between the transmission and reception angles for one pulse in every 32 pulses The decrease of the echo signal strength by the change of the satellite altitude from 350 km to 402.5 km is 1.21dB (= 20log(402.5/350)). From a simulation result, we estimate a decrease of 0.50 % in the surface rainfall estimates by the sensitivity degradation. Because the heights of clutter free bottom have increased at the off-nadir by the increase of the footprint size from 4.3 km to 5.0 km, the sampling range bins for near-surface rain have raised and the PR misses low rain systems more often after the boost than before the boost. A beam mismatch correction is executed in the current 1B21 algorithm, but the correction algorithm still has a small problem and rain rate estimates near the surface in the second-half of the PR scan tend to be smaller than those in the first-half on average. The changes due to the second and third causes can be estimated by comparing the estimates at off-nadir bins with those at near-nadir bins with the assumption that rain rates at all range bins should be the same on average. We estimate that the estimated surface rain decreased by 2.47 % by the increase of the foot print size and by 2.93 % by the beam mismatch after the boost. In summary, it is necessary to add a 5.90 % (= 0.50 + 2.47 + 2.93 %) correction to compensate the biases in the surface rain estimates caused by the boost.