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

抄録

  The Precipitation Radar (PR) on the Tropical Rainfall Measuring Mission (TRMM) satellite, which was launched on 28 November 1997, has been collecting global rainfall data for more than 10 years. The monthly estmated-surface rainfall (e_SurfRain) amounts estimated by PR have decreased on average since the satellite altitude was changed from 350 km to 402.5 km in August 2001 to extend its lifetime. There is no significant decrease in e_SurfRain in the five angle bins around the nadir (near-nadir) or in rainfall amounts at a 2-km height. The major causes of the changes in rain estimates due to the orbit boost are (1) sensitivity degradation, owing to the increase of satellite altitude, (2) an increase in the range of surface clutter owing to the increase in the footprint size, and (3) a mismatch between the transmission and reception angles of adjacent radar beams for one in every 32 pulses.
  The decrease in the echo from a distributed target near the surface attributable to the change of the satellite altitude from 350 km to 402.5 km is 1.21 dB (= 20log(402.5/350)). From a simulation result, we estimate a decrease of 0.5% in surface-rainfall estimates due to the sensitivity degradation. Because the height of the clutter-free bottom increases at off-nadir angles due to the increase in the footprint size from 4.3 km to 5.0 km, the sampling range bins for near-surface rain have been raised, causing the PR to miss low-rain systems more often after the boost than before the boost. Although a beam-mismatch correction is executed in the current 1B21 algorithm, the correction algorithm still results in a negative bias in rain-rate estimates near the surface in the second half of the PR scan. The changes in the surface-rainfall estimates after the boost due to the second and third causes can be estimated as −2.5% and −2.9% by comparing the estimates at off-nadir bins with those at near-nadir bins with the assumption that the true rain rates measured at all angle bins should be the same, on average. It is concluded that the total effect of the orbit boost is a decrease of 5.9% (= 0.5 + 2.5 + 2.9%) in the e_SurfRain amounts in PR version 6 products.