Changes in Tropical Rainfall Measuring Mission (TRMM) retrievals due to the orbit boost estimated from rain gauge data

Abstract

During the first three-and-a-half years of the Tropical Rainfall Measuring Mission (TRMM), the TRMM satellite operated at a nominal altitude of 350 km. To reduce drag, save maneuvering fuel, and prolong the mission lifetime, the orbit was boosted to 403 km in August 2001. The change in orbit altitude produced small changes in a wide range of observing parameters, including field-of-view size and viewing angles. Due to natural climatic variability, it is not possible to evaluate possible changes in precipitation retrievals from the satellite data alone. We estimate changes in TRMM Microwave Imager (TMI) and the Precipitation Radar (PR) precipitation retrievals due to the orbit boost by comparing them with surface rain gauges on ocean buoys operated by the NOAA Pacific Marine Environment Laboratory (PMEL). For each rain gauge, we compute the bias between the satellite and the gauge for pre- and post-boost time periods. For the TMI, the satellite is biased ~12% low relative to the gauges during the pre-boost period and ~1.5% low during the post-boost period. The mean change in bias relative to the gauges is approximately 0.4 mm day^-1. The PR is biased significantly low relative to the gauges during both boost periods. The change in bias is rain rate dependent, with larger changes in areas with higher mean precipitation rates.