Final Updates of Tropical Rainfall Measuring Mission Microwave Imager Emissive-Reflector Bias Correction for the Global Precipitation Measurement V07 Reprocessing

Abstract

Typically, every 3 - 5 years, the brightness temperature data products for the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) were reprocessed to incorporate improvements in signal processing algorithms. In 2017, the TMI 1B data product, Global Precipitation Measurement (GPM) version 5 (V05), was updated with an improved correction for a slightly emissive main reflector antenna that was physics-based and it incorporated empirical coefficients derived from additional deep-space calibration tests performed. Subsequently, it was discovered that TMI brightness temperatures still contain a consistent bias of the order of 0.6 K in all channels that varied with orbital phase and sun angle in a way that resembled the orbital pattern of the estimated main reflector physical temperature. In this paper, we present an analysis of this bias and provide a correction, which has been included in GPM version 7 (V07) reprocessing. First, an updated inter-satellite radiometric calibration (XCAL) radiative transfer model was used to derive the physical temperature of TMI’s main reflector for all possible solar orientations throughout the 17-year mission. Then, the corrected TMI brightness temperatures were derived using these new physical temperature tables. Finally, these updated Tbs were validated by intercomparison with collocated 13-month GMI measurements using the XCAL double difference technique. A quantitative and qualitative analysis of the improvements is provided.