An Uncertainty Estimation Model for Radiometric Intercalibration Between GPM Microwave Imager and TRMM Microwave Imager

The Global Precipitation Measurement (GPM) Microwave Imager (GMI) is the radiometric calibration transfer standard for the intersatellite radiometric calibration of the NASA GPM constellation radiometers. Because these radiometers are not identical, the GPM Intersatellite Calibration (XCAL) Working Group has developed a robust double difference technique to estimate the brightness temperatures (Tb) bias, which is applied to the brightness temperatures of constellation radiometers before being input into a single satellite radiometer rain retrieval algorithm (GPROF). Since the radiative transfer models and input geophysical parameters are not perfect, errors (uncertainties) in the estimates of the Tb biases will result. Further, the microwave sensors observations are not coincident in time nor exactly spatially collocated, and this also contributes to the Tb bias uncertainty. Therefore, it is important to quantify the bias uncertainty estimates, considering the various sources aforementioned and more, and to include them with the associated Tb bias before producing science products. A generic uncertainty quantification model is developed herein. For illustration purposes, we use the XCAL between GMI and the TRMM Microwave Imager (TMI), and results show that, after removing the biases, the residual uncertainty between GMI and TMI Tb's are <;0.3 K.