Correction of Time-Varying Radiometric Errors in TRMM Microwave Imager Calibrated Brightness Temperature Products

This letter presents an empirical correction for a time-varying radiometric calibration error (2.5 K peak to peak) present in the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) 1B11 calibrated brightness temperature (T_b) data product. The T_b error is modeled as a function of the spacecraft orbit position and the Sun elevation above the orbit plane (solar beta angle) based on the observed systematic differences between the TMI 10-GHz vertically polarized (10-V) channel oceanic T_b´s as compared to a radiative transfer model with collocated numerical weather model environmental parameters. Assuming a slightly emissive reflector, the errors in the other TMI channels are estimated from the 10-GHz error model. This method is evaluated using before- and after-correction comparisons of 1B11 data over the ocean and Amazon, and results show that this method effectively eliminates the time-varying calibration error. This correction technique will be implemented in version 7 of the TRMM 1B11 data product, which is scheduled to be released in 2010.