Future Calibration Challenges for IMERG V08 and Beyond

NASA’s Integrated Multi-satellitE Retrievals for GPM (IMERG) is a widely used high resolution global precipitation product. IMERG relies on retrievals from passive microwave (PMW) sensors as the primary input precipitation estimates, so it is vital that they are homogenously calibrated. In IMERG, the PMW precipitation estimates from both GPM and partner satellites are calibrated by the GPM Combined Radar–Radiometer Analysis (CORRA-G/T) for the GPM/TRMM eras respectively. Inspections of IMERG V07 indeed illustrate a very close calibration of the PMW estimates to V07 CORRA-G/T as result of several improvements of the CORRA-G/T calibration of PMW GPROF that were implemented in V07.

Despite the improved calibration procedure, systematic biases and inhomogeneities remain in the satellite precipitation products used as input for IMERG, especially in the CORRA-G/T product used as the calibration standard for IMERG. Early evaluations of V07 CORRA-T indicate discontinuities exist at the August 2001 TRMM boost, and V07 CORRA-G discontinuities at the November 2023 GPM Core Observatory boost. Specifically, spatiotemporal matches of other satellite GPROF precipitation to both pre-boost and post-boost TRMM CORRA-T and GPM CORRA-G quantify a substantial post-boost loss in the unconditional mean of the respective CORRA products.

Other IMERG record periods under CORRA-G/T inhomogeneity investigation include the TRMM PR hardware switch from the A-side to B-side on 29 May 2009 and the June 2014 transition from TRMM to GPM calibration in IMERG. We plan to work with the GPROF and CORRA teams to develop corrections for these change points as part of V08.