Covering the TRMM and GPM Eras with IMERG

Extensive work in the Global Precipitation Measurement (GPM) mission has created a relatively homogeneous record of passive microwave and radar data for both the sensor and precipitation levels (Levels 1 and 2) that spans both the Tropical Rainfall Measuring Mission (TRMM) and GPM eras. Version 06 of the U.S. GPM team's Integrated Multi-satellitE Retrievals for GPM (IMERG) merged precipitation productstarts with these precipitation retrievals, makes additional intercalibration with the TRMM and GPM Core Observatory sensors as the TRMM- and GPM-era calibrators, respectively, and incorporates monthly surface gauge data. One key upgrade is that we have shifted computing the motion vectors used in the Lagrangian interpolation ("morphing") to numerical model information from MERRA2 and GEOS5, namely using vertically integrated vapor fields (TQV). One benefit is that TQV-based vectors provide globally complete coverage, expanding IMERG's morphing coverage beyond the 60°N-S latitude band provided by the IR, although we continue to mask out precipitation over snowy/icy surfaces as unreliable.

The processing status for the new IMERG version will be summarized, together with examples of performance. On the average, the TQV motion vectors are slightly better than the IR-based vectors at all latitudes. One of the key design points is a smooth transition across the TRMM/GPM data boundary. At the monthly scale, this is facilitated by a scaling by GPCP monthly data, which is necessary to provide realistic averages in high latitudes. The switch from TRMM to GPM changes the latitudinal extent of calibration data for individual precipitation rates from the 35°N-S latitude band to 65°N-S, which requires additional approximations for the latitudinal band 35-65° in each hemisphere. We will briefly discuss approximations for higher latitudes and their effectiveness.