Validation of IMERG V07 Precipitation using the GPM Validation Network

NASA recently released Version 7 (V07) of its global-gridded merged satellite precipitation product, the Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (GPM) mission (IMERG) Final Run, in September 2023. IMERG V07 supersedes all previous versions and the changes in V07 are expected to collectively improve the accuracy of the precipitation estimates. To assess the IMERG V07 Final Run performance, we use the GPM Validation Network’s 118 ground-based radars distributed across North America, Brazil, and Pacific Islands. One novelty of using the GPM Validation Network for IMERG validation is the capability to trace errors from the Level 3 (gridded) IMERG algorithm back through to the input Level 2 (satellite footprint) GPM Microwave Imager (GMI) estimates provided by the Goddard Profiling (GPROF) algorithm, which facilitates future algorithm improvements.

In this presentation, we assess the impact of different algorithm steps including IMERG’s gridding and calibration of the input GPROF-GMI precipitation retrievals, as well as the V07 update which combines instantaneous GMI retrievals with propagated passive microwave precipitation fields for the first time. Specifically, we show that fixing the spatial offset in the gridding resulted in an appreciable improvement in its random error. Of particular interest is whether IMERG V07 exhibits an improvement in the systematic error over oceans at high latitudes as seen by the Alaskan radar. We also use the Validation Network to examine the three-dimensional characteristics of precipitation in regions where GPROF estimates are a cause of error in IMERG.

The GPM Validation Network provides the first IMERG V07 validation results across global land, oceanic and coastal radars under a consistent framework, and traces their errors to aid algorithm improvements for IMERG V08.