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The Integrated Multi-satelliE Retrievals for GPM (IMERG) is the unified U.S. algorithm that provides the Day-1 multi-satellite precipitation product for the U.S. GPM team. The precipitation estimates from the various precipitation-relevant satellite passive microwave (PMW) sensors comprising the GPM constellation are computed using the 2014 version of the Goddard Profiling Algorithm (GPROF2014), then gridded, intercalibrated to the GPM Combined Instrument product, and combined into half-hourly 0.1x0.1 degree fields.
The IMERG system is run twice in near-real time: "Early" multi-satellite product ~6 hr after observation time (goal of 4) and "Late" multi-satellite product ~18 hr after observation time (goal of 12). Once the monthly gauge analysis is received, ~3 months after the observation month, a "Final" satellite-gauge product is produced.
The IMERG Early Run, which is the near-realtime product in the IMERG suite of products currently runs about 6 hours after observation time. Many of the qualitative comments about the Final Run are also true for the Early Run. One difference is that the Early Run necessarily uses calibrations based on trailing accumulations of match-ups, since these cannot be computed into the future. In addition, the Early Run only has forward propagation of the microwave data (unlike both the Late and Final Runs), and it has climatological calibration to the monthly gauge data (as does the Late, but unlike the Final, which uses actual monthly gauge analyses). The beginning of the Early Run record is being computed with "seed" calibrations based on October 2014 data. Accordingly, users should expect the start of the Early Run record to be less accurate than following months of data that will have fully populated recent calibrations.
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