In the previous TMPA data set, each data value provided a precipitation rate based on one (or perhaps two) satellite snapshots during the TMPA’s 3-hour analysis period. IMERG values are based on a single microwave snapshot during its half-hour analysis period, or a morphed/Kalman filter interpolation if no microwave values are available.  The values are expressed in the intensive units mm/hr; it is usually best to assume that this rate applies for the entire half-hour period.

Q1: How closely should the monthly satellite-gauge combined precipitation datasets follow the gauge analysis?

Compared to previous versions, Version 06 IMERG introduces additional coverage at the high latitudes for the precipitation fields in all Runs -- Early, Late, and Final.  IMERG continues to use a merged geosynchronous infrared brightness temperature analysis to provide IR-based precipitation estimates.  The requisite analysis (provided by NOAA/NWS/Climate Prediction Center) covers the latitude band 60°N-S, so a "full" IMERG analysis is possible there.

The following table provides a quick reference for the IMERG variables that can be visualized using Giovanni.