In what sense is the entire record for each Run (Early, Late, Final) processed “consistently”?
The entire record for each Run (Early, Late, or Final) is computed with the same version of processing, with the same input steams specified for that Run. The data that are processed afresh are called “Initial Processing” (IP) and those that processed after the fact when the new version starts are called “retrospective processing” (RP). The RP have a couple of simplifications that the team doesn’t believe are important, but should be mentioned for completeness:
- The precipitation from the individual satellites is computed with the GPROF algorithm using a different source of temperature and humidity ancillary data. In the aggregate, the results are very similar.
- The entire set of input data is used in RP, whereas in IP it is possible that some of the input will have been delayed and there are not available to create the IP products. The latency for computing the Early, in particular, is set to wait long enough that we usually capture 85-90% of the eventual input.
A final point is that the complement of satellites and (sometimes) their overpass times change over time. To the extent that different sensors “see” different interpretations of the precipitation, and the IMERG intercalibration is not entirely able to overcome this, the team expects at least slight variations over time in quality and performance. This issue is less critical if you are looking at mean behavior, but more important if you are trying to estimate extremes.