How are the TMPA and IMERG algorithm designs the same and different?
Both TMPA and IMERG use a constellation of passive microwave satellites, and within the general umbrella groups of “sounder” and “imager” the inputs are much the same, although at the end of the TRMM era the TMPA was not upgraded to include the newer satellites. The direct inputs of the TMI and GMI are swamped by the amount of data from the rest of the microwave sensors, so the absence of TMI in the last 4.5 years of TMPA was not a major problem. At the back end of the multi-satellite algorithms, both TMPA and IMERG use the same scheme for combining satellite data with the GPCC analysis, although IMERG uses the GPCC Final analysis up through 2018, which tends to be more accurate than the GPCC Monitoring analysis that the TMPA used for the last ~9 years. What’s different? The algorithms for the Combined products are very different (2B31 for TMPA and CORRA for IMERG), and that is what provides calibration. The GPROF algorithm has been upgraded for use in IMERG – still Bayesian, but with the libraries of profiles sourced and organized differently. The IR scheme has shifted from VAR to PERSIANN-CCS – very different approaches. Compared to the simple chunking of data into 3-hour intervals in TMPA, note the massive amount of time spent in IMERG on morphing and the Kalman filter. The goal is two-fold:
- Provide a finer time resolution so that system evolution is more accurately captured, compared to the 3-hour interval in TMPA. This improved evolution not only provides morefrequent data values, but it should also make the IMERG time averages (such as daily) more accurate, since precipitation changes so rapidly in space and time.
- Reduce the use of IR estimates, which have low quality, by time-interpolating the microwave estimates, which have better quality. The hard part here is that the interpolation has to be done in a quasi-Lagrangian framework because the rain systems move. So, IMERG’s morphing/Kalman framework is intended to minimize the IR contribution, even though IR is still seen as necessary in regions with long microwave gaps.
Please view the document The Transition in Multi-Satellite Products from TRMM to GPM (TMPA to IMERG) for further details.