The calendar-window discovery

Daily satellite-gauge correlation is convention-dependent. IMERG-L is dated by the UTC calendar day; the BMKG gauges report on the local-day morning-observation window. So the two references peak at different aggregation-window offsets. Drag the dial and watch the peaks separate - the low daily correlation is a clock problem, not a rainfall problem.

Key finding Re-aggregating IMERG-L from the UTC day to the local-day window (offset −23 h) lifts the daily correlation against BMKG from 0.20 to 0.57 - with no change to the correction itself. Most of the r ≈ 0.34 ceiling was a calendar artefact, not a limit of the retrieval.

vs BMKG (validation)

at h = h

vs CPC-UNI (UTC label)

the calibration reference

vs CPC-UNI (relabelled +1 day)

harmonised to the local day

What the dial shows. At h = 0 (the UTC day) IMERG-L agrees best with CPC-UNI but poorly with BMKG; at h = −23 h (the local day) the picture flips, and BMKG recovers to about 0.57. No single calendar label maximises agreement with both references at once, so a corrected product is best distributed on an explicit local-observation day with all references harmonised to it. The residual after harmonisation is the genuine retrieval-timing limit - it cannot be relabelled away.

Daily vs monthly

The window fix matters only at the daily scale. Aggregate the same product to calendar months and the shift vanishes - a one-day window change just moves a day or two across each month boundary. Pooled over the GPM era (n = station-months):

Finding The same product scores r = 0.20 daily but 0.80 monthly at the UTC-day window. Re-windowing to the local day lifts the daily r to 0.57 but leaves the monthly value essentially unchanged (0.80 → 0.81). The daily ceiling lives in the day-by-day pairing - the timing - not in how much rainfall the product captures.
The evidence behind this recovery - when it emerged (the satellite-era transition), that it holds in every timezone band and season, station by station, and across the whole 0.5° domain - is on Whole-domain & era.