Comparison of IMERG Precipitation Estimates and PACRAIN Atoll Gauge Observations

Abstract:

Monthly satellite-based precipitation estimates from Version 06B of the Integrated Multi-SatellitE Retrievals for GPM (IMERG) Final run are compared with observations from 38 Pacific Rainfall Database (PACRAIN) atoll gauge stations for the period June 2000 – January 2019. The goal of this analysis is to assess the quality of IMERG over open ocean, where validation is historically challenging, using low-lying atoll station gauges as a proxy. Over the entire period, IMERG estimates are higher than the atoll observations by 7.8% with a correlation of 0.68. Seasonally, DJF and JJA offer comparable statistics with biases and correlations of 7.7% and 0.72, and 6.2% and 0.69, respectively. IMERG and the atolls show the best agreement in MAM with a bias of 2.3%, while SON exhibits the worst performance at 14.3% bias. IMERG is consistently lower at light precipitation rates and consistently higher for intense precipitation, with best agreement at intermediate rates. The highest precipitation rates in IMERG dictate the overall positive bias when compared to atolls. The bias in IMERG varies over time but does not exhibit any discernible trend or dependence on atoll population which varies widely over the analysis span. The PACRAIN atoll gauges are not wind-loss corrected, so application of an appropriate adjustment would increase the observed amounts, bringing them more in line with the IMERG estimates.