An evaluation of GPM-IMERG and ERA5 quantitative precipitation estimates over the Southern Ocean using shipborne observations

Quantitative precipitation estimates over the Southern Ocean (SO) continue to suffer from a high degree of uncertainty for both satellite-based and reanalysis products with large differences present amongst them. This uncertainty arises from the absence of long-term, high-quality observations of precipitation suitable for evaluation across a range of temporal and spatial scales. Recent voyages on the Australian RV Investigator, however, have provided new observations of precipitation across this region made with both the OceanRAIN maritime disdrometer and a dual-polarized C-band weather radar (OceanPOL).The present study employs these unprecedented observations to evaluate the Global Precipitation Mission (GPM) Integrated Multi-satellitE Retrievals (GPM-IMERG) and the ECMWF reanalysis (ERA5) precipitation products. Working at a resolution of 60 minutes and 0.25° (~25 km), the GPM-IMERG product overestimated precipitation intensity when evaluated against the OceanRAIN but captured the frequency well. Looking at the synoptic/process scale, GPM-IMERG was found to be least accurate (overestimate frequency and accumulation) under warm frontal and high-latitude cyclone conditions when cloud-top heights (CTT) were commonly between 6-8 km. In comparison, ERA5’s skill was more consistent across various synoptic conditions, except for high-pressure conditions where the precipitation frequency was highly overestimated. Using the OceanPOL radar, an area-to-area analysis (fractional skill score) finds that ERA5 has greater skill than the GPM-IMERG.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)