Assessments of IMERG precipitation over Europe

Accurate estimation of precipitation is essential in weather prediction, climate change research, and hydrologic applications. However, unlike temperature and pressure, precipitation fields can be spatially patchy and consequently extremely difficult to measure and predict. Many efforts have been made to measure precipitation since the 18th century, but building a global, consistent, and continuous database of rainfall is still challenging. The launch of the Global Precipitation Measurement Core Observatory (GPM-CO) in February 2019 emerged as a promising alternative to measure precipitation at global scale. After five years in orbit, the GPM Mission has produced enough quality-controlled data to allow a validation of their precipitation estimates over Europe.

This study evaluates Integrated Multi-Satellite Retrievals from GPM (IMERG) over Europe in order to evaluate application of the retrievals to hydrology. IMERG is compared with a pan-European precipitation dataset built on rain gauge stations, the ENSEMBLES OBServation (E-OBS) gridded dataset. Although there is overall agreement in the spatial distribution of mean precipitation (R² =0.8), important discrepancies are revealed in mountainous regions, specifically the Pyrenees, the Alps, west coast of the British Isles, Scandinavia, the Italian and Iberian peninsulas, and the Adriatic coastline. The results show that the strongest contributors to poor performance are pixels where IMERG has no gauges available for adjustment. If rain gauges are available, IMERG yields results similar to those of the surface observations, although the performance varies by region. However, even accounting for gauge adjustment, IMERG systematically underestimates precipitation in the Alps and Scandinavian mountains. Conversely, IMERG overestimates precipitation in the British Isles, Adriatic coastline, Italian Peninsula, and eastern European plains. Additionally, the research shows that gauge adjustment worsens the spatial gradient of precipitation because of the coarse resolution of Global Precipitation Climatology Centre data (GPCC).