Definition and impact of a quality index for radar-based reference measurements in the H-SAF precipitation product validation

Abstract

The EUMETSAT Satellite Application Facility on Support to Operational Hydrology and Water Management (H-SAF) provides rainfall estimations based on infrared and microwave satellite sensors on board polar and geostationary satellites. The validation of these satellite estimations is performed by the H-SAF Precipitation Product Validation Group (PPVG). A common validation methodology has been defined inside the PPVG in order to make validation results from several institutes comparable and understandable. The validation of the PR-OBS-3 (blended infrared–microwave (IR–MW) instantaneous rainfall estimation) product using radar-based rainfall estimations as ground reference is described herein. A network of C-band and Ka-band radars throughout Europe ensures a wide area coverage with different orographic configurations and climatological regimes, but the definition of a quality control protocol for obtaining consistent ground precipitation fields across several countries is required. Among the hydro-meteorological community, the evaluation of the data quality is a quite consolidated practice, even though a unique definition of a common evaluation methodology between different countries and institutions has not been set up yet. Inside H-SAF, the first definition of the quality index of the radar rainfall observations has been introduced at the Italian Civil Protection Department (DPC). In the evaluation of the DPC quality index, several parameters are considered, some measured by the radar itself (static clutter map, range distance, radial velocity, texture of differential reflectivity, texture of co-polar correlation coefficient and texture of differential phase shift) and some obtained by external sources (digital elevation model, freezing layer height). In some cases, corrections were applied for clutter and beam blocking. The DPC quality index was calculated and applied to some relevant meteorological events reported by a radar test site in Italy. The precipitation field derived by radar data was compared with the PR-OBS-3 precipitation product, with varying thresholds of quality index: the impact of the introduction of the quality index defined on the statistical results of the satellite product validation as well as their sensitivity to the threshold choice were thus evaluated. Results show that PR-RMSE (a relative RMSE here introduced) is reduced from values between 2.5 and 3 to values around 1 when the quality threshold is increased from 0 (no threshold) to 0.8. Fractional standard error also decreases, from values around 2 to values around 1.5 in the same span of the quality threshold.