Use of a GNSS-PRO forward operator for the evaluation of global and km-scale NWP models

Abstract. A new formulation for a Polarimetric Radio Occultation (PRO) forward operator was recently developed and evaluated with the Weather Research and Forecasting (WRF) model and the European Center for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS), representing a promising step toward the assimilation of PRO observations in numerical weather prediction (NWP) systems and a novel tool for evaluating frozen-hydrometeor microphysical parameterizations. In this study, the operator is applied for the first time across two operational Météo-France systems: the convection-permitting Applications of Research to Operations at Mesoscale (AROME) model and its overseas configuration, AROME-OM, and the global Action de Recherche Petite Échelle Grande Échelle (ARPEGE) model. Twenty-seven PRO occultations, each co-located with simulations from both AROME/AROME-OM and ARPEGE, are analyzed, spanning metropolitan France and the French overseas territories (Antilles, Southwest Indian Ocean, French Polynesia and New Caledonia) across both mid-latitude convective and stratiform precipitation, and tropical convective regimes. The PRO observations are first cross-checked against the ARAMIS ground-based radar observations over metropolitan France, providing an observation-only reference independent of either forward operator. For all AROME cases, the PRO operator is then systematically compared with the polarimetric radar forward operator (operadar) driven by the same AROME fields. The PRO operator globally achieves a better agreement with the observations than operadar across all regions, mainly because of its optimized nature, although a remarkably good agreement is found between the two operators and the observations over the metropolitan France domain. The PRO operator further serves as a diagnostic of how each NWP model partitions frozen water content among its prognostic species, and help identify the species that are the most difficult to accurately simulate with operadar (snow and wet graupel). These findings establish the PRO operator as a versatile multi-model diagnostic for frozen-hydrometeor microphysics and underline the operational potential of PRO observations as a complement to ground-based polarimetric radar networks, especially over seas.