Assessment and application of melting-layer simulations for spaceborne radars within the RTTOV-SCATT v13.1 model

Abstract

Because of their high sensitivity to hydrometeors and high vertical resolutions, spaceborne radar observations are emerging as an undeniable asset for numerical weather prediction (NWP) applications. The EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) NWP SAF (Satellite Application Facility for Numerical Weather Prediction) released an active sensor module within version 13 of the RTTOV (Radiative Transfer for TIROS Operational Vertical Sounder) software with the goal of simulating both active and passive microwave instruments within a single framework using the same radiative transfer assumptions. This study provides an in-depth description of this radar simulator. In addition, this study proposes a revised version of the existing melting-layer parameterization scheme of Bauer (2001) within the RTTOV-SCATT v13.1 model to provide a better fit to observations below the freezing level. Simulations are performed with the revised and default schemes for the Dual-frequency Precipitation Radar (DPR) instrument on board the Global Precipitation Measurement (GPM) mission using the ARPEGE (Action de Recherche Petite Echelle Grande Echelle) global NWP model, operational at Météo-France for two different 1-month periods (June 2020 and January 2021). Results for a case study over the Atlantic Ocean show that the revised melting scheme produces more realistic simulations much closer to observations compared to the default scheme both at Ku (13.5 GHz) and Ka (35.5 GHz) frequencies. A statistical assessment covering several cases shows significant improvement of the first-guess departure statistics. As a step further, this study showcases the use of melting-layer simulations for the classification of precipitation (stratiform, convective, and transition) using the dual-frequency ratio (DFR) algorithm. The classification results reveal a significant overestimation of the rain reflectivities in both hemispheres, which could indicate either an overproduction of convective precipitation in ARPEGE or a misrepresentation of the convective precipitation fraction within the forward operator.