Evaluating Polarimetric Radio Occultations for constraining precipitation microphysics in NWP

Accuretaly representing the microphysical structure of precipitating systems remains a major challenge in Numerical Weather Prediction (NWP). Cloud and precipitation processes occur at spatial and temporal scales that are not explicitly resolved by current models and must therefore be described through simplified microphysical parameterizations. These parameterizations have a strong impact on key model outputs, such as precipitation intensity, and their improvement requires observations that are sensitive to the vertical structure and microphysical properties of hydrometeors.

Polarimetric Radio Occultations (PRO) provide a complementary observational capability to address this gap. As with standard GNSS Radio Occultations, PRO delivers high vertical resolution thermodynamic profiles under all-weather conditions. In addition, PRO is sensitive to the presence and vertical distribution of hydrometeors through the differential phase shift (ΔΦ), defined as the phase difference between horizontally and vertically polarized GNSS signals. When these signals propagate through non-spherical and/or oriented hydrometeors, differential propagation effects arise, leading to a positive differential phase shift. As a result, PRO measurements offer direct sensitivity to the microphysical structure of precipitating systems.

The accuracy of simulated PRO observables depends on the formulation of the forward operator, particularly on how the relationship between differential phase shift and hydrometeor water content is represented. Recent work has proposed a new forward operator based on a linear relationship that enables the inclusion of scattering-related information as a function of hydrometeor type. In this study, we evaluate the performance of this updated PRO forward operator under Atmospheric River (AR) conditions, which are characterized by intense moisture transport and strong precipitation. Simulations are compared with those produced using the offline forward operator currently implemented at ECMWF to assess whether the new formulation could serve as a viable replacement for operational applications.

Beyond this potential operational impact, the new forward operator enables the use of PRO as a constraint on cloud and precipitation microphysics. By exploiting the Atmospheric Radiative Transfer Simulator (ARTS) scattering database, we analyze the sensitivity of PRO observables to the scattering properties of different particle habits, providing insight into the extent to whether PRO measurements can discriminate between microphysical assumptions and improve the representation of precipitating systems in NWP models.