Analysis of Polarimetric Radio Occultation Data from ROHP after Six Years in Orbit and their Use for NWP

Conventional radio occultations (RO) are gathered by a single circularly-polarized receiving antenna, tracking the GNSS phase delay induced across the propagation path. Polarimetric RO (PRO) enhances standard RO by receiving the GNSS signals in two orthogonal linear (H/V) polarizations. The induced cross polarization ΔΦ represents a cumulative effect arising from the presence of aspherical hydrometeors along each ray path, augmenting the conventional thermodynamic (temperature, moisture, and pressure) profile. Data from the Radio Occultations and Heavy Precipitation (ROHP) experiment, orbiting onboard the Spanish PAZ satellite since May 2018 and complemented with recent commercial PRO, have revealed the expected depolarization from liquid phase precipitation hydrometeors. These same data also reveal positive ΔΦ signatures extending into and well above the freezing level, indicating possible sensitivity to the full top-to-bottom precipitation vertical structure along the identical air mass sampled for the conventional RO profile.

Currently, only conventional PRO products (e.g., bending angle) are assimilated into Numerical Weather Prediction (NWP) forecast models. Methods to simulate the polarimetric RO profile are being developed by operational weather agencies, for eventual implementation into a data assimilation system. While the comparisons between PRO forward operator simulations and observed PRO profiles is promising, further assessment is needed to identify which types of weather systems one could expect a future PRO assimilation to yield the most impact.

However, the fact that the ΔΦ signal is not currently assimilated allows its use as an independent diagnostic. This knowledge of the presence of heavy precipitation and its associated vertical extent is useful for the evaluation and diagnosis of precipitation as revealed by forecast, climate and reanalysis data products. Furthermore, the polarimetric profile can be independently compared to the precipitation structure as revealed by near-coincident complementary satellite data, such as the large suite of passive microwave (MW) data (e.g., ATMS, GPM).

This presentation will highlight selected science topics that emerged at the recent (November 2023) 2^nd Polarimetric Radio Occultations User Workshop. For example, polarimetric RO data will be highlighted as one means to evaluate the different precipitation vs water vapor relationships that arise when conditioning these same data on different pressure level layers in the lower free troposphere (LFT), thereby evaluating sensitivity of convection to the moisture structure and the entrainment of environmental air. Cluster classification analyses will be described for different types of vertical ΔΦ and refractivity profiles that can occur during precipitation scenarios.