Understanding the Polarimetric Radio Occultation observable differential phase shift with the help of the NEXRAD polarimetric weather radars

Lack of knowledge about the physical processes controlling heavy precipitation arises from the limited number of simultaneous observations of the vertical structure of precipitation and its thermodynamic environment. These limitations are caused by the degradation that the signals of some spaced-based sensors suffer in presence of thick clouds or the lack of high vertical resolution thermodynamic measurements.
To overcome this problem, the ROHP (Radio Occultation and Heavy Precipitation) experiment provides high-quality thermodynamic profiles (temperature, pressure, water vapor pressure, etc.) and vertical information of the hydrometeors, simultaneously. This proof-of-concept experiment led by the Institut de Ciències de l’Espai (ICE-CSIC, IEEC) in collaboration with NOAA, UCAR, and NASA/Jet Propulsion Laboratory, is carried out aboard the Spanish low earth orbiter (LEO) PAZ. Its objective is to test the new Polarimetric Radio Occultation (PRO) concept, and it has been operating since 2018. The standard radio occultation technique consists of tracking the signals emitted by a Global Navigation Satellite System (GNSS) satellite from a LEO satellite that is rising or occulting behind the Earth’s limb. The novelty that PRO offers is that GNSS signals are collected using two different linearly polarized antennae (horizontal and vertical) as opposed to the standard technique, where GNSS signals are acquired using a circularly polarized antenna. Consequently, we can obtain an observable called the differential phase shift, defined as the difference in the accumulated phase delay between both polarizations (H-V). Since the hydrometeors surrounding heavy precipitation events stand out for being oblate spheroid-like, we will have an associated accumulated phase shift if rays are crossing heavy precipitation.
For the sake of continuing with the validation of the PRO technique, we make use of the polarimetric weather data provided by the Next Generation Weather Radars (NEXRAD). NEXRAD is the network of dual-polarized Doppler radars operating at the S-band, that covers all the United States territory. By comparing the differential phase shift obtained with PAZ and the observables from NEXRAD, we can analyze the polarimetry of both systems. In this study, we focus on the vertical structures of NEXRAD-provided specific differential phase shift (Kdp) that can be compared to the PAZ observable accounting for some geometry and frequency factors. This comparison will help us to better understand the PAZ observables, and ultimately to better understand the microphysics underlying heavy precipitation events.