Calibration, Validation, and Science Results from PAZ Polarimetric Radio Occultations

The Radio Occultations and Heavy Precipitation (ROHP) experiment on board the Spanish satellite PAZ was launched in February 2018 to demonstrate the concept of polarimetric radio occultation (PRO) in detecting heavy precipitation. Soon after its activation, it became clear that the signals exhibited signatures of precipitation structures, therefore confirming the hypothesis and its main science goal. Since high vertical-resolution tropospheric water vapor profiles can be retrieved simultaneously through clouds and precipitation, the PAZ data offers a unique view of water vapor in extreme weather conditions that pose significant difficulties to passive infrared and microwave sounder retrievals.

The PRO technique consists in measuring the phase difference between the horizontal (H) and vertical (V) components of the electromagnetic field coming from GPS satellites in an occulting geometry. H and V components are measured independently, yet synchronously, with a dual linear polarized antenna placed on a Low Earth Orbiter and pointing towards the limb of the Earth in the satellite’s anti-velocity direction. In the presence of asymmetric hydrometeors (flattened during their fall by air drag), the phase delay in the H component is larger than for the V component. PAZ data confirm that this phase delay difference can be measured.

In this talk, I will describe the calibration and validation of the PAZ PRO data. Unlike standard RO processing, PRO measurements must be carefully calibrated to remove non-hydrometeor contributions (e.g., antenna cross-polarization, ionospheric effects) to the H-V phase difference observable. Validation of the PAZ polarimetric phase difference relies on global precipitation products such as IMERG and collocated GPM precipitation radar measurements. Forward simulations of PAZ observables in convective environments based on TRMM/GPM and CloudSat retrievals show that PAZ measurements are sensitive not only to heavy precipitation but also to frozen hydrometeors in the tropical mid-troposphere. We will discuss implications of these results and our ongoing retrieval efforts.