Sensing of Heavy Precipitation aboard the PAZ Satellite: Validation of Polarimetric Radio Occultation Precipitation Observations with GPM Constellation Products

The Spanish satellite PAZ was successfully launched on February 22, 2018 from Vandenberg Air Force Base in California. This satellite is collecting, for the first time ever, Polarimetric Radio Occultations (PRO) from space. The experiment is called Radio-Occultation and Heavy Precipitation (ROHP) and its novelty is the ability to acquire Global Navigation Satellite System (GNSS) signals at two linear orthogonal polarizations (horizontal and vertical) in an independent yet synchronous way. The phase measurement difference between the Horizontal and Vertical ports can be used to obtain precipitation information, augmenting the current capabilities of GNSS Radio Occultations.

The concept behind the precipitation measurement is similar to how Polarimetric Weather Radars work, but in a forward scattering propagation fashion (occultation geometry - i.e. limb sounding). The differential phase measurement of the horizontal with respect to the vertical port provides the integral along the ray-path of the specific differential phase shift, induced by the oblate-shaped heavy precipitating droplets. Hence, we can obtain vertical profiles of the contribution of heavy precipitation along each RO ray. In addition, the measurements at each polarization port can be recombined in order to obtain the standard thermodynamic products (vertical profiles of temperature, pressure, and water vapor) provided by standard RO. This allows us to obtain unique co-located precipitation and thermodynamic vertical profiles, with high vertical resolution, globally distributed, regardless of cloud coverage or thickness, and seamlessly over ocean and over land.

These novel PRO observations are being obtained since May 10^th 2018. As part of the calibration and validation phase, each PRO profile is checked against the GPM dual polarization radar (DPR) observations and GPM Integrated Multi-SatellitE Retrievals (IMERG) looking for spatial and temporal coincident observations. An extensive dataset of 3D precipitation, surface precipitation, and brightness temperature is being built and compared to the PRO observations. So far, these studies have shown good correlation between PRO precipitation observations and GPM precipitation retrievals.

In this presentation we are going to discuss the calibration and validation phase of the mission, the quality of the PRO precipitation observations, its limitations, the potential applications and the complementarity with other precipitation retrievals. In addition, other potential uses of this technique are going to be described, such as the potential detection of large and horizontally oriented ice crystals in deep convective precipitating structures.