Forward operator for polarimetric radio occultation measurements

Abstract

Global Navigation Satellite System (GNSS) polarimetric radio occultation (PRO) observations sense the presence of hydrometeor particles along the ray path by measuring the difference of excess phases in horizontally and vertically polarised carrier waves. As a first step towards using these observations in data assimilation and model diagnostics, a forward operator for the GNSS-PRO observable φDP (polarimetric differential phase shift) has been implemented by extending the existing two-dimensional forward operator for radio occultation bending-angle observations. Evaluation of heavy-precipitation cases showed that the implemented forward operator can simulate the observed φDP in synoptic-scale atmospheric river (AR) cases very accurately. For tropical cyclone cases it is more challenging to produce reasonable φDP simulations, due to the high sensitivity of φDP with respect to displacement of the position of the tropical cyclones. It was also found that snow is the dominant contributor to the simulated φDP and that the ability to compute the ray paths in two dimensions is essential to accurately simulate φDP.