On the global relationship between polarimetric radio occultation differential phase shift and ice water content

Abstract

The Radio Occultations and Heavy Precipitation (ROHP) experiment aboard the Spanish PAZ satellite was deployed in 2018 with the objective of demonstrating the ability of the polarimetric radio occultation measurement (PRO) concept in detecting rain (liquid-phase precipitation). Analysis of these data has also demonstrated the ability of PRO to detect horizontally oriented frozen-phase precipitation. To verify these observations, a global climatological comparison is performed using the CloudSat (94 GHz) radar as a reference for different heights and taking into account the radio occultation (limb-based) viewing geometry. A robust relationship (e.g., high correlation coefficient) is found between the polarimetric radio occultation observable differential phase shift (δφ) and the integrated ice water content (IWC) along the rays, globally, and for heights above freezing level. PAZ observations capture the major precipitation features and the signatures of the Intertropical Convergence Zone. The correlation coefficients maximize for tropical oceans, for a range of heights between 5 and 12 km. The ratios between δφ and along-ray integrated IWC mean that climatologies are found to be within the range 0.03-0.09 mm kg-1 m2, although uncertainty due to the IWC retrieval algorithm must be noted. Further evaluation of the ratios with single-particle forward-scattering simulations indicate that the scattering by frozen hydrometeors with a combination of effective densities, axis ratios, and tilt angle distribution yield results that agree well with the observations. Overall, the global climatological results obtained in this study show the presence of horizontally oriented particles across the whole globe and for a wide range of height layers.