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

Abstract

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.

The PAZ Polarimetric Radio Occultation Research Dataset for Scientific Applications

“…There are two main reasons behind this shape: (1) The first one is that the PRO observable ∆Φ is sensitive to frozen hydrometeors, as long as they are horizontally oriented (e.g. Padullés et al, 2022Padullés et al, , 2023, and these are placed above the freezing level;…”

“…The fact that ∆Φ is sensitive to frozen hydrometeors, and that these are horizontally oriented, globally, and for different types of clouds (e.g. Gong and Wu, 2017;Zeng et al, 2019;Padullés et al, 2023) implies that the ∆Φ signature should have a relationship with the tops of the clouds. Being a measurement at L-band, it is expected that sensitivity to clouds starts below the cloud top height (as inferred from the infrared, for example).…”

“…Validation of these observations has been performed extensively: Cardellach et al (2019) and Padullés et al (2020) showed the good agreement with two-dimensional precipitation products, such as the NASA's Integrated Multi-satellitE Retrievals for Global Precipitation Mission (GPM, IMERG, Huffman, 2017) and the merged product of infrared brightness temperatures (T B ) at 10.8 µm from geosatationary satellites (Janowiak et al, 2017). In addition to precipitation, sensitivity to the upper layers of convective clouds has been demonstrated (Padullés et al, 2022), and a good climatological agreement with Ice Water Content (IWC) retrievals is observed (Padullés et al, 2023). More recently, precise validation of the vertical structure of the signal has been performed using the US Next Generation Weather Radar (NEXRAD) polarimetric observations (Paz et al, 2024).…”

Evaluating the Polarimetric Radio Occultation Technique Using NEXRAD Weather Radars

“…Furthermore, it has also been shown that the PRO observable ∆Φ increases with higher precipitation rain rates, indicating also sensitivity to precipitation intensity. Moreover, subsequent studies have shown that PRO is not only sensitive to precipitation, but also to horizontally oriented frozen hydrometeors found in various vertical layers of convective clouds [8,9]. This sensitivity is particularly pronounced for snow, where aggregated ice crystals produce relatively large hydrometeor particles.…”