Probability of intense precipitation from polarimetric GNSS radio occultation observations

Abstract

There is currently a gap in satellite observations of the moisture structure during heavy precipitation conditions, since infrared and microwave sounders cannot sense water-vapour structure near the surface in the presence of intense precipitation. Conversely, Global Navigation Satellite System (GNSS) radio occultations (RO) can profile the moisture structure with high precision and vertical resolution, but cannot indicate the presence of precipitation directly. Polarimetric RO (PRO) measurements have been proposed as a method to characterize heavy rain in GNSS RO, by measuring the polarimetric differential phase delay induced by large size hydrometeors. Previous studies have shown that the PRO polarimetric phase shift is sensitive to the path-integrated rain rate under intense precipitation scenarios, but there is no current method to invert PRO measurements into quantitative estimates of the path-averaged rain rate. In this manuscript, a probabilistic inversion approach to the GNSS PRO observables is proposed, where the GPM precipitation products are used for the construction of an a priori look-up table (LUT) database. The performance of the LUTs is assessed for use in the inversion of satellite-based GNSS PRO observations, based on synthetically generated PRO data of actual events, which correspond to co-locations between GNSS RO profiles and the TRMM observations. The synthetic data include end-to-end propagation effects of the polarimetric observables and a simple separation algorithm to isolate the hydrometeor component of the observation. The assessment results in agreement better than ±1 mm h −1 between the reference LUT and the actual rain statistics of the synthetic data, proving the suitability of the GPMbased probabilistic inversion tool. These findings indicate that the GNSS PRO products are capable of extending the current GNSS RO ones by associating indications of rainrate probabilities at different altitudes, at ∼250 m vertical resolution and under intense precipitation scenarios with the standard vertical thermodynamic profiles.

Calibration and validation of the Polarimetric Radio Occultation and Heavy Precipitation experiment aboard the PAZ satellite

“…3. Using the strategy described above we can reduce the cases in which the ray does not cross precipitation, but it would be labeled as rainy in a 2 • circular colocation and reduce the uncertainty in the colocations that might have appeared in Cardellach et al (2019).…”

“…Once the receiving system has been calibrated and the data accordingly corrected, these new observables are validated using the GPM products mentioned above. The results of the validation are compared with what was obtained in Cardellach et al (2019) and also with the predicted performance from the simulations in Cardellach et al (2014).…”

Assessment of global navigation satellite system (GNSS) radio occultation refractivity under heavy precipitation

“…These models are known to have issues with the parameterization of convective processes; hence, further investigation in this direction is required. This is the aim of polarimetric radio occultations, which will provide joint products of temperature, pressure, and moisture and an indication of the amount of precipitation (mostly sensitive to the heaviest) at each vertical level (Cardellach et al, 2017) with the objective of advancing the understanding of heavy precipitation events, closely linked with high specific-humidities conditions. Alternatively, further investigations are being conducted with the aim to make the RO retrievals less dependent on models, which would improve the retrievals itself and provide more independent information on such scenarios.…”

“…Usually, these a priori values are obtained from global meteorological analyses or reanalyses. On the other hand, bending angle and refractivity profiles are directly assimilated into NWP (e.g., Healy et al, 2005;Cucurull et al, 2007), with a high positive impact in the weather forecasts (Cardinali and Healy, 2014). …”

“…The RO technique offers unique potential to study the interaction between heavy precipitation and vertical thermodynamic processes within the atmosphere, since RO signals can penetrate into thick clouds and RO products have high vertical resolution. Recent investigations by Cardellach et al ( , 2017 and Padullés et al (2016) have shown potential to retrieve vertical precipitation information adapting RO receivers to collect polarimetric observables (Pol-ROs). Therefore, PolROs emerge as a technique that could provide relevant simultaneous information of precipitation and thermodynamics (e.g., moisture) to advance the understanding of the processes linking the vertical structure of moisture and heavy precipitation.…”