Advances in the Use of Global Navigation Satellite System Polarimetric Radio Occultation Measurements for NWP and Weather Applications (2024)

Keywords: Atmosphere; Cloud microphysics; Convective-scale processes; Thermodynamics; Numerical weather prediction/forecasting; Convective parameterization

Hybrid Meeting “2nd GNSS Polarimetric Radio Occultations User Workshop”

1. Introduction

Conventional radio occultations (ROs) track the phase delay induced by water vapor and air density gradients in Global Navigation Satellite System (GNSS) signals as they propagate through Earth’s atmosphere. Standard RO products such as bending angle profiles from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC-2) (Schreiner et al. 2020), the EUMETSAT MetOp satellites (von Engeln et al. 2011), and numerous other publicly and commercially available RO products are routinely assimilated into weather models for operational numerical weather prediction (NWP). The positive impact to forecasts of temperature and winds when RO data are included in the suite of observations ingested into data assimilation (DA) systems has been previously demonstrated (recent references include Ruston and Healy 2021; Singh et al. 2021; Cucurull 2023).

Polarimetric RO (PRO) enhances the standard RO by receiving the GNSS signals with a dual-polarization RO receiver. The PRO concept has been flight-proven via the Radio Occultations and Heavy Precipitation (ROHP) experiment, operating onboard the Spanish Paz satellite since May 2018 (Cardellach et al. 2019). In addition to the capabilities of the conventional RO, PRO can provide an indication of precipitation at each vertical level, offering a potential to advance the assimilation of satellite data in precipitating¹ conditions. Furthermore, these observations also provide unique diagnostics and observations for investigating the thermodynamic environment in and near precipitation (Neelin et al. 2023) and assumptions underlying parameterizations used by operational NWP forecast models (Murphy et al. 2019).

The hybrid format “2nd GNSS Polarimetric Radio Occultations User workshop” followed on to the first (online-only) workshop from 2020. It represented a forum for addressing multiple uses and applications that have been studied in the nearly 6 years that PRO data have been available. The objectives of the workshop were threefold: 1) To provide potential users with an understanding of the PRO measurement concept, the available data, and the geophysical content within the PRO profile; 2) to offer data providers a forum to discuss and better understand the requirements of scientific, NWP, and climate model users; and 3) to connect these communities to develop new products and further use of PRO in science and weather applications.

2. The PRO concept: Available data and geophysical content

The workshop started by providing a brief background on the development of the PRO concept. Since many readers may be unfamiliar with the measurement, a brief tutorial is provided in the sidebar.

The idea for the PRO measurement extends to 2009, using precipitation profile products from the joint NASA/JAXA Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) to simulate the expected magnitude of the Δϕ signal through realistic GNSS propagation distances and precipitation intensities (Cardellach et al. 2015) including ionospheric conditions (Tomás et al. 2018). A modified Integrated GPS Occultation Receiver (IGOR) was installed as a demonstration secondary payload, named ROHP on the Spanish Paz satellite (X-band synthetic aperture radar) (Cardellach et al. 2018). The Paz payload was deployed in February 2018, orbiting at an altitude of 514 km in a sun-synchronous polar orbit (Cardellach et al. 2019), providing ∼200 RO per day. PRO has since been examined for use on airborne platforms (Murphy et al. 2019).

This introduction to ROHP was followed by presentations and discussion on the more recent availability of PRO from commercial RO providers. These include NASA’s Commercial Satellite Data Acquisition (CSDA) Program, the NOAA Commercial Data Program, the EUMETSAT commercial radio occultation data service, and ESA’s commercial radio occultation Third Party Mission (TPM) data service. In early 2023, PRO capability was added to three spacecraft in the RO constellation operated by Spire Global, including one funded by the European Space Organization, producing over 2000 profiles per day by tracking signals from all four major GNSS constellations [GPS, Galileo, Global Navigation Satellite System (GLONASS), and BeiDou]. Future PROs are planned in mid–late 2024 for two spacecraft in development by PlanetIQ and other small companies providing weather observations and services.

The calibration and validation of the ROHP Δϕ profile were discussed. Space/time coincident Integrated Multi-satellitE Retrievals for GPM (IMERG) data (Kidd et al. 2021), averaged along the lowest level ray paths, were used (i) where IMERG indicated no precipitation, to assure that there was no bias in the Δϕ signal under known nonprecipitating conditions, and (ii) under increasing intensities of IMERG precipitation rate, to characterize the structure of the Δϕ profile (Padullés et al. 2020). Figure 1 shows good agreement between locations of the top-most height of the Δϕ profile and IMERG precipitation climatology during the Northern Hemisphere summer months, agreeing with not only tropical locations but also with the shallower convection in the Southern Oceans. The majority of the largest Δϕ (largest symbol diameter) occur in the west Pacific Ocean basin.

Given the limb-viewing geometry between the moving transmitting and receiving satellites, the contribution to Δϕ at a tangent height includes potential contributions from many upper vertical levels. This complicates the estimation of geophysical content from the Δϕ signal, requiring the use of ray-tracing models to estimate parameters such as ray-path-averaged precipitation rate. Analysis of ROHP data revealed the expected cross polarization from lower-level rain (liquid phase) hydrometeors. While most of the conventional RO bending angle can be attributed to the nearest 200 km surrounding the closest Earth tangent point, the cumulative contributions to the Δϕ can occur anywhere along the ray path where precipitation-sized hydrometeors are intercepted (Turk et al. 2021). To explain large positive Δϕ signatures extending above the freezing level, it was necessary to include K_DP contributions from ice and mixed phase, indicating sensitivity to the precipitation vertical structure from all water phases (Padullés et al. 2022a).

ROHP data have been available since May 2018. Currently, conventional level-1a/1b (excess phase/bending angle) processing is performed and distributed by UCAR (https://data.cosmic.ucar.edu/gnss-ro/paz/postProc). The polarimetric profile processing builds on top of these level-1a data, and these data are available from the ROHP-PAZ data portal (https://paz.ice.csic.es) and at the Global Environmental and Earth Science Information System (GENESIS) (https://genesis.jpl.nasa.gov).

3. Requirements of scientific and numerical forecast modeling users

Despite its natural coarse resolution along the ray propagation path, an RO measurement possesses a very high resolution in the “across-ray” (∼1 km) and in the vertical (<500 m) dimensions. This characteristic was noted in ROHP data whose propagation path intersected the edge of deep convective clouds, often encompassing widespread anvil cloud structures for many hundreds of kilometers, revealing high Δϕ signals (often exceeding 20 mm) at altitudes above the freezing level (Padullés et al. 2023).

Ongoing cluster classification analyses were described for the different types of vertical Δϕ and refractivity profiles that can occur during precipitation scenarios. Current sampling limits the analyses to mid- to high latitudes. Work on progress is trying to provide thermodynamical context behind each cluster to the vertical changes in temperature lapse rates, and the top of clouds as detected using Δϕ and the transition to the tropopause. The ability of Δϕ to detect the top of clouds appears to statistically coincide with the transition to the bottom of the tropical tropopause layer (TTL) to the tropopause.

A major challenge for numerical models for climate and weather has been parameterizing the cloud microphysical process, with several techniques currently employed in the numerous microphysical parameterization (MP) schemes used in the NWP community (Grabowski et al. 2019; Morrison et al. 2020). There are relatively few means to validate vertical microphysical processes in numerical models, particularly over open ocean (Hristova-Veleva et al. 2021), and PRO observations could be exploited for this purpose as one of the most immediate applications for numerical weather prediction. During the 2020 PRO workshop, comprehensive numerical experiments to evaluate MP schemes using PRO observations were proposed, based on a pilot study by Murphy et al. (2019). In that pilot study, two double-moment MP schemes were used in simulations of an atmospheric river (AR) event over the northeastern Pacific Ocean employing the Weather Research and Forecasting (WRF) Model. The MP schemes examined resulted in very different characteristics of simulated convection. The Morrison double-moment scheme (Morrison et al. 2009) produced deeper convection associated with a higher mass of frozen hydrometeors, compared with the WRF double-moment 6-class scheme (Lim and Hong 2010) that produced shallower convection associated with higher mass of rain below the freezing level.

Initial results from the microphysics experiment were presented at this workshop, examining tropical cyclone (TC) and additional AR case studies contributed by multiple participants. Outputs from global NWP models from the European Centre for Medium-Range Weather Forecasts (ECMWF), the Japan Meteorological Agency (JMA), and research and near-real-time area WRF Model outputs (Martin et al. 2018) were used to simulate the Δϕ vertical profile. A simplified K_DP–M forward operator (M is the specific water content) was developed for simulating the Δϕ vertical profile from the model output (Padullés et al. 2022a). These different parameterization schemes were shown to produce detectable differences in the Δϕ vertical profile, despite relatively modest convection associated with the AR event (Hotta et al. 2024). However, in other cases, particularly for the TC cases, the simulated and ROHP-observed Δϕ profile resulted in similar Δϕ values but with a notable displacement in the altitude where the peak Δϕ was noted. This offset was attributed to differences in aligning the relatively instantaneous nature of the PRO with the time-evolving weather feature of interest, which may be represented in the model at the slightly different location than in nature. One very encouraging result was the comparison of the PRO observations with model simulations using seven different microphysical parameterizations for a typhoon case. The result showed that one scheme had a better match with PRO observation than the other schemes. This shows the promise of using PRO observation to evaluate different microphysics schemes for numerical models. Obviously, considerably more work is needed to ensure statistical robustness of this finding.

Comparisons between the levels of the Δϕ profile that lies well above the freezing level (where the intercepted hydrometeors are assumed to be all frozen) were shown to be related to the ice water path (IWP) estimates provided by the CloudSat W-band (94 GHz) cloud radar (Padullés et al. 2023). The coincident thermodynamic and precipitation-sized IWP information from PRO may augment the cloud ice IWP information provided by high-frequency limb- and nadir-viewing observations that collect dual-polarization data (Gong et al. 2020). Better constraints on cloud ice may improve ways to balance top-of-atmosphere radiation and precipitation at the surface (Waliser et al. 2009).

Different methods to incorporate PRO information into NWP systems were discussed. One option would be through direct assimilation of the Δϕ profile (in either time or impact parameter domain), as well as using one of the forward operators for bending angle observations already employed for conventional RO in DA systems (Wang et al. 2022). In this case, there is a need to simulate the Δϕ profile from the model’s diagnosed and/or parameterized precipitation structure, along the same ray paths used to simulate the bending angle profile from the model state variables. Initial development of a PRO simulator built upon the existing RO processing package (ROPP) (Culverwell et al. 2015) 2D forward operator (Healy et al. 2007) was described. The Δϕ profile was simulated from two specific weather events (a tropical cyclone and a midlatitude AR event) that were nearly time coincident with one or more ROHP observations (Hotta et al. 2024). Better agreement in the shape/structure of the Δϕ profile was noted for the AR event, given the more widespread horizontal nature of precipitation that is a characteristic of baroclinic weather systems.

Since the PRO signal is not currently assimilated into models, the Δϕ profile could be used as an independent diagnostic. For example, comparison between the observed Δϕ structure and height to the location and height of the precipitation systems generated by the model microphysical parameterization could be used to identify and refine the choice of the parameterization scheme.

4. Furthering the use of PRO in science and applications

While the ROHP instrument is currently operating nominally, it provides too few PRO profiles to enable the design of meaningful observing system experiments (OSEs), such as the evaluations conducted to assess the impact of COSMIC-2 bending angles (Cucurull 2023), to demonstrate the impact of PRO assimilation on forecast skill. A minimum 3-month collection period capturing at least 4000 PRO per day was recommended. In addition to the existing PAZ and Spire observations, future PRO observations planned by PlanetIQ and others could be considered for this collection period so that impact experiments could be conducted. For example, one type of OSE could utilize the PRO signal solely to select (or reject, depending upon the OSE design) the associated bending angle data that are known to intercept precipitation. Impact performance results would then better reflect weather forecast skill due to RO sampling within precipitation-affected areas, rather than an average skill that is heavily weighted by the relatively large fraction of observations that cover nonprecipitating areas.

While the comparisons between simulated (using the simplified K_DP–M PRO forward operator discussed above) and observed PRO profiles are promising, participants agreed that further assessments are needed for the simulated PRO interactions with clouds and precipitation so that it will better replicate the observed profile. By identifying and quantifying different types of weather systems in the PRO path, the Δϕ vertical profile can be linked more accurately to cloud ice and precipitation in the atmosphere. Furthermore, these PRO measurements will provide added values to the future weather/climate missions such as MetOp SG and NASA’s planned Atmosphere Observing System (AOS).

In the satellite DA community, considerable effort is directed toward assimilation of precipitation-impacted MW radiances at higher frequencies (89 GHz and higher), typically provided by passive MW sounders such as ATMS (Kalluri 2021 and references within). At these short wavelengths, the complex morphology of frozen hydrometeor characteristics that influence ice scattering processes leads to wide variability in the simulated brightness temperatures (TBs) (Geer et al. 2021). This controls the amount by which the radiance observation can adjust the model background state. Since the K_DP–M relation is largely dependent upon the particle aspect ratio and bulk density (Padullés et al. 2022a; Turk et al. 2021), it was suggested that the Δϕ vertical profile could be used to physically constrain the selection of the ice crystal types into the forward MW radiance operator. Joint simulations of passive MW and PRO will better quantify the information about hydrometer content in the vertical and the relative contribution from the different hydrometeor species.

Outside of short/medium-range weather model applications, the topic turned to the use of PRO for process-oriented diagnostic observations, to evaluate convection in global climate models (GCMs) (Neelin et al. 2023). Near-surface observations under combined heavy precipitation and high-water vapor conditions are challenging to obtain from conventional satellite observations, owing to factors such as radar beam attenuation. Previous investigations (Padullés et al. 2022b) used IMERG and COSMIC RO data to demonstrate the different precipitation versus water vapor relationships that arise when conditioning these same data on different pressure level layers in the lower free troposphere (LFT), thereby evaluating sensitivity of convection to the moisture structure. The combination of PRO and other satellite data (e.g., wide swath passive microwave MW radiometers, which can provide large area coverage) was discussed to evaluate the impact of stability and subsaturation of the surrounding air on the intensity and depth of convection that develops (Emmenegger et al. 2022, 2024).

Current RO observational strategies are designed to equalize global sampling patterns across local observing times. A constellation observing system (e.g., 2-min separation) was described, whereby each receiver senses the same GNSS transmitting satellite. Such a constellation could capture a closely spaced (e.g., within 100 km) sequence of PRO profiles along the same azimuth direction, thereby sampling independent airmass quantities in and near the thermodynamic environment surrounding convection (Turk et al. 2022). A further study could refine this observing strategy for operational NWP applications, for example, the proper mixture of PRO vs conventional RO and data orbit schemes that would enable some fraction of RO in close proximity to one another.

Since the fundamental PRO measurement is based upon a stable differential time delay, the magnitude of the Δϕ signal for the ray paths nearest the surface was suggested to provide an independent reference to evaluate global satellite precipitation products (such as IMERG) on a precipitation threshold basis (Kirstetter et al. 2015). This would be valuable over open oceans where independent validation data from radars or rain gauges are practically nonexistent (Gorooh et al. 2023).

Adapting existing RO instrumentation to PRO capability requires minor receiver modifications to receive the two orthogonal linearly polarized signals. During the workshop, the RO products produced from the Spire PRO demonstration satellites were shown to produce standard RO products without any degradation induced by the dual-polarization processing. This promising finding suggests that PRO should be considered as the default configuration for future RO systems.

Last, it was noted that future constellations of small satellites capable of both reflectometry (GNSS-R) and PRO measurements could be part of an affordable system for observing certain interconnected components of the global hydrological cycle. For example, availability of collocated measurements of water vapor (a first-order determinant of evaporation from saturated surfaces), precipitation, and soil moisture and comparing those wetlands and inundation extent would help in understanding wetland hydrologic dynamics (Chew et al. 2023).

The participants discussed the need to develop a roadmap document to capture the current PRO challenges and the steps to overcome them. The goal is to guide the research and operational communities as well as the funding agencies toward full exploitation of the GNSS PRO concept [International Radio Occultation Working Group (IROWG) 2023]. Further information on this roadmap will be updated on the PAZ site (https://paz.ice.csic.es).