Toward Quasi-Global Gap-Free Ocean Wind Speed Mapping Enabled by Rapid-Sampling GNSS Reflectometry Satellites

Validation of TerraSAR-X satellite neutral radio occultation profiles across the equatorial region

Impact of TRITON GNSS-R wind speed assimilation on convective rainfall prediction: a case study of a nearshore rainfall event on April 23, 2024

The TRITON satellite, developed by the Taiwan Space Agency (TASA) and launched in October 2023, is equipped with a global navigation satellite system (GNSS) reflectometry receiver and operates in a high-inclination orbit. The ocean surface wind speed (OSW) is retrieved from GNSS signals reflected off the ocean surface. This study examines the impact of assimilating TRITON OSW in addition to radar observations on convective-scale precipitation forecasts, focusing on a nearshore heavy rainfall event on 23 April 2024 characterized by high position uncertainty. Given the track-based sampling of GNSS-R, the TRITON OSW data available for this event fortuitously cover two critical areas, including weak southwesterly flow over the Taiwan Strait and the Meiyu front, northeast of Taiwan. The TRITON OSW assimilation is conducted using a Weather Research and Forecasting (WRF)-based Radar Ensemble Data Assimilation framework on a 4-km analysis grid with a rapid update cycle of 15 min. Despite the limited temporal availability of OSW data, a positive impact on the wind analysis and very short-term forecast is identified. The assimilation of TRITON OSW data effectively corrects low-level wind, successfully complementing ground-based radar data where near-surface observations are unavailable. Consequently, the weak winds over the Taiwan Strait and offshore southwestern Taiwan, as well as the strong, front-associated winds northeast of Taiwan can be better represented. The OSW assimilation also enhances moisture in the southwestern offshore region. These adjustments yield more accurate predictions for the intensity and location of linear-type nearshore heavy rainfall systems. The improvement stems from enhanced convergence within and above the planetary boundary layer, as well as greater convective instability. Furthermore, the incremental assimilation of TRITON OSW observations with successive cycles provides the possibility of increasing the value of the TRITON OSW, given its dense distribution along a sequence of specular points. This study serves as an initial case-specific evaluation exploring the dynamic mechanisms through which TRITON OSW data can complement the radar assimilation framework and improve nearshore convective representation.

Wide-swath satellite altimetry and novel subsurface temperature observations improve predictions in a dynamic western boundary current: System optimization and performance

Evaluating Tropospheric Mapping Functions for GPS-Derived PWV in a Tropical Region: Insights from Southwestern Mexico

Global Navigation Satellite Systems (GNSS) have emerged as a reliable and cost-effective tool for estimating atmospheric precipitable water vapor (PWV), particularly in regions with limited meteorological instrumentation. In this study, the performance of three tropospheric mapping functions—the Global Mapping Function (GMF), Niell Mapping Function (NMF), and Vienna Mapping Function 1 (VMF1)—was evaluated for PWV estimation using GPS observations collected during the 2009–2011 period in southwestern Mexico, a region characterized by high atmospheric variability and frequent extreme weather events. GPS data from three stations (TECO, COL2, and PENA) were processed using the GAMIT/GLOBK 10.71 software, and the resulting PWV estimates were validated against independent radiosonde observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) Fifth-Generation Reanalysis (ERA5) data. The results show that GPS-derived PWV successfully captures the seasonal variability of atmospheric water vapor, with maximum values during the summer rainy season. High correlations were obtained with both radiosonde and ERA5 data, particularly at the TECO station (R = 0.95–0.99), where RMSE values ranged from 3.27 to 5.46 mm and BIAS values from −2.73 to −1.47 mm. In contrast, larger discrepancies were observed at COL2 and PENA, mainly due to horizontal separation and altitude differences relative to the radiosonde site, highlighting the importance of spatial representativeness during validation. Among the evaluated mapping functions, no single model consistently outperformed the others across all stations, years, and reference datasets. Nevertheless, GMF and NMF generally exhibited more stable and consistent performance, whereas VMF1 showed greater variability under the adopted processing strategy. Additionally, a clear relationship was identified between PWV and precipitation records, indicating that increases in PWV coincided with periods of intense rainfall and suggesting its potential as an indicator of atmospheric conditions favorable for precipitation events. Overall, this study shows that GPS-derived PWV can reproduce the seasonal variability of atmospheric water vapor under the adopted processing strategy and demonstrates the importance of mapping function selection and spatial representativeness for accurate PWV estimation.

Tropical Diurnal Cycle of Ocean Surface Winds From CYGNSS Observations

The diurnal cycle of ocean surface wind reflects the daily response of air–sea thermal and dynamical processes. Using 1‐hr wind speed data from the Cyclone Global Navigation Satellite System (CYGNSS, 2018–2024), this study refines the characterization of tropical ocean diurnal winds and evaluates their consistency with ERA5 reanalysis. Compared with conventional 6‐hourly satellite records, the hourly sampling of CYGNSS provides sufficient temporal coverage for harmonic fitting, yielding diurnal phase estimates with sub‐hour precision and revealing coherent nighttime wind maxima between 23:00 and 06:00 local time. Both data sets show stronger amplitudes near coasts, straits, and bays, and weaker but detectable signals over open oceans. ERA5 amplitudes are approximately 1.4 times those derived from CYGNSS, with a correlation of 0.75 and a root‐mean‐square phase difference of 1.54 hr. Notably, strong diurnal signals frequently appear in regions lacking buoy observations, highlighting the importance of spaceborne measurements. By providing independent, all‐weather, and high‐frequency observations, CYGNSS offers a unique capability for resolving global diurnal wind variability and validating model‐based reanalysis.

Sea-Ice Thickness Retrieval Using Integrated Grazing-Angle and Near-Nadir GNSS-R Reflectivity From the Spire Constellation

Accurate estimation of sea-ice thickness (SIT) is essential for understanding polar climate processes and supporting operational monitoring. This study demonstrates, for the first time, the use of grazing-angle global navigation satellite system reflectometry (GNSS-R) reflectivity from the Spire CubeSat Constellation to retrieve SIT, alongside sea-ice salinity and density. We extend previous work on near-nadir GNSS-R by integrating both near-nadir and grazing-angle observations using a three-layer ice reflectivity model and inversion framework. Weekly SIT products are generated and validated against the CryoSat-2/SMOS (CS2SMOS) reference dataset. The combined retrieval reduces the unbiased root-mean-square error (ubRMSE) to 0.540 m (0.376 m within the 0.5–1 m SIT range), compared with individual full-Arctic product ubRMSEs of 0.609–0.743 m. Within the 0.5–1 m SIT range, where individual Soil Moisture and Ocean Salinity (SMOS) and CryoSat-2 retrievals are less reliable, the combined retrieval achieves lower ubRMSE than every individual GNSS-R product, although a slight positive bias remains. Retrieved salinity and density fields, derived as by-products of the inversion, show spatial patterns consistent with known geophysical behavior. While some seasonal biases remain, especially underestimation during early and late ice growth periods, this study demonstrates the potential of Spire GNSS-R as a complementary observation source for sea-ice remote sensing, offering high revisit frequency and moderate spatial resolution that may contribute to future fused SIT records as the technique matures.