Physics-Informed Transformer Networks for Interpretable GNSS-R Wind Speed Retrieval

Abstract

Highlights: What are the main findings? Physics-informed Transformer-GNN achieves 32% overall improvement in GNSS-R wind speed retrieval (RMSE reduced from 1.98 to 1.35 m/s) with improved performance in extreme weather conditions. Mathematical equivalence between Transformers and Graph Neural Networks enables interpretable attention mechanisms that quantify spatiotemporal physical influences in ocean-atmosphere interactions. What are the implications of the main findings? Attention weights provide physically meaningful interpretations of multi-scale atmospheric processes from local (25–100 km) to synoptic (>500 km) scales without sacrificing prediction accuracy. The framework addresses the fundamental accuracy-interpretability trade-off in operational meteorology, enabling both improved extreme weather forecasting and actionable insights for meteorologists. Global Navigation Satellite System Reflectometry (GNSS-R) provides all-weather, high-resolution ocean wind speed monitoring that offers additional benefits for forecasting tropical cyclones and severe weather events. However, existing GNSS-R wind retrieval models often lack interpretability and suffer accuracy degradation during high wind conditions. To address these limitations, we leverage a mathematical equivalence between Transformers and graph neural networks (GNNs) on complete graphs, which provides a physically grounded interpretation of self-attention as spatiotemporal influence propagation in GNSS-R data. In our model, each GNSS-R footprint is treated as a graph node whose multi-head self-attention weights quantify localized interactions across space and time. This aligns physical influence propagation with the computational efficiency of GPU-accelerated Transformers. Multi-head attention disentangles processes at multiple scales—capturing local (25–100 km), mesoscale (100 km–500 km), and synoptic (>500 km) circulation patterns. When applied to Level 1 Version 3.2 data (2023–2024) from four Asian sea regions, our Transformer–GNN achieves an overall wind speed RMSE reduction of 32% (to 1.35 m s−1 from 1.98 m s−1) and substantial gains in high-wind regimes (winds >25 m s−1: 3.2 m s−1 RMSE). The model is trained on ERA5 reanalysis 10 m equivalent-neutral wind fields, which serve as the primary reference dataset, with independent validation performed against Stepped Frequency Microwave Radiometer (SFMR) aircraft observations during tropical cyclone events and moored buoy measurements where spatiotemporally coincident data are available. Interpretability analysis with SHAP reveals condition-dependent feature attributions and suggests coupling mechanisms between ocean surface currents and wind fields. These results demonstrate that our model advances both predictive accuracy and interpretability in GNSS-R wind retrieval. With operationally viable inference performance, our framework offers a promising approach toward interpretable, physics-aware Earth system AI applications.