Global-Scale Diagnoses of Tropical Cyclone Maximum Wave Heights from 5 Km GPM Retrievals and Their Implications for Reanalysis Bias

Tropical cyclone (TC) inner-core maximum wave height (MWH) measurements are virtually absent, forcing reliance on reanalyses such as ERA5. Using a GPM-retrieved dynamic MWH product on a 5 km pixel grid, 165 global TC–GPM overpass cases of tropical-storm intensity and above, drawn from 143 unique tropical cyclones (maximum sustained wind ≥17 m s−1; the weakest sampled storm peaked at 20 m s−1), are analyzed to evaluate ERA5 MWH in both magnitude and location. The inner-core MWH/significant wave height (SWH) ratio (~2.0 vs. ERA5’s ~1.9) declines steeply to ~1.78 at 250–300 km—a decline ERA5 entirely misses. The underestimation Δ reaches 5.4 m for super typhoons (median 0.44 m). ERA5 also misplaces the MWH point: GPM places it 1.3 km inside the radius of maximum wind (median) versus 12.5 km outward for ERA5—a 13.8 km outward bias (p < 0.0001). ERA5 shows an artificially narrow azimuthal spread (circular variance 0.376 vs. GPM’s 0.538; cluster-bootstrap p = 0.0005). Physical consistency (steeper gradient, rain-rate collocation) supports GPM MWH realism; because the retrieval uses ERA5 SWH as a background, the comparison is complementary rather than fully independent. The bias is about half of ERA5’s output grid spacing (~28 km), indicating a resolution-related origin.