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-based dynamic MWH product at 5 km resolution, 165 global TCs (maximum wind ≥ 25 m s⁻¹) are analyzed to evaluate ERA5 MWH in both magnitude and location. GPM reveals an inner-core MWH/significant wave height (SWH) ratio of ~2.0, higher than ERA5’s ~1.9, and—critically—exhibits a steep radial decline that ERA5 entirely misses. The absolute underestimation Δ = GPM_hmax − ERA5_hmax reaches 5.4 m for super typhoons (median 0.44 m). ERA5 also misplaces the MWH point: the GPM MWH point lies 1.3 km inside the radius of maximum wind (median), whereas ERA5 places it 12.5 km outward — a 13.8 km systematic outward bias (p < 0.0001). ERA5 shows an artificially narrow azimuthal spread (circular variance 0.499 vs. GPM’s 0.647, bootstrap p = 0.0005) and a flatter radial gradient. Physical consistency (steeper gradient, rain-rate collocation) supports GPM MWH realism. The bias magnitude is comparable to ERA5’s effective resolution (~28 km), indicating a resolution-related origin.