Inverting Tropical Cyclone Eye Geometry via the Deep‐Ocean Acoustic Notch

Abstract

The radius of maximum wind is a key descriptor of tropical‐cyclone structure, but it remains difficult to observe continuously over the open ocean. This study examines whether can be inferred from deep‐ocean low‐frequency ambient noise generated by storm‐forced surface processes. The central observable is an acoustic notch, a local minimum in vertical‐array response associated with the weaker source region of the storm eye relative to the surrounding eyewall. A forward framework couples a parametric vortex wind model, wind‐ and rain‐related surface‐noise parameterizations, and parabolic‐equation acoustic propagation. Idealized simulations show that notch distance is strongly related to storm scale, yielding a near‐linear calibration with an RMSE of 1.2 km. Evaluation against ERA5‐derived radial wind slices from three typhoons confirms that the notch retains meaningful structural sensitivity under realistic forcing, yielding a correlation of , though with expectedly larger scatter (RMSE of 27.3 km). Sensitivity tests indicate that frequencies near 50 Hz provide a practical balance of propagation efficiency and robustness to idealized rain perturbation. The retrieval remains interpretable over practical array apertures and moderate geometric mismatch, but is sensitive to array depth and storm‐induced sound‐speed perturbations. These results support the acoustic notch as a physically interpretable low‐frequency proxy for tropical‐cyclone inner‐core scale in favorable deep‐ocean environments, rather than as a standalone operational retrieval system.