2026/06/08 by Yu Liu, Y Q Liu, Danyang Wang +2
Earth and Planetary Sciences · Environmental Science · #Tropical and Extratropical Cyclones Research #Meteorological Phenomena and Simulations #Wind and Air Flow Studies
paper · doi:10.1175/jas-d-25-0160.1
Abstract The radius of maximum wind (RMW) of a tropical cyclone (TC) plays a critical role in shaping the wind field and associated hazards. Observations report RMW values as small as 5 km, yet the existence and magnitude of a physical lower bound remain unclear. Using an axisymmetric boundary layer diagnostic model, we examine how small the RMW can become before the inner core becomes unstable and undergoes rapid expansion. A nondimensional parameter defined as the horizontal mixing length normalized by the RMW, , emerges as a key control in the model. We identify a critical threshold of , above which horizontal turbulent diffusion controls the boundary layer outflow structure and induces an abrupt outward shift of the boundary layer ascent center. This ascent shift acts as a robust indicator of structural instability and imposes a strong constraint on the minimum achievable RMW. For a typical horizontal mixing length of l h = 1000 m, the implied lower bound of the RMW is approximately 5 km. The ascent shift resembles eyewall replacement cycles and arises from the horizontal-turbulent-diffusion-induced modification of the supergradient wind distribution. These results highlight a potentially underappreciated pathway for inner-core reorganization in compact TCs and suggest that horizontal turbulent diffusion contributes to the self-regulation of the TC inner-core structure. Significance Statement The radius of maximum wind (RMW) sets the eyewall location and the extent of damaging winds and rainfall in tropical cyclones. Although very small RMWs have been observed, whether a physical lower limit exists remains unclear. Using a boundary layer diagnostic model, we show that horizontal turbulent diffusion imposes a lower bound on the RMW of about 5 km, thereby preventing storms from sustaining unrealistically compact eyewalls and highlighting its role as a key regulator of tropical cyclone structure.