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,

Tropical Cyclone,Horizontal Turbulent Diffusion,Radius of Maximum Wind