Comment on “On the Physics of the New Time-Dependent Theory of Tropical Cyclone Intensification” by Smith and Montgomery (2025)

Smith and Montgomery (2025, hereafter SM25) questioned the physical basis of the time-dependent theory of tropical cyclone (TC) intensification proposed by Wang et al. (2021, hereafter W21). Their main criticisms concern the locality of the closure parameter A′, the lack of inflow above the boundary layer (BL), and the use of an empirically calibrated parameter in model validation. Here we clarify these issues and restate the physical framework of W21. The theory applies local thermodynamic quasi-equilibrium (TQE) at the contracting radius of maximum wind (RMW). Eyewall heating, which is represented implicitly rather than prognosed explicitly, induces weak inflow in the mid-lower troposphere, leading to inward RMW contraction as a process of the gradient wind adjustment, a balanced response consistent with classical vortex spin-up mechanism. The apparent inward migration of absolute angular-momentum ( M ) surfaces can equivalently be viewed in an Eulerian tendency sense, where the vertical-advection term −ω ∂M/∂p dominates, reconciling the interpretation with the balanced dynamics framework. The parameter A′ represents the degree of non-congruence between M and saturated-moist-entropy (s*) surfaces at the top of the boundary layer and can be interpreted physically as a “ventilation” parameter in the eyewall. Its empirically determined intensity dependence, confirmed using extended full-physics axisymmetric model ensemble simulations (Li et al. 2024) and best-track data (Xu and Wang 2022), captures consistent intensification behavior across independent datasets. Moreover, a recent extension of the theory (under review) eliminates A′ entirely by explicitly resolving the vortex structure. With these clarifications, the W21 framework remains a compact yet physically consistent model that reconciles thermodynamic and dynamic perspectives of TC intensification.