The maximum intensity of tropical cyclones in axisymmetric numerical model simulations

An axisymmetric numerical model is used to evaluate the maximum possible intensity of tropical cyclones. As compared with traditionally formulated nonhydrostatic models, this new model has improved mass and energy conservation in saturated conditions. In comparison with the axisymmetric model developed by Rotunno and Emanuel, the new model produces weaker cyclones (by ∼10%, in terms of maximum azimuthal velocity); the difference is attributable to several approximations in the Rotunno-Emanuel model. Then, using a single specification for initial conditions (with a sea surface temperature of 26°C), the authors conduct model sensitivity tests to determine the sensitivity of maximum azimuthal velocity (V_max) to uncertain aspects of the modeling system. For fixed mixing lengths in the turbulence parameterization, a converged value of V_max is achieved for radial grid spacing of order 1 km and vertical grid spacing of order 250 m. The fall velocity of condensate (V_t) changes V_max by up to 60%, and the largest V_max occurs for pseudoadiabatic thermodynamics (i.e., for V_t > 10 ms^-1). The sensitivity of V_max to the ratio of surface exchange coefficients for entropy and momentum (C_E/C_D) matches the theoretical result, V_max - (C_E/C_D)^1/2, for nearly inviscid flow, but simulations with increasing turbulence intensity show less dependence on C_E/C_D; this result suggests that the effect of C_E/C_D is less important than has been argued previously. The authors find that V_max is most sensitive to the intensity of turbulence in the radial direction. However, some settings, such as inviscid flow, yield clearly unnatural structures; for example, V_max exceeds 110 m s^-1, despite a maximum observed intensity of ∼70 m s^-1 for this environment. The authors show that turbulence in the radial direction limits maximum axisymmetric intensity by weakening the radial gradients of angular momentum (which prevents environmental air from being drawn to small radius) and of entropy (which is consistent with weaker intensity by consideration of thermal wind balance). It is also argued that future studies should consider parameterized turbulence as an important factor in simulated tropical cyclone intensity.