The Intensity-dependence of Tropical Cyclone Intensification Rate in Observations and in a Simple Energetic Model

In this study, the simple energetic model is modified to understand the observed intensity-dependence of the tropical cyclone (TC) intensification rate (IR) on storm intensity, with an IR peak occurring when the maximum sustained surface wind speed of the TC is around 35-40 m s-1. According to the modified conceptual model, the TC IR is controlled by the intensification potential (IP) and the weakening rate due to surface friction under the eyewall. The IP is determined primarily by the potential energy available for a TC to develop, which is a function of the thermodynamics of the atmosphere and the underlying ocean, and the dynamical efficiency of the TC system, which depends strongly on the degree of convective organization in the eyewall and the inner-core inertial stability of the storm. At relatively low TC intensity, the IP of the storm is often larger than the weakening rate due to surface friction, leading to an increase in the TC IR with TC intensity. As the storm reaches an intermediate intensity of 35-40 m s-1, the difference between the IP and weakening rate due to surface friction reaches the maximum, corresponding with the maximum IR. Later on, the IR decreases as the TC further intensifies because surface friction increases with TC intensity at a faster rate than the IP. Finally, the storm approaches its maximum potential intensity (MPI) when IR becomes zero. The modified conceptual model is further validated with results from idealized simulations with a nonhydrostatic, cloud-resolving TC model.