The Role of Boundary Layer Dynamics in Tropical Cyclone Intensification - Part II: Results from the Full-physics Model TCM4

In part I, results from a simplified framework, which can help isolate and quantify the thermodynamic effect of boundary layer dynamics in tropical cyclone (TC) intensification, are presented. In this part II, results from full-physics simulations using the TC model TCM4 are discussed with the focus on the relative importance and the combined dynamical and thermodynamic effects of boundary layer dynamics due to the presence of surface friction. Our results demonstrate that larger surface friction can enhance the radial inflow and stronger upward motion (Ekman pumping), leading to faster moistening of the inner core, larger and more inward penetrated eyewall updrafts and thus diabatic heating inside the radius of maximum wind (RMW). This largely shortens the initial spin-up of the TC vortex and earlier establishment of the rapid intensification (RI). However, larger surface friction has little effect on the subsequent intensification rate of the simulated storm, suggesting that the positive contribution of the indirect thermodynamic effect by surface friction is largely offset by the negative dynamical effect due to surface frictional dissipation. The results thus strongly suggest that boundary layer dynamics contributes significantly to the onset of RI but has little effect of the subsequent intensification rate. Our further results show that the storm with slower radial decay rate of tangential wind has higher intensification rate due to stronger inward penetration of boundary layer inflow and thus more inward eyewall updrafts and eyewall heating. As a result, the initial vortex structure is the key to the intensification rate because it can modify the strength and inward penetration of boundary layer inflow and thus the radial location of diabatic heating relative to the RMW in the eyewall.