Effects of the reduced air-sea drag coefficient in high winds on the rapid intensification of tropical cyclones and bimodality of the lifetime maximum intensity

The air-sea drag coefficient (Cd) is closely related to tropical cyclone (TC) intensification. Several recent studies suggested that the Cd decreases in winds greater than 33 m s−1. The effects of Cd reduction in high winds on TC intensity, especially rapid intensification (RI) and the lifetime maximum intensity (LMI) distribution, were investigated by analyzing the wind-dependent Cd-based ocean vertical mixing and the energy budget. In addition, to consider the uncertainty of the Cd in extreme winds (above 50 m s−1), three types of Cd fitting that decrease after 33 m s−1, which show different trends after 50 m s−1 (increase, flat, and decrease), were adopted. The results were then compared with those for the control fitting (saturated after 33 m s−1) and show that the reduced Cd in high winds drives an increase in net energy by reducing frictional dissipation and suppressing sea surface cooling. This extra energy prevents the TC from achieving a steady-state, causing the bimodality of simulated maximum potential intensity. The observed steady-state probability (intensification rate and RI probability) in the Cd reduced wind range were significantly lower (higher) than in the others. These results suggest that Cd reduction might potentially induce the RI and LMI bimodality.