AUTHOR=Kim Sung-Hun , Kang Hyoun-Woo , Moon Il-Ju , Kang Sok Kuh , Chu Pao-Shin 

TITLE=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

JOURNAL=Frontiers in Marine Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.1032888

DOI=10.3389/fmars.2022.1032888

ISSN=2296-7745

ABSTRACT=<p>The air-sea drag coefficient (<italic>C<sub>d</sub></italic>) is closely related to tropical cyclone (TC) intensification. Several recent studies suggested that the <italic>C<sub>d</sub></italic> decreases in winds greater than 33 m s<sup>−1</sup>. The effects of <italic>C<sub>d</sub></italic> 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 <italic>C<sub>d</sub></italic>-based ocean vertical mixing and the energy budget. In addition, to consider the uncertainty of the Cd in extreme winds (above 50 m s<sup>−1</sup>), three types of C<italic><sub>d</sub></italic> fitting that decrease after 33 m s<sup>−1</sup>, which show different trends after 50 m s<sup>−1</sup> (increase, flat, and decrease), were adopted. The results were then compared with those for the control fitting (saturated after 33 m s<sup>−1</sup>) and show that the reduced <italic>C<sub>d</sub></italic> 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 <italic>C<sub>d</sub></italic> reduced wind range were significantly lower (higher) than in the others. These results suggest that <italic>C<sub>d</sub></italic> reduction might potentially induce the RI and LMI bimodality.</p>