Interaction of Cloud Dynamics and Microphysics During Tropical Cyclone Rapid Intensification

Tropical cyclone (TC), among the most devastating natural disasters globally, represents one of the most critical and challenging research foci in meteorology, particularly for intensity forecasting. A pivotal source of uncertainty lies in the rapid intensification (RI) process, defined as an increase in maximum sustained winds of at least 15 m/s within 24 hr. The TC RI events become more frequent in recent years, but their forecasts still remain challenging. Better understanding of the physical processes associated with RI of TCs would essentially improve its forecasting capability. The cloud dynamical and microphysical processes, especially their interactions that respond to RI are not well explored. In this study, the cloud macro and micro characteristics associated with RI of TC Nanmadol (2022) over the western Pacific are investigated using multiple-satellites observations. The storm underwent RI during 15–16 September 2022, and it has wreaked havoc on Japan's most cities as it moved across the Japanese island afterward with a track length of about 1,120 km. It is found inside Nanmadol as well as other TCs that a few particularly-large particles tend to occur in the outer rainbands during RI process. We further found that the rapidly-intensifying TCs possess a distinct upper-level outflow structure, which would attract cloud particles to accumulate and grow in the outer rainbands. This suggests that the large particles form in the outer rainbands due to the interaction of cloud dynamical and microphysical processes, which likely play a more substantial role in the RI process than previously acknowledged. Moreover, such unique features of particle distribution and upper-level outflow could be useful indicators for TC RI.