Persistent Convective-Dominated Inner-Core Clouds: A Key Driver of Tropical Cyclone Genesis

This study investigates Typhoon Ma-on (2022) genesis in a high-resolution simulation. Results show that multiple peaks of convection occur in the inner core (within 100 km) of Ma-on, accompanied by periodic evolution of convective and stratiform clouds. A new concept, key period, is defined as a period that starts with increasing vertical motion associated with convective bursts, followed by the development of stratiform clouds and dissipation of convective clouds. To identify potential indicators of TC genesis, the key period of genesis is compared with an earlier key period.

Diagnosis of the vorticity equation reveals that convective clouds make most contributions to vorticity growth through vertical advection and stretching. The results further indicate that deeper convection is not necessarily more conducive to genesis; rather, persistent convection with its maximum upward motion at lower to middle levels more effectively drives lower-level spin-up. Additionally, diagnosis of water vapor equation shows that, convective-dominated inner-core clouds enhance the secondary circulation through diabatic heating, thereby ensuring the radial inflow of moisture. In contrast, when stratiform clouds occupy large areas in the inner core, lower-level divergence becomes dominant, which may cause moisture outflow and therefore insufficient moisture supply.

These confirm the crucial role of persistent convective-dominated inner-core clouds during the key period approaching TC genesis. That requires a strengthened mid-level vortex, to which temperature responds to maintain thermal wind balance, forming a cold anomaly near the disturbance center, below the mid-level vortex. Consequently, convective instability increases in the boundary layer, favoring more sustained convective clouds and new convective bursts. That maintains convective-dominated inner-core clouds and ultimately promotes TC genesis.