Diurnal Variations in Tropical Cyclone Formation and Associated Convective Features

This study investigates the diurnal variations in the formation of tropical cyclones (TCs) and their linkage to convective features. Using best-track data from 2001 to 2020, it was found that TC formation shows a clear diurnal variation, peaking between 0300 and 0900 local solar time (LST) in most regions. Before a TC forms, the radial gradient in infrared (IR) brightness temperatures (BT) is mainly influenced by the expansion of clouds with IR BT below 208 K. These clouds are most abundant between 0300 and 0600 LST and least abundant between 1500 and 1800 LST. Clouds within 200 km of the circulation center have a higher fraction and greater diurnal variation compared to those located at radii of 200-500 km. A higher fraction of early morning clouds with IR BT < 208 K within 200 km of the circulation center increases the radial IR BT gradient, enhancing the likelihood of TC formation during morning hours.