Impact of Solar Radiation on Low-Level and Mid-Level Vortices during Tropical Cyclogenesis

This study attempts to investigate how the diurnal cycle of radiation will affect low-level and mid-level vortices in the tropical depression and then lead to tropical cyclogenesis. The three experiments with distinct solar radiation conditions were conducted: the control experiment with real diurnal radiation, the day-only, and the night-only experiments. It is shown that the daytime radiative conditions were more favorable for mid-level vorticity development, while nighttime conditions were more conducive to low-level vorticity development. The daytime shortwave radiation promoted the formation of stratiform clouds and increased the vertical gradient of diabatic heating at the mid-troposphere, thereby facilitating the development of mid-level vorticity. The vorticity budget analysis showed that the changes in local low-level vorticity are primarily driven by horizontal convergence rather than vertical transport. This implies that the mid-level vorticity is hardly transported to the lower levels. The daytime shortwave radiation suppressed convective development, weakening low-level convergence and limiting low-level vorticity enhancement. These experiments demonstrated that the strong mid-level vorticity caused by solar radiation does not necessarily promote the development of low-level vorticity. Therefore, the acceleration of tropical cyclogenesis by the diurnal radiation cycle is primarily contributed from the low-level vorticity enhancement, which tends to occur at night.