Relative Importance of Mid-Level and Low-Level Vortices in Tropical Cyclogenesis Inferred From Experiments on Sensitivity to Radiation

The associated dynamics of mid-level and low-level vortices and their relative importance in tropical cyclogenesis (TCG) are less studied and understood to date. The issues are studied using the sensitivity of TCG to radiation as a tool. Three groups of idealized full-physics ensemble experiments with distinct solar insolation conditions are conducted: the control group with real diurnal radiation, the day-only, and the night-only groups for comparison. New findings are that the daytime radiative condition is more favorable for mid-level vorticity development than the nighttime one. This is because stratiform clouds cover a large area and dominate radiative heating, inducing a greater vertical gradient of diabatic heating at altitudes of 4–6 km through increased evaporation cooling most at about 4 km and decrease of latent heat release above about 6 km. However, the vorticity budgets show that the stronger mid-level vortex cannot extend downward to enhance the low-level vortex. Moreover, daytime solar radiation increases the low-level stability and suppresses convective development at the edge of cold pools, weakening low-level convergence of advective vorticity flux, and limiting low-level vorticity enhancement. Therefore, TCG is delayed in day-only experiments compared to others. These results reveal that the acceleration of TCG by longwave radiation is primarily caused by the enhancement of low-level convergence of advective vorticity flux, which tends to occur at nighttime without solar insolation. The study suggests that the development of low-level vortex is more important to TCG relative to mid-level vortex with radiative effects.