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

Abstract 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.