Two Types of Transitions to Relatively Fast Intensification in Tropical Cyclone Simulations with Weak to Moderate Environmental Vertical Wind Shear

Tropical cyclone intensification is simulated with a cloud-resolving model under idealized conditions of constant SST and unidirectional environmental vertical wind shear maximized in the middle troposphere. The intensification process commonly involves a sharp transition to relatively fast spinup before the surface vortex achieves hurricane strength in the azimuthal mean. The vast majority of transitions fall into one of two categories that are labeled S and A. Type S transitions initiate quasi-symmetric modes of fast spinup. They occur in tropical cyclones after a major reduction of tilt and a pronounced azimuthal spread of inner-core convection. In general, the lead-up to a type S transition also entails gradual contractions of the radii of maximum wind speed (r_m) and maximum precipitation. Type A transitions start prior to vertical alignment, normally in somewhat weaker tropical cyclones with broader inner cores. Instead of enabling the transition, alignment is an integral part of the initially asymmetric mode of fast spinup that follows. On average, type S and type A transitions respectively occur well-after and once the cyclonically rotating tilt vector becomes perpendicular to the shear vector. Prominent temporal peaks of lower tropospheric convective available potential energy and low-to-midlevel relative humidity averaged over the whole inner-core region of the low-level vortex characteristically coincide with type S transitions but not with type A transitions. Prominent temporal peaks of precipitation and midlevel vertical mass flux in the meso-β scale vicinity of the low-level convergence center characteristically coincide with type A transitions but not with type S transitions. Despite such differences, in both cases the transitions tend not to begin before the distance between the low-level convergence and vortex centers divided by r_m reduces to unity.