Evolution of convective characteristics during tropical cyclogenesis

Four idealized, high‐resolution (500 m horizontal grid spacing), numerical simulations are used to investigate the evolution of convective structures during tropical cyclogenesis. The simulations all begin with a weak initial axisymmetric cloud‐free vortex in a quiescent environment, but differ in the moisture level of the initial sounding and whether or not ice microphysical processes are considered. Irrespective of experimental setup, there is only a short period where shallow or congestus clouds dominate. The shallow cloud phase is slightly extended with the drier initial environmental sounding. The composite structure of the convective elements sampled changes markedly throughout the genesis period. For much of the genesis phase, vertical profiles of the mean convective cell show significant amounts of anticyclonic vorticity produced in cells in the inner core. Towards the end of the genesis phase, there is a large increase in the production of cyclonic vertical vorticity in inner‐core convection, and cyclonic vorticity becomes dominant at low‐mid levels. The evolution from roughly equal strength vertical profiles of cyclonic/anticyclonic vorticity at low‐mid levels to profiles where cyclonic vorticity dominates occurs at relatively low system wind speeds (Vmax less than 10 m·s−1). This finding indicates a change in the structure of vortical convection prior to rapid intensification. In outer‐core convection, there are roughly equal strength vertical vorticity dipoles produced throughout the genesis period. Four idealized, high‐resolution (500 m horizontal grid spacing), numerical simulations are used to investigate the evolution of convective structures during tropical cyclogenesis. The composite structure of the average convective element sampled changes markedly throughout the genesis period, and towards the end of the genesis phase there is a large increase in the production of cyclonic vertical vorticity in inner‐core convection, which becomes dominant at low‐mid levels. Irrespective of microphysical scheme and environmental sounding used, there is a short period where cumulus congestus clouds dominate, that is, when the mean cloud top is between 5 and 9 km height, a finding which is at odds with a prior theory claiming that tropical cyclogenesis can be viewed as a two‐stage process in which congestus clouds have a large dynamical and thermodynamical contribution at early stages.