Tropical cyclone formation: A synopsis of the internal dynamics

Although the conditions necessary for tropical cyclone (TC) formation have been recognised for decades, an understanding of the dynamical and thermodynamical processes responsible for the formation of a TC scale warm-cored vortex is still uncertain. Recent modelling studies have provided new kinematic and thermodynamic insight, some of which has been confirmed by observations. In this presentation we will draw on modelling results and observations, and revisit theoretical work to construct a conceptual model of the internal dynamics of TC formation.

The conceptual model assumes a pre-existing cyclonic disturbance with a thermodynamic environment conducive to deep convection. Precipitating convection implies a net upward tropospheric mass flux, which must be accompanied by outflow aloft and inflow below. The inflow converges vorticity on two scales: (i) the scale of the individual ascent regions, and (ii) the scale of the entire system. The interaction of anomalous cyclonic vorticity from (i) tends towards the development of a central upright vortex core. The concentration of vorticity by (ii) increases the vorticity reservoir of the developing system and focuses the convection on an increasingly smaller area, which results in a non-linear increase in the efficiency of system scale intensification for a given upward mass flux.

Critical to TC genesis is the concentration of low-level vorticity. Downdrafts that develop in response to evaporation of rain tend to be greatest in the lower troposphere. In the mean they oppose the “up” part of the convective in-up-out circulation, and thus reduce or even eliminate the inflow and hence vorticity convergence at low-levels. In a relatively dry mid-level environment in which the potential for evaporation of rain is high, formation will be retarded, and the reduction of downdrafts will be essential to TC formation. Additionally, entrainment of dry environmental air into convective updrafts reduces the updraft buoyancy due to evaporative cooling, and thus drier environments weaken the mean upward mass flux. However, the processes of vertical mixing in overturning convective up- and downdrafts, and the environmental moistening from cloud water detrainment, reduce the potential for downdrafts and increase the potential for stronger, deeper updrafts. Thus sustained convection increases the efficiency of the in-up-out circulation particularly at low levels.

In this presentation we introduce the conceptual model and justify the inherent assumptions using theoretical and observational arguments.