On Formation and Intensification of Tropical Cyclones Through Latent Heat Release by Cumulus Convection

The effect on large scale motions of latent heat release by deep cumulus convection in a conditionally unstable atmosphere is investigated and a method devised to include this effect directly in the equations for large scale flow. This method is then applied to the hurricane formation problem by incorporating it into time-dependent, circular symmetric dynamic hurricane models, either in gradient-wind balance or unbalanced. Numerical integrations of a two-level approximation of the balanced model have been carried out for two different formulations of the problem (including or not including a frictional radial flow), both starting from a hypothetical initial state characterized by a weak barotropic circular vortex with a maximum tangential velocity of 10 m sec−1 at a distance of 141.2 km from the center. The results obtained without frictional radial flow showed slow intensification of the tangential flow, to about 25 m sec−1, and establishment of a strong radial temperature gradient in the upper troposphere, from sixteen to twenty-four hours after the initial time, after which a steady state ensued. The radial flow obtained from this model remained less than 2 m sec−1. On the other hand, the results obtained with a superimposed frictional radial flow either decayed after reaching a moderate tangential velocity, or developed very rapidly after attaining higher velocity, and did not approach any steady state. The results further show that while the two-level approximation of the balanced model is able to reveal many important aspects of the development problem, it is not able to describe the further development associated with the upper level temperature gradient.