Hurricane Formation in Diabatic Ekman Turbulence

This study examines the emergence of hurricanes from chaotic swirling motions in a 3-layer model of the tropical troposphere that includes basic parameterizations of cumulus convection and air-sea interaction. The chaotic flow is referred to as Diabatic Ekman Turbulence (DET), in order to emphasize that cumulus convection and Ekman pumping are critical to its behavior. The time required for tropical cyclogenesis in DET is examined over a broad range of sea-surface temperatures, tropical latitudes, and surface exchange coefficients for moist entropy and momentum. The mean results show sensible trends, but the specific genesis time can vary significantly with subtle details of the initial turbulence. Moreover, hurricanes do not always form. In the event that a tropical depression develops into a hurricane, the process is highly asymmetric. Intensification involves a shear-flow instability, the production of mesovortices, and contraction of the basic circulation. Despite the complex evolution, the intensification rate is largely consistent with the expectations of a quasi-linear stability analysis. Properties of mature hurricanes and the nature of their fluctuations are briefly addressed in the context of the model.