The Dynamics and Energetics of Mature Tropical Cyclones

Rapid progress toward the understanding of tropical cyclones has been made during the past 10 years, largely as a result of the development of numerical models. The dynamics and energetics of the mature tropical cyclone are reviewed in this article. First, the pressure, wind, temperature, and moisture structures of the hurricane are summarized. Then a scale analysis is applied over four separate regions of the hurricane domain to emphasize the most important dynamic processes in each region. The energetics of the tropical cyclone is examined in detail. The storm is shown to be a self-sustaining quasi-steady thermodynamic heat engine that is driven primarily by latent heat release. The relationship between latent heating and production of kinetic energy is discussed from the available potential energy viewpoint. The possibility of a steady state axisymmetric hurricane in a closed domain is considered. Although such a model appears possible from an energetic viewpoint, angular momentum considerations reveal that a steady axisymmetric closed system is impossible on any scale. For small domains (radius •500 km), in which axisymmetric circulations are possible, the system must be open in order that angular momentum be imported from the environment. For larger domains, axisymmetric circulations are unrealistic. The water vapor budget of the hurricane is examined to indicate the relative importance of evaporation and horizontal transport of water vapor in maintaining the moisture supply. Evaporation is shown to be an important percentage of the precipitation in the mature storm. Finally, the numerical modeling of tropical cyclones is summarized. An important problem in the development of hurricane models is the treatment or parameterization of the cumulus cloud scale and the hurricane scale interactions. The implications of the concept of conditional instability of the second kind on hurricane modeling are discussed. Results from several axisymmetric (two-dimensional) hurricane models, including attempts to simulate hurricane modification experiments, are summarized. Some recent results from a time-dependent three-dimensional hurricane model are presented.