Water Vapor Budget in a Developing Tropical Cyclone and Its Implication for Tropical Cyclone Formation

Evolution of the water vapor budget from the tropical wave to the tropical cyclone stage is examined using a high-resolution numerical model simulation. We focus on a time window from 27 hours prior to genesis to 9 hours after genesis, and the diagnoses were carried out in the framework of the marsupial paradigm. Analysis shows that the vertically integrated inward moisture flux accounts for a majority of the total condensation and that its fractional contribution increases from the tropical wave to the tropical cyclone stage. The fractional contribution of the local evaporation is much smaller and decreases from the tropical wave to the tropical cyclone stage. It is also shown that the radial moisture flux above 850 hPa is rather weak prior to genesis but increases significantly after genesis due to the deepening of the inflow layer. The decrease in the fractional contribution of the local evaporation, or the increase in the fractional contribution of the vertically integrated inward moisture flux, is due to the strengthening of the low-level convergence associated with the secondary circulation. The intensification of the secondary circulation can be attributed to the organized convection and concentrated diabatic heating near the circulation center. The results suggest that the local evaporation and its positive interaction with the primary circulation may not be as important as generally appreciated for tropical cyclone development. By contrast, the increase in the fractional contribution by the inward moisture flux with the storm intensification implies the importance of the positive feedback among the primary circulation, the secondary circulation and convection for tropical cyclone development.