Water Vapor Budget in a Developing Tropical Cyclone

The water vapor budget of mature tropical storms has been examined in many previous studies based on numerical model simulations or radar observations, but it has not been well studied at the pre-genesis stage. In this study, a high-resolution numerical model simulation of Tropical Storm Fay (2008) is used to examine the water vapor budget and respective changes in the overall moisture contribution before and after genesis in the marsupial framework. Analysis shows that the mean vertical moisture transport plays the dominant role in moistening the free atmosphere. The horizontal moisture influx is found to account for a majority of the moisture transport from the marine boundary layer to the free troposphere, while the ocean source of water vapor contributes only a small portion. The ocean source of water vapor is shown to increase after genesis but remains relatively small compared to the horizontal moisture influx. The water vapor budget analysis also suggests that dry air intrusion is weak in the middle troposphere due to the weak mid-level radial inflow. A comparison of the budget analysis of ex-Gaston (2010), a system that failed to re-develop, shows that vertical transport of dry air from the upper troposphere, where a well-defined closed circulation is absent, contributes to the mid-level drying and hinders the re-development of ex-Gaston.