A Numerical Model Study about the Impacts of Dry Air on Tropical Cyclone Formation

The impacts of dry air on tropical cyclone formation are examined in the high-resolution numerical simulations of ex-Gaston (2010) and pre-Fay (2008). The former can be regarded as a non-developing system as it failed to redevelop after being downgraded into a remnant low, and the latter developed into a tropical cyclone despite lateral dry air entrainment and transient upper-level dry air intrusion. Backward trajectory analysis and water budget analysis show that vertical transport of dry air from the upper troposphere, where a well-defined wave pouch is absent, contributes to mid-level drying near the pouch center and the non-development of ex-Gaston. Water vapor budget analysis at the pre-genesis stage shows that vertical moisture advection plays the dominant role in moistening the free atmosphere. Persistent mid-level drying is shown to suppress deep convection and reduce moisture supply to the free atmosphere. Three-dimensional trajectory analysis based on the numerical simulation of Fay suggests that dry air entrained at the pouch periphery does not penetrate to the pouch center due to the weak mid-level inflow. Moreover, it is shown that dry air gets moistened as it is being wrapped into the wave pouch, and lateral entrainment in the middle troposphere thus does not suppress convection near the pouch center or prevent development of tropical storm Fay.