Impacts of the Mid-level Dry Air on the Formation of Tropical Storm Fay (2008) over the Atlantic
The Advanced Research Core of Weather Research and Forecasting model (WRF-ARW) is used to simulate the formation of Tropical Storm Fay (2008). The model simulation starts from the wave stage of the precursor disturbance to one day after the formation of the storm. The water vapor budget is examined at the different stages of the storm evolution within the framework of the marsupial paradigm (Dunkerton et al. 2009) in order to better understand the impacts of the Saharan Air Layer (SAL) on tropical cyclone formation. Previous studies have suggested that the SAL suppresses convection and inhibits tropical cyclone formation, however, the mechanisms regarding the impacts of the SAL remain unclear. The marsupial paradigm provides a framework to systematically examine the dynamic and thermodynamic evolution of precursor disturbances and provides a new perspective on this issue. Based on the high-resolution (1 km) model simulation, the contribution of evaporation and low-level inflow to moisture supply, the role of deep convection in moistening the air column, the lateral entrainment of dry air, and the importance of a closed circulation in retaining moisture are examined.