The Role of Moisture and the Saharan Air Layer in the Tropical Wave to Cyclone Transition of Hurricane Ernesto (2006)

WRF model simulations representing the evolution of the pre-Hurricane Ernesto African Easterly Wave (2006) are performed with model initial moisture altered, with simulations featuring sub- and super-moist air in the wave environment compared to a Control run representing the actual tropical cyclogenesis of Hurricane Ernesto. With a focus on the analysis of the wave pouch area in a wave-relative framework, and on the generation and maintenance of potential vorticity in this area, the ability for the wave pouch area to buffer the incipient tropical cyclone against various amounts of dry air is explored. The effect of dry air in the simulated wave track region is analagous to the effect of Saharan Air Layer air which is often present in tropical wave environments in the North Atlantic Ocean. The result of the WRF modeling study is that simulations with a dryer-than-normal Saharan Air Layer still exhibit strong tropical cyclogenesis, while simulations with a more moist Saharan Air Layer, in general, do not exhibit strong tropical cyclogenesis for this case. Through a thorough analysis of the development of nine simulated easterly waves, including a focus on the potential vorticity budget, this difference is attributed to the strength of the wave pouch circulation within the easterly wave in this case, such that simulations with a very strong wave pouch circulation had strong tropical cyclogenesis, while simulations with a weaker wave pouch circulation did not have tropical cyclogenesis.