Tropical cyclone genesis and surges in the Saharan Air Layer

Recent observations of tropical cyclone genesis in the Atlantic during NAMMA, have indicated that interaction of a MCV with plumes of the Saharan Air Layer (SAL) seem to spawn explosive growth of convection causing rapid genesis of a tropical storm (Koren et all, 2005, Pratt et al., 2007). This is in contrast with previous observations of Dunion and modeling studies of Tripoli and Dunion (2007) that suggest the SAL actually inhibits storm development. The SAL is characterized by warm mid level temperatures, low relative humidity, high concentrations of Saharan dust and a strong easterly flow near 700 hPa. The previous studies of Mediterranean storms by Tripoli et al (2005) have demonstrated that the SAL can act to suppress convection while destabilizing the middle tropoisphere, creating a loaded gun type thermodynamic profile that leads to intense convection when triggered. Presumably, the storm circulation may act as such a trigger. Hashino and Tripoli have also shown that the increased ice nuclei of the SAL can impact the microphysical process, altering rain rates of Mediterranean storms. More recently Tripoli and Dunion (2007) show that the shear associated with the SAL can strongly suppress the genesis process. However, they also found that when expressed as a plume advancing into the trades, the shear created along the southern SAL front acts to spawn cyclonic eddies that may play a role in cyclone genesis. These eddies have both baroclinic and barotropic aspects to them resulting from the thermodynamic and dynamic contrasts across the plume periphery. In this study, we employ idealized cloud resolved modeling experiments to investigate the dynamic interaction of the SAL with the trades and the potential for this process to lead to tropical cyclone genesis.