Atlantic tropical cyclone formation: Pre-genesis evolution of tropical easterly waves and impacts of the middle to upper tropospheric dry air.

摘要

This study first provides an overview of the dynamic and thermodynamic evolution of tropical easterly waves (TEWs) for 164 named tropical storms over the Atlantic during 1989-2010 July-October. The evolution of precipitation and the low-level convergence suggests that convection begins to organize near the center of the wave critical layer about one day prior to genesis, along with the rapid intensification of vorticity. The composites derived from the ERA-Interim reanalysis reveal higher specific humidity and equivalent potential temperature near the center of the wave critical layer, especially in the middle troposphere within one day prior to genesis.;The study then focuses on the formation of the Cape Verde storms over the East Atlantic. There are two groups of easterly waves over West Africa, one to the south and the other to the north of the African Easterly Jet (AEJ), which sometimes merge near the coast of West Africa. Three groups of waves are identified in order to determine the role of wave merger in tropical cyclogenesis over the East Atlantic: non-merger developers, merger developers, and merger non-developers. Relative to non-mergers, it is found that merger developers have a weaker circulation near the surface at the early stages but the merger of a southern wave with a northern wave leads to a stronger and deeper wave pouch, which is more conducive to tropical cyclogenesis. It is also found that dry air intrusion west of the wave trough in the middle and upper troposphere inhibits deep convection and leads to the nondevelopment of some mergers, but that boundary layer dry air in the northern waves moistens quickly over the ocean and does not impede development.;The interannual variability of the middle and upper tropospheric dry air and its impacts on tropical cyclone activity over the Atlantic are further examined using the EOF analysis and composite analysis. It is found that the interannual variability of the upper-tropospheric (300-500 hPa) dry events is to some extent independent of that in the middle troposphere (600-850 hPa), and both have impacts on the interannual variability of Atlantic tropical cyclone activity. The sources of dry air and its association with African dust outbreaks are also examined.