Tropical Storm Development from African Easterly Waves in the Eastern Atlantic: A Comparison of Two Successive Waves Using a Regional Model as Part of NASA AMMA 2006

Abstract

Abstract Two successive African easterly waves (AEWs) from August 2006 are analyzed utilizing observational data, the European Centre for Medium-Range Weather Forecasts reanalysis, and output from the National Center for Atmospheric Research–National Oceanic and Atmospheric Administration Weather Research and Forecasting model (WRF) to understand why the first wave does not develop over the eastern Atlantic while the second wave does. The first AEW eventually forms Hurricane Ernesto over the Caribbean Sea, but genesis does not occur over the eastern Atlantic. The second wave, although weaker than the first over land, leaves the West African coast and quickly intensifies into Tropical Storm Debby west of the Cape Verde islands. This study shows that the environmental conditions associated with the first AEW’s passage inhibited development. These conditions include strong low- and midtropospheric vertical wind shear owing to a stronger than normal African easterly jet, lower than normal relative humidity, and increased atmospheric stability. All of these are characteristics of an intensification of the Saharan air layer (SAL), or SAL outbreak, over the eastern Atlantic. The environmental conditions were more favorable for genesis 2½ days later when the second wave left the African coast. Additionally, a strong low-level southwesterly surge develops over the eastern North Atlantic in the wake of the passage of the first wave. This westerly surge is associated with an enhancement of the low-level westerly flow, low-level cyclonic vorticity, large-scale low-level wind convergence, and vertical motion conducive for development over the region. While the initial westerly surge is likely associated with the passage of the first wave, over time (i.e., by 1600 UTC 20 August 2006) the development of the second wave becomes influential in maintaining the low-level westerly surge. Although SAL outbreaks are also associated with the addition of dust, the different cyclogenesis histories of the two systems are simulated without including dust in the regional model.