Variability of African Easterly Waves and Their Interactions with Saharan Mineral Dust Aerosols

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Just as weekly weather reports for the US focus on warm fronts and cold fronts, a summer weather report for West Africa might feature African Easterly Waves (AEWs), weather patterns with troughs and ridges separated by perhaps 1,500km that move eastward over the Sahel and across much of the downwind Atlantic. The report might also mention a vast plume of airborne Saharan dust moving along a similar trajectory to the AEWs. The proximity of Saharan dust plumes and AEWs suggests that the two might interact, since winds associated with an AEW could stir up dust and transport it, while solar radiation absorbed by the dust could cause atmospheric heating that would affect the evolution of the AEW. But the interaction of AEWs and dust plumes has not been extensively studied, and the extent to which dust lofted and transported by AEWs in turn affects the AEWs is not well understood. Previous work by the Principle Investigators (PIs) of this award shows, in an idealized context, that radiative heating from dust can contribute to the development of AEWs (see award #1524767).Work under this award uses a combination of observational analysis and numerical model experiments to better understand the interactions of AEWs and Saharan dust plumes. On the observational side the project takes advantage of the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) produced by the NASA Global Modeling and Assimilation Office). MERRA-2 is a valuable resource for the work because it tracks desert dust based on satellite observations and provides dust optical depth and radiative heating along with the standard meteorological variables. The modeling component uses the Weather Research and Forecast (WRF) model in an idealized configuration to identify unstable modes corresponding to AEWs. The simulations are performed with and without dust radiative heating so that its effects on the wavelength, phase speed, and growth rates of the AEW modes can be assessed. Additional experiments are performed using the Unified Forecast System model, again with and without dust radiative heating, to assess the influence of dust heating in a more realistic setting.The work has societal relevance due to its potential contribution to improving weather and air quality forecasts. Better forecasts of AEWs would benefit the US as well as the nations of West Africa since AEWs play a key role in generating hurricanes, including those that make landfall on the eastern seaboard. Saharan dust is also an intercontinental problem as it affects air quality in Africa, the Caribbean, and the US. The PIs work with the Africa Desk of the National Weather Service's Climate Prediction Center to incorporate research results into weekly weather outlooks and short-range dust forecasts. In addition, the project provides support and training to a graduate student.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.