Investigating the Direct Radiative Impact of Saharan Dust Aerosols on African Easterly Waves

Aerosols produce some of the largest uncertainties in numerical weather prediction due to their efficient absorption and scattering of solar and terrestrial radiation, which modifies the thermodynamic structure and energy budgets of the atmosphere. During summertime, the hot and dry Central West Sahara (15-25⁰N, 15-5⁰W) has some of the largest dust emissions globally. In this region, dry convection mixes the dust throughout the Saharan boundary layer (1000-500 hPa). Meanwhile, differential heating by the Saharan Desert sets up a midtropospheric African easterly jet (AEJ) across North Africa (at ≈650 hPa, 15⁰N) that transports the dust westward across the Atlantic. The AEJ also facilitates the development of synoptic scale African easterly waves (AEWs), which can directly interact with the dust aerosols.

For this study we used the Unified Forecast System (UFS) model together with the Common Community Physics Package (CCPP) single-column model (SCM). The CCPP is a collection of atmospheric physical parametrizations that incorporates aerosol effects into the radiation scheme and is also now an integral part of the UFS. Both models were employed to examine an AEW that developed by the end of July 2020 and interacted with a synoptic-scale dust plume near the coast of West Africa. The main goal is to quantify the impact of the dust aerosol radiative effects on the synoptic weather system. First, the CCPP-SCM was used to make faster sensitivity experiments, in order to understand how different dust concentrations can modify the atmospheric vertical profiles and the surface conditions around the AEW trough. Secondly, we performed UFS experiments with different aerosols configurations, which allowed us to have a better comprehension of the impact of the dust aerosol over the AEW and the atmospheric circulation around it. Preliminary results from the UFS experiments at the level of 850hPa (where dust influence is expected to be stronger) indicate that the amount of aerosols play an important role in the spatial and temporal evolution of the AEW. For instance, differences in the AEW's amplitude and the horizontal configuration of the cyclonic relative vorticity are seen. This also leads to variations in the precipitation forecast.