Assessing Dust Radiative Direct Effects on African Easterly Waves Environment over the Atlantic Ocean Using the CCPP Single Column Model

Aerosols produce some of the largest uncertainties in numerical weather prediction due to their efficient absorption and scattering of solar and terrestrial radiation. During summertime, the hot and dry Central West Sahara has some of the largest dust emissions globally. In this region, dry convection mixes the dust throughout the Saharan boundary layer. 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.

In this study, we used the Common Community Physics Package (CCPP) single-column model (SCM), a collection of atmospheric physical parameterizations that incorporate aerosol effects into the radiation scheme. The CCPP is now an integral part of the Unified Forecast System (UFS) model. We employed this model to determine and quantify the direct radiative effects of various dust vertical profiles and concentrations on African Easterly Waves (AEWs). First, we analyzed the variability of dust conditions and AEWs over the eastern tropical Atlantic, defining the average conditions of dust transport through the AEWs. Afterward, we identified a composite of AEWs with low dust levels in their surroundings. From this composite, we selected at least nine different columns corresponding to various phases of the wave. These columns provided the initial conditions and the dynamical forcings needed for experiments considering different dust concentrations and vertical dust profile shapes.

Preliminary results indicate that the maximum heating in the shortwave radiation scheme occurs around the altitude of maximum dust concentration. Conversely, in the longwave radiation scheme, dust exerts a cooling effect around the dust plume. However, a heating effect is observed in the lower troposphere, as the dust plume generates greater longwave downward radiation. It is important to note that low cloud conditions can alter these results, particularly in the lower troposphere.

Additionally, it is hypothesized that if a dust intrusion with a peak in the lower troposphere (below 700 hPa) occurs in a more trough-oriented direction of the AEW, the thermal structure of the trough could be diminished, as the dust would heat its associated colder region in the lower troposphere. The opposite could be expected if the intrusion occurs in a more ridge-oriented direction of the AEW.