The Impact of Saharan Dust on Atlantic Convective Systems: A Case Study

Tropical weather systems such as the African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones usually interact with the Saharan Air Layer (SAL) and mineral dust aerosols embedded in the SAL. Previous studies have debates about the strength and even the direction of the impact from the dust and SAL on the development of the tropical systems. In this study, the sole impact from the dust on a hurricane is quantified using carefully designed numerical experiments.

Hurricane Earl (2010) was originated from an AEW disturbance over Africa. It was influenced by the dusty SAL especially in its early development stage. We conduct numerical experiments using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate Hurricane Earl (2010) starting from a weak disturbance.

Experiments with 36-km resolution show that without moist convection, dust slightly weakens the low-pressure system in North Africa by cooling the atmosphere. This scenario mainly results from a reduction of the boundary layer heating during the daytime and from the enhancement of the outgoing longwave radiative cooling during the nighttime. The zonal mean of the AEJ is intensified slightly (less than 1m/s) in its southern part and weakened in its northern part when dust is included in the model. Adding dust weakens the AEWs at 850 hPa and 600 hPa levels and the tropical cyclone.

High-resolution (4-km) cloud-resolving model experiments show that dust deepens the system slightly but does not affect the track. In the tropical low stage, dust increases the low-level cloud at 1-2 km height. It reduces rainfall in the outer edge of SAL region between 250 km to 350 km radii while it increases rainfall in the inner edge at around 200 km. The associated latent heat release shifts the updraft radially inward. The dynamical consequence is the acceleration of the tangential wind and the mid-level circulation. In the tropical depression stage, adding dust increases the number of cloud droplets in most of the regions and enhances the convection around the center, which generates more ice, snow, and rain. Overall, the impact of dust aerosols on Earl is small, the center minimum sea-level pressures only differ by less than 1 hPa at the end of the simulation. This suggests that pure meteorological model may be sufficiently accurate to forecast hurricanes if the dynamical and thermodynamical features of the SAL are properly described.

In order to predict aerosols, one can use a simple chemistry model such as GOCART which only has aerosol direct effects. Using the Ensemble Kalman Filter (EnKF) to assimilate conventional observations and MODIS aerosol optical depth (AOD) data, both forecasts of the track of the storm and the aerosol can be improved.