Enhancement of Tropical Cyclones by Aerosols: Mineral Dust's Role in Tropical Depression Formation

Saharan Air Layers (SAL) are layers of dry, dusty air that originate from North Africa and propagate over the Atlantic Ocean. Presence of SALs over the Tropical Atlantic are known to impede the formation of Tropical Cyclones (TC) through three primary mechanisms:

• Dust within SALs warms the middle troposphere and cools the surface, stabilizing lapse rates and hampering convection.
• Dry SAL air evaporates clouds, weakening convection.
• SALs increase local wind shear, impeding organization of TCs.

These processes are well-supported by both observations and numerical simulations. However, the authors have found at least one instance where dust within a SAL is required for TC development.

The Weather Research and Forecasting with Chemistry (WRFChem) model is used to simulate the 2017 Tropical Depression number four (TD4). Two experiments are conducted: the Control and the Aerosol Free. The Control experiment consists of a seven day WRFChem simulation beginning July 3^rd, 2017 using the Goddard mineral dust emission parameterization and ocean mixed-layer model. The Aerosol Free simulation is identical but without mineral dust emissions.

TD4 formed on July 5^th, 2017 just west of Cape Verde and moved westwards towards Venezuela. It gradually strengthens and develops organized convection and rainfall until July 7^th, when it began to dissipate. Late on July 7^th, the National Hurricane Center downgraded the depression due to lack of organization. WRFChem captures this behavior well when dust emission is included. A closed low forms on July 5^th as recorded by the NHC, and the simulation shows that maximum wind speeds within the TC exceed the 33 knot requirement for tropical depression status. The TC is trackable in WRFChem until it exits the domain on the 7^th. This is in contrast to the Aerosol Free simulation where the TC never meets the 33 knot threshold and dissipates on the 6^th of July 2017.

To explain the differences in these simulations, it is necessary to reconsider the result of aerosol warming within the atmospheric column. While such warming does increase stability, our simulations suggest that it also reduces TC central pressure by up to 4 hPa. This larger pressure gradient enhances convergence within the TC, promoting the development of convection through enhanced moisture transport. This mechanism may be more significant for tropical depressions on the edge of development compared to more powerful tropical storms and hurricanes.

This case demonstrates that more complex interactions between TCs and SALs exist than previously known. The authors present analysis detailing the impacts of SAL dust on TC development and the mechanism by which it occurs.