Aircraft Observations and Process Denial in Dust Climate Modelling to Improve Understanding of Coarse Dust Transport

The difficulty faced by dust models in accurately representing the long-range transport of coarse (d>2.5 microns) dust is a long standing issue, and impacts the ability of climate models to adequately represent the impacts of dust in the Earth system. Here we apply vertically resolved aircraft observations of dust from the Sahara to the Caribbean to validate the Met Office Unified Model HadGEM3A for sizes up to 63 microns. We find that the model underestimates coarse dust transport, which is exacerbated by up to six orders of magnitude with trans-Atlantic transport.

Next we exclude or vary transport processes to better understand the dominant transport mechanisms in the model: gravitational sedimentation, impaction scavenging, convective mixing, turbulent mixing and shortwave absorption. Although all these processes can change the horizontal and vertical distribution of coarse dust across the Atlantic, only reductions (of 50-80%) in sedimentation velocity are sufficient to reconcile observational and model size distributions. Potential physical explanations which might allow such reductions will be discussed.