Speaker: Oscar Guillemant (NASA/GISS/Columbia)
Topic: Processes controlling the downwind transport of African dust aerosols over the Atlantic, as calculated by the NASA GISS Earth System Model

Dust constitutes one of the main masses of natural aerosols and therefore has a crucial role in several aspects of the Earth system. Such roles include direct and indirect radiative effects, impacts on atmospheric chemistry as well as an ecological role by delivering nutrients to distant ecosystems. All of these contributions depend strongly on the Particle Size Distribution (PSD) of dust. In particular, super-coarse dust, with a diameter above 10 microns, has been historically less studied in model and in situ observations, partly because it was assumed to be removed quickly from the atmosphere by gravitational settling. However, in-situ ocean sediment trap measurements in the sub-tropical Atlantic have shown that super-coarse dust from North Africa is present downwind, all the way to the Caribbean, (van der Does et al. (2016)) whereas in models these particle sizes are mostly absent. We also show that the Dust Optical Depth (DOD) is higher over Africa and lower across the Atlantic in the model compared to CALIPSO observations, which suggest a similar bias in the long range transport of dust. Therefore, our goal in this study is to tackle potential sources of size bias in the NASA GISS Earth system model (ModelE2.1). We show that updating the prescribed size distribution for emissions and the gravitational settling speed by assuming aspherical particles have a significant effect on the long-range transport of dust, but not enough to match the observed PSD downwind over the Atlantic. The current numerical scheme for gravitational settling is shown to be more diffusive and less accurate than a conservative Semi-Lagrangian scheme. The latter might allow more transport of dust downwind.