Quasi-Lagrangian Large-Eddy Simulation of Dust Transport in the Saharan Air Layer

Abstract:

Mineral dust constitutes a great fraction of aerosol mass in the atmosphere. Not only it delivers key micronutrients to ecosystems (such as phosphorus to the Amazon soil), but it also interacts with radiation and acts as cloud condensation nuclei, producing important impacts on climate, weather, and the hydrological cycle. Despite its importance, measurements show that large-scale models persistently underestimate the atmospheric lifetime of coarse dust, i.e., dust with diameter larger or equal to 5 μm. This implies that some relevant process is either missing or not accurately represented in such models.

Using high-resolution large-eddy simulations (LES) in a semi-Lagrangian box traveling above the tropical North Atlantic in the Saharan Air Layer (SAL), we estimate the possible impact of turbulence in increasing the airborne lifetime of these particles, which would allow them to be transported over greater distances. To that end, we use the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) driven by MERRA-2 reanalysis data to determine the Lagrangian trajectory of the LES box. The reanalysis data is also used to determine the mean vertical wind shear within the SAL, which is used to force the LES via geostrophic balance. Since the box is located above the marine boundary layer, we adapt the LES code to simulate a layer of turbulence between two stratified layers in geostrophic balance. Simulation results are used to characterize time evolutions of turbulence and dust concentrations within the SAL, providing useful information for the parametrization of coarse dust settling in large-scale models.