Shear-Induced Vertical Mixing in a Stratified Saharan Air Layer

Abstract Recent studies have suggested vertical turbulent mixing in the Saharan air layer (SAL) as a mechanism for the long-range transport of coarse Saharan dust. Nevertheless, buoyancy profiles measured in the SAL typically exhibit some degree of stable stratification, implying that any turbulence in this elevated layer must be stratified. In this paper, we aim to quantify the impact of weak stratification on turbulent mixing rates in an idealized SAL flow, in which turbulence is triggered by shear instabilities. We combine analytical solutions for this idealized flow with large-eddy simulation (LES) data and results from the stratified turbulence community to produce simple parameterizations for the momentum and scalar eddy diffusivities as a function of layer depth, shear magnitude, and gradient Richardson number Ri g for implementation in climate models. The diffusivities calculated for our elevated SAL setup decay faster with Ri g than typical ABL models, highlighting the importance of employing appropriate parameterization schemes to correctly model slow atmospheric processes (which can be significantly affected by small diffusivity values). When applied to the transport of dust, our results suggest that even small eddy diffusivities (associated with Ri g ≳ 0.10) are enough to significantly impact the airborne lifetime of particles as large as supercoarse dust (with a diameter greater than 10 µ m). Therefore, even after accounting for the stabilizing effect of buoyancy, turbulent mixing in the SAL remains a reasonable explanation for the long-range transport of coarse Saharan dust.