Skip to main content
Dryad

Data from: Shear-induced vertical mixing in a stratified Saharan Air Layer

Rodakoviski, Rodrigo1; Chamecki, Marcelo1

Published Sep 30, 2024 on Dryad. https://doi.org/10.5061/dryad.sn02v6xdp

Data files

Sep 30, 2024 version files 15.55 MB

Abstract

Recent studies have suggested vertical turbulent mixing in the Saharan Air Layer (SAL) as a possible mechanism explaining the observed long-range transport of coarse Saharan dust particles. Nevertheless, buoyancy profiles measured in the SAL typically display some degree of stable stratification, implying that any turbulence in this elevated layer must be stratified. In this paper, we idealize the SAL as a stratified shear layer where turbulence is triggered by the instability of the African Easterly Jet, and stratification occurs due to lateral entrainment of stratified, non-SAL air from the surroundings. Analytical solutions obtained for this idealized set-up are combined with LES data and results from the stratified turbulence community to produce a simple parameterization of the eddy diffusivity in a weakly stratified SAL as a function of layer depth, shear magnitude, and gradient Richardson number $\Ri_g$. Our results suggest that even $O\pth{1}\,\mathrm{m^2\,s^{-1}}$ eddy diffusivities (associated with relatively large $\Ri_g$) are enough to significantly impact the airborne lifetime of particles as large as super-coarse dust (with a diameter greater than $10\,\mathrm{\mu m}$). Therefore, even after accounting for the stabilizing effect of buoyancy, turbulent mixing in the SAL remains a likely explanation for the long-range transport of coarse Saharan dust. Moreover, our diffusivities decay faster with $\Ri_g$ than in typical ABL models, highlighting the importance of employing proper parameterization schemes in climate models to represent slow processes (affected by small diffusivities values) accurately.