DOI: 10.5061/dryad.sn02v6xdp
title: Data from: Shear-induced vertical mixing in a stratified Saharan Air Layer

description: 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.
description: # Data from: Shear-induced vertical mixing in a stratified Saharan Air
 Layer Dataset obtained from Large-Eddy Simulations (LES) of the Saharan
 Air Layer including different scenarios for shear and stratification. The
 simulations refer to a free-shear, stratified, 1-km thick air layer
 contained between free-slip walls, in a horizontal domain of 4 km x 4km
 with a resolution of 320^3 grid points. LES runs are labeled PiHj, where i
 is a number from 1 to 4 labelling the pressure gradient force (PGF)
 magnitude, whereas j is a number from 0 to 3 labelling the stratifying
 heating rate. Higher numbers indicate stronger PGF and faster
 stratification. Files contain statistics (planar, time-averages)
 corresponding to the published results. See main text named
 "Shear-induced vertical mixing in a stratified Saharan Air
 Layer" by R. Rodakoviski and M. Chamecki for more details. ##
 Description of the data and file structure Each file is named after the
 corresponding figure in the paper. Files are either self-explanatory (with
 top row containing variable names and units, when applicable) or contain a
 README file describing their structure. A file-by-file description of
 their structure is also given below. ## fig2.tar.gz Contains: *
 tropical-theta.dat: height above MSL z (km) and potential temperature
 theta (K) for SAL average levels. Calculated based on standard tropical
 profile defined in Anderson et al. (1986). * AER_D contains 2 files: *
 all-profs-T.out: first column is normalized z, following 10 columns are
 normalized potential temperature measurements from dropsondes, and last
 column is normalized ensemble average * all-profes-u.out: first column is
 normalized z, following 10 columns are zonal wind speed (minus its
 vertical average) measurements from dropsondes, and last column is
 ensemble average * SALTRACE contains 11 files: * mean-soundings.out:
 normalized z and normalzied ensemble averaged potential temperature *
 may##.txt: sounding measurement on day ## of may at SAL levels. first row
 describes the variables in each columns (potential temperature, wind
 speed, water vapor) * README.md ### fig3a.dat Measurement profile. Columns
 are height above MSL z (km), buoyancy and shear frequencies squared, and
 local gradient Richardson number. ### fig3b.dat LES profile. Columns are
 height z (m), buoyancy and shear frequencies squared, and local gradient
 Richardson number. ## fig4.dat Production and buoyancy flux normalized by
 dissipation rate (as described in the text) for different LES runs
 (labeled row by row). ### fig5a.dat Potential temperature (K) from LES in
 a matrix format (xz plan) for case P1H1. ### fig5b.dat Potential
 temperature (K) from LES in a matrix format (xz plan) for case P4H1. ###
 fig5c.dat Potential temperature (K) from LES in a matrix format (xz plan)
 for case P1H3. ### fig5d.dat Potential temperature (K) from LES in a
 matrix format (xz plan) for case P4H3. ### fig6a.dat Vertical wind speed
 (m/s) from LES in a matrix format (xz plan) for case P1H1. ### fig6b.dat
 Vertical wind speed (m/s) from LES in a matrix format (xz plan) for case
 P4H1. ### fig6c.dat Vertical wind speed (m/s) from LES in a matrix format
 (xz plan) for case P1H3. ### fig6d.dat Vertical wind speed (m/s) from LES
 in a matrix format (xz plan) for case P4H3. ## fig7.tar.gz Contains: *
 fig7a.dat: z/h, u velocity (m/s) * fig7b.dat: z/h, theta/(318.0 K) *
 fig7c.dat: z/h, K_u (m^2/s) * fig7d.dat: z/h, w variance (m^2/s^2) *
 fig7e.dat: z/h, Ri_g * README.md ### fig8a.dat alpha and normalized shear
 frequency (as described in the text) for different LES runs (labeled row
 by row). ### fig8b.dat gamma and normalized buoyancy frequency (as
 described in the text) for different LES runs (labeled row by row). ###
 fig9a.dat Froude number and mixing coefficient for different LES runs
 (labeled row by row). ### fig9b.dat Buoyancy Reynolds number and mixing
 coefficient for different LES runs (labeled row by row). ### fig10a.dat
 Turbulence kinetic energy and vertical velocity variance for different LES
 runs (m2/s2, labeled row by row). ### fig10b.dat Product between rms
 vertical velocity (sigma_w) and dissipation lengthscale (ell_epsilon),
 momentum diffusivity, and heat diffusivity for different LES runs (m2/s,
 labeled row by row). ### fig11a.dat Product Mh and vertical velocity
 variance for different LES runs (m2/s2, labeled row by row). ###
 fig11b.dat Gradient Richardson number and normalized dissipation
 lengthscale for different LES runs (labeled row by row). ### fig12a.dat
 Gradient Richardson number and normalized momentum and eddy diffusivities
 for different LES runs (labeled row by row). ### fig12b.dat Gradient
 Richardson number and stability function from different LES runs (labeled
 row by row). ### fig13a.dat First column: gradient Richardson number.
 Remaining columns: relative increase in airborne lifetime of dust
 particles for different size particles (given in micrometers in the first
 row of the file). ### fig13b.tar.gz Contains: * theory-variablerig.dat:
 first column is particle geometric diameter in micrometers, remaining
 columns are concentration fraction for different Richardson numbers (given
 in the first row of the file). * limits.dat: first column is particle
 geometric diameter in micrometers, second and third columns are
 concentration fraction for laminar flow (no turbulence, K=0) and instant
 mixing (K=infinity) respectively. * saltrace-data.dat: first column is
 particle geometric diameter in micrometers, second is concentration
 fraction calculated from SALTRACE data as described in the text. *
 README.md # Figures from supplement ## figS1.tar.gz Contains: *
 thetaxy-p#h#.dat: 4 files containing potential temperature (K) from LES in
 a matrix format (xy plan) for case P#H#. * wxy-p#h#.dat: 4 files
 containing vertical velocity (m/s) from LES in a matrix format (xy plan)
 for case P#H#. * README.md ## figS2.dat First column is sigma_w^* squared,
 second column is dimensionless shear rate. Each row is labeled according
 to the simulation name described in the main text. ## figS3.tar.gz
 Contains: * CDF_AERD.dat: Cumulative distribution function for AERD data.
 * cdfles-P1H3.dat: Cumulative distribution function for LES data (case
 P1H3). * cdfles-P4H1.dat: Cumulative distribution function for LES data
 (case P4H1). * CDF_SALTRACE.dat: Cumulative distribution function for
 SALTRACE data. * PDF_AERD.dat: Probability distribution function for AERD
 data. * pdfles-P1H3.dat: Probability distribution function for LES data
 (case P1H3). * pdfles-P4H1.dat: Probability distribution function for LES
 data (case P4H1). * PDF_SALTRACE.dat: Probability distribution function
 for SALTRACE data. * README.md ## figS4.tar.gz Every file has 4 columns,
 containing normalized z, N2 (1/s2), S2 (1/s2), Rig=N2/S2 as described in
 the header. Contains: * figS4ab_dz.dat: used for both figures S4a and S4b
 * figS4a_#dz.dat: used for figure S4a with data smoothed at #dz *
 figS4b_#dz.dat: used for figure S4b with data coarsened at #dz *
 figS4cd_dz.dat: used for both figures S4c and S4d * figS4c_#dz.dat: used
 for figure S4c with data smoothed at #dz * figS4d_#dz.dat: used for figure
 S4d with data coarsened at #dz * README.md ### figS5a.dat Vertical
 profiles measured in SALTRACE campaign. Columns are height above MSL z
 (km), potential temperature theta (degC), zonal wind speed u (m/s),
 meridional wind speed v (m/s), specific humidity q (g/kg) as labeled in
 the header. ### figS5b.dat Vertical profiles measured in AER-D campaign.
 Columns are height above MSL z (km), zonal wind speed u (m/s), meridional
 wind speed v (m/s), potential temperature theta (K), water vapor mixing
 ratio r (g/kg) as labeled in the header. ## Sharing/Access information
 Links to other publicly accessible locations of the data: * Not
 applicable. Data was derived from the following sources: * Large-eddy
 simulations (see paper & description above). * AER-D data (described
 in Ryder, 2021) can be obtained from the Center for Environmental Data
 Archive at
 [https://catalogue.ceda.ac.uk/uuid/d7e02c75191a4515a28a208c8a069e70](https://catalogue.ceda.ac.uk/uuid/d7e02c75191a4515a28a208c8a069e70). * SALTRACE sounding data can be obtained as described in Rittmeister et al. 2017.