Quantifying particle size and turbulent scale dependence of dust flux in the Sahara using aircraft measurements

• Published in: Journal of geophysical research. Atmospheres 2014-06, Vol.119 (12), p.7577-7598
• Main Authors: Rosenberg, Philip D., Parker, Douglas J., Ryder, Claire L., Marsham, John H., Garcia-Carreras, Luis, Dorsey, James R., Brooks, Ian M., Dean, Angela R., Crosier, Jonathon, McQuaid, James B., Washington, Richard
• Format: Article
• Language: English
• Subjects: Aircraft; aircraft measurements; Boundary layers; Dust; Eddies; eddy covariance flux; emission; Emission measurements; Flow separation; Fluctuations; Fluid dynamics; Flux; Fluxes; Geophysics; Kinetic energy; Marine; mineral dust; Saltation; subgrid parameterization; Terrain; topography; Turbulence
• ISSN: 2169-897X; 2169-8996; 2169-8996
• DOI: 10.1002/2013JD021255

The first size‐resolved airborne measurements of dust fluxes and the first dust flux measurements from the central Sahara are presented and compared with a parameterization by Kok (2011a). High‐frequency measurements of dust size distribution were obtained from 0.16 to 300 µm diameter, and eddy covariance fluxes were derived. This is more than an order of magnitude larger size range than previous flux estimates. Links to surface emission are provided by analysis of particle drift velocities. Number flux is described by a −2 power law between 1 and 144 µm diameter, significantly larger than the 12 µm upper limit suggested by Kok (2011a). For small particles, the deviation from a power law varies with terrain type and the large size cutoff is correlated with atmospheric vertical turbulent kinetic energy, suggesting control by vertical transport rather than emission processes. The measured mass flux mode is in the range 30–100 µm. The turbulent scales important for dust flux are from 0.1 km to 1–10 km. The upper scale increases during the morning as boundary layer depth and eddy size increase. All locations where large dust fluxes were measured had large topographical variations. These features are often linked with highly erodible surface features, such as wadis or dunes. We also hypothesize that upslope flow and flow separation over such features enhance the dust flux by transporting large particles out of the saltation layer. The tendency to locate surface flux measurements in open, flat terrain means these favored dust sources have been neglected in previous studies. Key Points Aircraft based, size‐resolved, Saharan dust fluxes are measured The eddy covariance method is used with particles up to 300 micrometers Links to terrain, topography and vertical turbulent kinetic energy are discussed