UCLA

Desert dust plays an important role in several components of the Earth system, mainly due to its nutrient content and interactions with radiation and clouds. Nevertheless, the airborne lifetime of coarse dust particles and their atmospheric transport from source regions are severely underestimated by current climate models. For instance, Saharan dust particles with diameters as large as 30 µm have repeatedly been detected in the Americas, while models are unable to reproduce this phenomenon. In this dissertation, we investigated the effects of vertical turbulent mixing in elevated shear layers on particle deposition rates. We focused on the Saharan Air Layer (SAL), a dust-laden air mass situated above the Marine Boundary Layer (MBL) that periodically moves from the African continent towards the Caribbean region over the tropical North Atlantic. Using an analytical approach combined with large-eddy simulations (LES), we first present a theory for dust concentration profiles evolving under the effects of gravitational settling and turbulent mixing. We found that the increase in dust airborne lifetime due to turbulence is determined uniquely by the Peclet number Pe (the ratio of the mixing timescale to the settling timescale), and it is limited to a factor of 2 when compared with laminar flows. Afterwards, we quantify the decrease in shear-induced eddy mixing rates caused by stable buoyancy stratification (which is typically present in the SAL). Wefound that, under weak stratification, SAL eddy diffusivities can be calculated as a function of layer depth, shear magnitude, and gradient Richardson number Rig . Our estimates indicate that even small diffusivity values can significantly delay the deposition of particles as large as super coarse dust (with a diameter greater than 10 µm). In fact, we show that, together with particle asphericity, turbulent mixing can explain to a great extent the presence of super coarse Saharan dust in the Caribbean observed during the SALTRACE field campaign. Finally, we also note that the diffusivities calculated for our elevated SAL set-up decay faster with Rig than typical ABL models, highlighting the importance of employing appropriate parameterization schemes in climate models to represent slow processes (particularly affected by small diffusivities values) accurately.