Saharan air layers sampled by South Florida's Cloud-Aerosol-Rain-Observatory: What can we learn?

The areal coverage of Saharan dust over the Atlantic ocean is large enough to produce a climate-scale radiative forcing. Much of what is known about the Saharan Air Layer (SAL) is based on analysis from the eastern Atlantic, and SAL properties after long-range transport have been characterized less often. Here we examine the vertical structure, radiative impact, potential cloud impact and air mass history of Saharan air layers arriving at Miami. The analysis relies on measurements taken by a depolarization lidar, sun photometers, broadband radiometers, surface-based cloud condensation nuclei counter from South Florida's Cloud-Aerosol-Rain-Observatory (CAROb; http://carob.rsmas.miami.edu) since May 2013, combined with surface-based filter samples. The measurements to date show that approximately half of the Saharan air layers are incorporated into the boundary layer, with most of the aerosol presence located between 1-3 km in height. During those time periods, cloud condensation nuclei concentrations decrease relative to the background state, in contrast to increases noted at Barbados. This can be interpreted as a more polluted background state at Miami as compared to Barbados. Backtrajectories suggest the dust often benefits from large-scale ascent within the inter-tropical convergence zone during transport, before subsiding under the influence of the north Atlantic anticyclone. Mixing with air originally from the northeast Atlantic may explain a lower depolarization ratio compared to measurements closer to the source regions. The impact of the dust on the broadband surface radiation, both shortwave and longwave, will also be discussed.