Aerosol vertical distribution in dust outflow over the Atlantic: Comparisons between Global Earth Observing System (GEOS)-Chem and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

Vertically-resolved attenuated backscatter from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission and Aerosol Optical Thickness (AOT)from Moderate-resolution Imaging Spectroradiometer (MODIS) are used to characterize the Saharan dust outflow over the Atlantic and to evaluate results from a global chemical and transport model (GEOS-Chem). We first document two events of dust plume transport from the Sahara to the American coast representative of summer and winter conditions. The GEOSChem model reproduces fairly well the aerosol patterns over the Atlantic although observed AOT are overestimated close to the source regions and underestimated in the remote regions, suggesting a too weak transport toward the western Atlantic and/or a too strong deposition over the Caribbean sea. Observed and model-derived attenuated backscatter profiles agree qualitatively well, suggesting that the altitudes of the aerosol layers over the Atlantic is well reproduced. We find that dust plumes extend up to 6 km in summer and up to 4 km in winter over the Atlantic close to the source regions and that they gradually descent throughout their travel over the Atlantic. We then use our simulation to characterize the deposition flux of dust in this region. Half of the dust deposited on the surface of the open ocean in 2006 in this area occurs during summer, 20% during each of winter and spring, and 10% in the fall. During the July 2006 one-week dust episode that we investigated, 5 Tg of dust were deposited (4% of the annual total in the region).