Aerosol vertical distribution in dust outflow over the Atlantic: Comparisons between 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 the 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. Observed and model-derived attenuated backscatter profiles agree qualitatively well, suggesting that the altitudes of the aerosol layers over the Atlantic are well reproduced by the model. In both the model and the observations, dust plumes extend up to 8 km in summer and up to 4 km in winter over the Atlantic close to the source regions and gradually descend throughout their travel over the Atlantic. We find however that, in summertime, observed AOT are overestimated by the model 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. 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 a 1-week dust episode in July 2006 that we investigated, 5 Tg of dust were deposited (4% of the annual total in the region).