The Saharan Air Layer (SAL) is a well-mixed layer of warm, dry, and dusty air of nearly constant water vapor mixing ratio generated by the intense surface heating and strong, dry convection in the Sahara Desert. Although generated in the Sahara, the SAL can extend throughout North Africa and out into the Eastern Atlantic with both radiative and dynamical consequences on the surface energy balance and the development of organized convective systems (including tropical cyclones). Motivated by the strong heterogeneity in both the location and reporting frequency of the 55 North African rawinsondes stations, this study investigates how effectively Level 2 humidity, temperature, and aerosol optical depth data products (2002-2016) from the NASA Aqua satellite’s Atmospheric Infrared Sounder (AIRS), Atmospheric Microwave Sounder Unit (AMSU), and Moderate-resolution Imaging Spectroradiometer (MODIS) are able to resolve and characterize bulk SAL properties over North Africa.
To validate its SAL detection capabilities and indicated bulk properties, AIRS Level 2 support data are evaluated against rawinsondes and compared to model analysis at each of the 55 rawinsonde stations. SAL detection involved a two-step process: 1) algorithm-based detection of well-mixed layers (WMLs) in dry environments (< 7 g kg-1 mixing ratio) 2) identification of SALs by applying Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) back trajectories to determine the history of each WML. WML occurrence rates from AIRS closely resemble that from rawinsondes, yet rates from model analysis were up to 30% higher than observations in the Sahara due to model errors. Despite the overly frequent occurrence of WMLs from model analysis, HYSPLIT trajectory analysis showed that SAL occurrence rates (given a WML exists) from rawinsondes, AIRS, and model analysis were nearly identical. Thus, although the number of WMLs varied amongst the data sources, the proportion of those WMLs which were classified as SALs was nearly the same. SAL bulk properties showed that AIRS and model analysis exhibited a slight warm and moist bias relative to rawinsondes in non-Saharan locations, but model analysis was notably warmer than rawinsondes and AIRS within the Sahara. The latter result is likely associated with the dearth of available data to be assimilated by model analysis in the Sahara. The variability of SAL thicknesses were decently captured by both AIRS and model analysis, but the former favor layers than are thinner than observations. Finally, further analysis of HYSPLIT trajectories revealed that fewer than 10% and 33% of all SAL back trajectories passed through regions with notable precipitation (>100 mm accumulated along the trajectory path) or “dusty” air (> 35 optical depth units) on average, respectively.
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