Evaluation of Infrared and Microwave Sounding Data for Characterizing the Dynamics and Evolution of the Saharan Air Layer in West Africa

The Saharan Air Layer (SAL) is a well-mixed layer of warm, dry, and dusty air of nearly constant mixing ratio generated by the intense surface heating and strong, dry convection in the Sahara Desert. Although generated in the Sahara, this layer's influence extends throughout northern Africa and into the eastern Atlantic Ocean with both radiative and dynamical consequences on the surface energy balance and the development of organized convective systems including tropical cyclones. Given the limited coverage of radiosonde stations in and around the Sahara Desert, this study investigates how effectively Level 2 humidity, temperature, and aerosol optical depth data products (2003-2015) 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 the bulk SAL properties over northern Africa.

The Aqua AIRS, AMSU, and MODIS products were validated against the regional radiosonde network and compared to model re-analysis to quantify differences between native data products over assimilated model products. Level 2 AIRS/AMSU support data products provide 100 levels of temperature and specific humidity (near surface to 0.1 hPa) in conditions ranging from clear skies to 90% cloud cover. The support product is interpolated from the 28 level AIRS/AMSU standard product but contains additional experimental and diagnostic data useful for data analysis and interpretation. The MODIS Aqua instrument Deep Blue aerosol optical depth product (550 nm) were used to characterize the spatial and temporal variability of dust loads within the SAL.

For this study, we developed an algorithm that automates the detection of dry (< 7 g kg^-1 mixing ratio) isentropic layers of nearly constant mixing ratio in radiosonde, AIRS/AMSU, and reanalysis profiles over a range of climate zones (coastal, savanna, Sahel, Sahara). Because AIRS/AMSU standard product levels are generated at fixed pressure levels (e.g., 1000 hPa, 925 hPa, 850 hPa), most of the detected well-mixed layers occur in desert and Sahelian profiles where well-mixed layers extend more than 500 m rather than in coastal areas where radiosonde data indicates mixed layers as shallow as 50-70 m. Re-analysis and radiosonde data were more capable than AIRS/AMSU in detecting shallow layers due to their comparatively higher vertical resolution. For areas outside the Sahara Desert, we performed trajectory analysis to distinguish true Saharan air influx from local residual and subsidence layers. The trajectory analysis used the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model that ingests reanalysis wind data to discern the origin of all detected well-mixed layers. Although vertically coarse, AIRS/AMSU-based SAL data does provide a more spatially continuous view of the bulk properties of the SAL over the last 12 years than the sparse and often unreliable regional radiosonde network and a more accurate view of the height and depth of the SAL at inland stations than model reanalysis.