The seasonal vertical distribution of the Saharan Air Layer and its modulation by the wind

Abstract

Abstract. The Saharan Air Layer (SAL) influences largescale environment from western Africa to eastern tropical Americas, by carrying large amounts of dust aerosols. However, the vertical distribution of the SAL is not well established due to a lack of systematic measurements away from the continents. This can be overcome by using the observations of the spaceborne lidar CALIOP onboard the satellite CALIPSO. By taking advantage of CALIOP's capability to distinguish dust aerosols from other types of aerosols through depolarization, the seasonal vertical distribution of the SAL is analyzed at 1 • horizontal resolution over a period of 5 yr (June 2006-May 2011. This study shows that SAL can be identified all year round displaying a clear seasonal cycle. It occurs higher in altitude and more northern in latitude during summer than during winter, but with similar latitudinal extent near Africa for the four seasons. The south border of the SAL is determined by the Intertropical Convergence Zone (ITCZ), which either prohibits dust layers from penetrating it or reduces significantly the number of dust layers seen within or south of it, as over the eastern tropical Atlantic. Spatially, near Africa, it is found between 5 • S and 15 • N in winter and 5-30 • N in summer. Towards the Americas (50 • W), SAL is observed between 5 • S and 10 • N in winter and 10-25 • N in summer. During spring and fall, SAL is found between the position of winter and summer not only spatially but also vertically. In winter, SAL occurs in the altitude range 0-3 km off western Africa, decreasing to 0-2 km close to South America. During summer, SAL is found to be thicker and higher near Africa at 1-5 km, reducing to 0-2 km in the Gulf of Mexico, farther west than during the other seasons. SAL is confined to one layer, of which the mean altitude decreases with westward transport by 13 m deg −1 during winter and 28 m deg −1 , after 30 • W, during summer. Its mean geometrical thickness decreases by 25 m deg −1 in winter and 9 m deg −1 in summer. Spring and fall present similar characteristics for both mean altitude and geometrical thickness. Wind plays a major role not only for the transport of dust within the SAL but also by sculpting it. During winter, the trade winds transport SAL towards South America, while in spring and summer they bring dust-free maritime air masses mainly from the North Atlantic up to about 50 • W below the SAL. The North Atlantic westerlies, with their southern border occurring between 15 and 30 • N (depending on the season, the longitude and the altitude), prevent the SAL from developing further northward. In addition, their southward shift with altitude gives SAL its characteristic oval shape in the northern part. The effective dry deposition velocity of dust particles is estimated to be 0.07 cm s −1 in winter, 0.14 cm s −1 in spring, 0.2 cm s −1 in summer and 0.11 cm s −1 in fall. Finally, the African Easterly Jet (AEJ) is observed to collocate with the maximum dust load of the SAL, and this might promote the differential...

Profiling of Saharan dust from the Caribbean to western Africa – Part 1: Layering structures and optical properties from shipborne polarization/Raman lidar observations

“…Systematic studies of the east-to-west dust transport with the satellite lidar CALIOP (Cloud and Aerosol Lidar with Orthogonal Polarization) were then presented by Liu et al (2008a, b). Further Saharan dust studies over the tropical Atlantic based on CALIOP measurements can be found in Adams et al (2012) and Tsamalis et al (2013). The latter authors characterized the decay of the Saharan dust amount in terms of layer descent and deposition velocity.…”

Seasonal cycle of desert aerosols in western Africa: analysis of the coastal transition with passive and active sensors

“…In boreal summer, SAL is characterized by hot, dry, very dust-laden air, and it is located between 10 and 25 • N (Dunion and Marron, 2008;Tsamalis et al, 2013). This SAL is marked by very strong potential temperatures up to 40 • C and a radon presence ( 222 radon) indicating the desert origin of air masses (Carlson and Prospero, 1972).…”

“…In winter, the SAL is characterized by the transport of dust containing chemical elements such as aluminum (Al), silicon (Si), iron (Fe), titanium (Ti), and manganese (Mn; e.g., Formenti et al, 2001;Ben-Ami et al, 2010) and is located between 5 and 10 • N (e.g., Tsamalis et al, 2013). Some studies relating aerosols to their transport are generally a simple description of the vertical distribution of aerosols in the SAL (Generoso et al, 2008;Liu et al, 2008;Ben-Ami et al, 2009;Braun, 2010;Yu et al, 2010;Adams et al, 2012;Ridley et al, 2012;Yang et al, 2012) or a description of the seasonality of the SAL in connection with large-scale dynamics (Liu et al, 2012;Tsamalis et al, 2013).…”

Mineralogy and mixing state of North African mineral dust by on-line single-particle mass spectrometry

“…During the summer months, the westward moving SAL typically occurs 5 at relatively high altitudes (1-6km) in the Cabo Verde area (Tsamalis et al, 2013), and the warm, dry, dust-laden air creates a stable layer that is separated from the lower levels by a strong temperature inversion (Wong et al, 2009). …”