Abstract
Abstract. The Saharan Air Layer (SAL) influences large-scale 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 advection for SAL parts, especially during summer.
Highlights
It is well known that large quantities of Saharan dust are transported across the tropical Atlantic throughout the year as a result of large-scale Saharan dust outbreaks, with the maximum number of events occurring during summer
A first step to this end has been realized during the AMMA campaign, where the impact of Saharan Air Layer (SAL) on African Easterly Waves has been examined (Zipser et al, 2009; Ismail et al, 2010; Lafore et al, 2011)
Five-year CALIPSO observations of desert dust and polluted dust layers are used to depict the seasonal vertical distribution of SAL with 1◦ horizontal resolution. These results offer a better description of the SAL, not accessible up to now, at least vertically
Summary
It is well known that large quantities of Saharan dust are transported across the tropical Atlantic throughout the year as a result of large-scale Saharan dust outbreaks, with the maximum number of events occurring during summer. The SAL long-range transport is enhanced by the persistent temperature inversions that exist at its base and top, in part because of the interaction of dust aerosols with radiation, keeping the SAL relatively warm and stable in relation to its environment as it crosses the Atlantic (Karyampudi et al, 1999). 2008; Liu et al, 2008a; Ben-Ami et al, 2009; Braun, 2010; Yu et al, 2010; Adams et al, 2012; Ridley et al, 2012; Yang et al, 2012), restricted to the description of dust vertical distribution by presenting means at the regional scale or were less focused In these studies mainly previous versions of CALIPSO data have been used, which showed significant errors regarding the discrimination between clouds and aerosols.
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