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| Titel |
The seasonal vertical distribution of the Saharan Air Layer and its modulation by the wind |
| VerfasserIn |
C. Tsamalis, A. Chédin, J. Pelon, V. Capelle |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 22 ; Nr. 13, no. 22 (2013-11-19), S.11235-11257 |
| Datensatznummer |
250085823
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| Publikation (Nr.) |
 copernicus.org/acp-13-11235-2013.pdf |
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| Zusammenfassung |
| 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. |
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