Maintenance of Lower Tropospheric Temperature Inversion in the Saharan Air Layer by Dust and Dry Anomaly

Wong, Sun; Dessler, A. E.; Mahowald, N. M.; Yang, Ping; Qian, Feng

doi:10.1175/2009jcli2847.1

Cited by 60 publications

(83 citation statements)

References 72 publications

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“…These inversions help maintain the integrity of the layer. The dust that defines the layer also helps to maintain its thermal structure [164]. At the top of the SAL lies a region of high relative humidity where altocumulus and stratocumulus layers are often observed [162].…”

Section: Saharan Air Layermentioning

confidence: 99%

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

2013

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The West African Sahel is well known for the severe droughts that ravaged the region in the 1970s and 1980s. Meteorological research on the region has flourished during the last decade as a result of several major field experiments. This paper provides an overview of the results that have ensued. A major focus has been on the West African monsoon, a phenomenon that links all of West Africa. The characteristics and revised picture of the West African monsoon are emphasized. Other topics include the interannual variability of rainfall, the atmospheric circulation systems that govern interannual variability, characteristics of precipitation and convection, wave activity, large-scale factors in variability (including sea-surface temperatures), and land-atmosphere relationships. New paradigms for the monsoon and associated ITCZ and for interannual variability have emerged. These emphasize features in the upper atmosphere, as well as the Saharan Heat Low. Feedback mechanisms have also been emphasized, especially the coupling of convection with atmospheric dynamics and with land surface characteristics. New results also include the contrast between the premonsoon and peak monsoon seasons, two preferred modes of interannual variability (a latitudinal displacement of the tropical rainbelt versus changes in its intensity), and the critical importance of the Tropical Easterly Jet.

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Section: Saharan Air Layermentioning

confidence: 99%

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

2013

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“…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, thus keeping the SAL relatively warm and stable in relation to its environment as it crosses the Atlantic (Karyampudi et al, 1999). However, both the dust and low humidity are essential for maintaining the temperature structure from thermal relaxation in the SAL (Wong et al, 2009). …”

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confidence: 99%

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

Chédin²,

et al. 2013

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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...

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“…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). …”

Section: Potential Source Areas and Transport Historymentioning

confidence: 99%

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

Marsden¹,

Ullrich²,

Möhler³

et al. 2018

Preprint

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Abstract. The mineralogy and mixing state of dust particles originating from the African continent influences climate and marine ecosystems in the North Atlantic due to its effect on radiation, cloud properties and biogeochemical cycling. Single-particle mineralogy and mixing state is particularly important in many processes but is difficult to predict because of large temporal and spatial variability and the lack of in-situ measurements of dust properties during emission, transport and deposition. This lack of measurements is in part due to the remoteness of potential source areas (PSA) and transport pathways, but also because of 5 the lack of an efficient method to report the mineralogy and mixing state of single particles with a time resolution comparable to atmospheric processes.In this work, the mineralogy and mixing state of the fine fraction (< 2.5µm) in laboratory suspended dust from the Sahara and Sahel were made using novel techniques with on-line single-particle mass spectrometry (SPMS) and traditional off-line scanning electron microscopy (SEM). A regional difference in mineralogy was detected, with material sourced from Morocco 10 contained a high number fraction of illite like particles in contrast to Sahelian material which contains potassium and sodium depleted clay minerals like kaolinite. Applying the same methods to ambient measurement of transported dust in the marine boundary layer at Cabo Verde in the remote North Atlantic enabled the number fractions of illite/smectite clay mineral (ISCM), non-ISCM, and calcium containing particles to be reported at a 1 hour time resolution over a 20 day period alongside internal mixing with nitrate, sulphate and organic/biological material. The ISCM and nitrate content was found to change significantly 15 between distinct dust events, indicating a shift in source and transport pathways which may not be captured in off-line composition analysis or remote sensing techniques.The results show SPMS and SEM techniques are complimentary and demonstrate that SPMS can provide a meaningful high resolution measurement of single-particle mineralogy and mixing state in laboratory and ambient conditions. In most cases, the mineralogy varies continuously between particles rather than a collection of discrete mineral phases. These techniques 20 will be useful in resolving the complexity of mineral dust transport and in obtaining atmospherically relevant test material for laboratory experiments of dust properties.

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Maintenance of Lower Tropospheric Temperature Inversion in the Saharan Air Layer by Dust and Dry Anomaly

Cited by 60 publications

References 72 publications

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

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

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

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