Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar

Immler, Franz; Schrems, Otto

doi:10.5194/acp-3-1353-2003

Cited by 28 publications

(24 citation statements)

References 30 publications

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“…Airborne measurements near Dakar above ocean registered dust up to 6 km (McConnell et al, 2008). Off western Africa, NAMMA airborne observations pointed out that the SAL generally extends up to 4 to 6.5 km, with a characteristic temperature inversion at its base at about 2 km (Ismail et al, 2010), which agrees with shipborne lidar measurements in the same region (Immler and Schrems, 2003). Also, SAGE II climatological data reveal one dust layer in the eastern north tropical Atlantic located between 2 and 6 km (Zhu et al, 2007).…”

Section: Summer (Jja)supporting

confidence: 74%

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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Section: Summer (Jja)supporting

confidence: 74%

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

Chédin²,

et al. 2013

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“…The dust aerosol severely affects the local climate and environment by influencing the atmospheric radiation balance and decreasing atmospheric visibility (Wang et al, 2005). Recent years have witnessed numerous studies on the aerosol radiative properties (Blanco et al, 2003;Gobbi et al, 2003;Immler et al, 2003;Collaud Coen et al, 2004;Papayannis et al, 2005;Balkanski et al, 2007;Zhang et al, 2007;Su et al, 2008;Hong et al, 2008).…”

Section: Introductionmentioning

confidence: 99%

A case study of dust aerosol radiative properties over Lanzhou, China

et al. 2010

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Abstract. The vertical distribution of dust aerosol and its radiative properties are analysed using the data measured by the micropulse lidar, profiling microwave radiometer, sunphotometer, particulate monitor, and nephelometer at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) during a dust storm from 27 March to 29 March 2007. The analysis shows that the dust aerosol mainly exists below 2 km in height, and the dust aerosol extinction coefficient decreases with height. The temporal evolution of aerosol optical depth (AOD) during the dust storm is characterized by a sub-maximum at 22:00 (Beijing Time), 27 March and a maximum at 12:00, 28 March. The AOD respectively derived by lidar and sunphotometer shows a good consistency. The PM 10 concentration and aerosol scattering coefficient share similar variation trends, and their maximums both appear at 22:00, 27 March.The aerosol extinction coefficient and relative humidity have the similar trends and their maximums almost appear at the same heights, which presents a correlation between extinction coefficient and relative humidity known as aerosol hygroscopicity. The relative humidity is related with temperature, and then the temperature will affect the aerosol extinction properties by modifying the relative humidity condition.The aerosol extinction coefficient, scattering coefficient, and PM 10 concentration present good linear correlations. The correlation coefficients of the aerosol scattering coefficients of 450, 520, and 700 nm and PM 10 concentration, of aerosol extinction coefficient retrieved by lidar at 532 nm and PM 10 concentration, and of aerosol extinction and scattering coefficient are respectively 0.98, 0.94, and 0.96.

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“…The mean value of δ a for maritime aerosols in the planetary boundary layer over the entire observation period was about 0.1, while the mean value of δ a for a dust layer was about 0.4. Other observational studies with polarized‐lidar measurements also reported δ a values higher than 0.1 for a dust layer and lower than 0.1 for a nondust layer [e.g., Murayama et al , 1999; Liu et al , 2002; Immler and Schrems , 2003; Müller et al , 2003]. On the basis of these measurements, we choose the dust model when δ a > 0.1 and the sea salt model when δ a < 0.1.…”

Section: Algorithm To Estimate the Aerosol Optical Propertiesmentioning

confidence: 99%

“…The aerosol depolarization ratio ( δ a ) is defined as the ratio of the perpendicular component of the aerosol backscattering coefficient to the parallel component [e.g., Immler and Schrems , 2003]. δ a can be derived from lidar polarization measurement using the following equation: where δ obs is the total depolarization ratio, defined as the ratio of the perpendicular component to the parallel component of the power received by a detector from the linearly polarized transmitted beam [e.g., Kobayashi et al , 1985; Gobbi and Barnaba , 2003].…”

Section: Algorithm To Estimate the Aerosol Optical Propertiesmentioning

confidence: 99%

An algorithm that retrieves aerosol properties from dual‐wavelength polarized lidar measurements

Nishizawa¹,

Okamoto²,

Sugimoto³

et al. 2007

J. Geophys. Res.

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[1] We developed two types of algorithms, backward and forward, to estimate vertical profiles of extinction coefficient at a wavelength of l = 532 nm for each aerosol type, using information from three-channel Mie-lidar measurements, i.e., copolarization and cross-polarization components at l = 532 nm and total component (copolarization and cross polarization) at l = 1064 nm. The mode radii, standard deviations, and refractive index for each aerosol type are assumed in the algorithms. The algorithms have the following main features: (1) Extinction coefficient of total aerosols at a far end is estimated in the backward algorithm, while the value at a far end is prescribed as the boundary condition in the Fernald method. (2) They determine aerosol types, i.e., water soluble, sea salt, or dust, for each layer. (3) The vertical profiles of microphysical properties of aerosols such as lidar ratio are also estimated. The backward algorithm is first applied to the lidar signals only calibrated for the spectral ratio to the total components to derive the calibration constant and aerosol properties under clear-sky condition. Next, the forward algorithm is used to retrieve aerosol properties under cloud bottom. We performed intensive error analyses. The errors for each aerosol component in the extinction coefficient are found to be smaller than 20% (50%) for the backward (forward) algorithms, respectively, when measurement errors are ±5%. The validation of the algorithms from the comparison against sky radiometer measurements over ocean shows that the optical thickness agrees within 2% (10%) for the backward (forward) algorithms.Citation: Nishizawa, T., H. Okamoto, N. Sugimoto, I. Matsui, A. Shimizu, and K. Aoki (2007), An algorithm that retrieves aerosol properties from dual-wavelength polarized lidar measurements,

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Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar

Cited by 28 publications

References 30 publications

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

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

A case study of dust aerosol radiative properties over Lanzhou, China

An algorithm that retrieves aerosol properties from dual‐wavelength polarized lidar measurements

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