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Journal Article: BibTeX citation key:  Ansmann2008a
Ansmann, A., Tesche, M., Althausen, D., Müller, D., Seifert, P., Freudenthaler, V., Heese, B., Wiegner, M., Pisani, G., Knippertz, P. & Dubovik, O. (2008) Influence of Saharan dust on cloud glaciation in southern Morocco during the Saharan Mineral Dust Experiment. IN Journal of Geophysical Research - Atmospheres, 113. D04210.
Added by: Peter Knippertz 2010-08-20 14:13:09    Last Edited by: Fanny Lefebvre 2011-01-17 14:07:12
 B  
Categories: Atmospheric processes
Keywords: Aerosol, Atmospheric Boundary Layer, Clouds - Convection, Diurnal cycle, Observation system, Precipitation, Rainfall, Semi-arid
Creators: Althausen, Ansmann, Dubovik, Freudenthaler, Heese, Knippertz, Müller, Pisani, Seifert, Tesche, Wiegner
Collection: Journal of Geophysical Research - Atmospheres

Peer reviewed
Number of views:  924
Popularity index:  56.14%
Maturity index:  published

 
Abstract
Multiwavelength lidar, Sun photometer, and radiosonde observations were conducted at Ouarzazate (30.9°N, 6.9°W, 1133 m above sea level, asl), Morocco, in the framework of the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006. The field site is close to the Saharan desert. Information on the depolarization ratio, backscatter and extinction coefficients, and lidar ratio of the dust particles, estimates of the available concentration of atmospheric ice nuclei at cloud level, profiles of temperature, humidity, and the horizontal wind vector as well as backward trajectory analysis are used to study cases of cloud formation in the dust with focus on heterogeneous ice formation. Surprisingly, most of the altocumulus clouds that form at the top of the Saharan dust layer, which reaches into heights of 4–7 km asl and has layer top temperatures of −8°C to −18°C, do not show any ice formation. According to the lidar observations the presence of a high number of ice nuclei (1–20 cm−3) does not automatically result in the obvious generation of ice particles, but the observations indicate that cloud top temperatures must typically reach values as low as −20°C before significant ice production starts. Another main finding is that liquid clouds are obviously required before ice crystals form via heterogeneous freezing mechanisms, and, as a consequence, that deposition freezing is not an important ice nucleation process. An interesting case with cloud seeding in the free troposphere above the dust layer is presented in addition. Small water clouds formed at about −30°C and produced ice virga. These virga reached water cloud layers several kilometers below the initiating cloud cells and caused strong ice production in these clouds at temperatures as high as −12°C to −15°C.
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Added by: Fanny Lefebvre    Last Edited by: Fanny Lefebvre