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        Evolution of the ice phase in tropical altocumulus : SAMUM lidar observations over Cape Verde

        Author
        Ansmann, A.
        Tesche, Matthias
        Seifert, P.
        Althausen, D.
        Engelmann, R.
        Fruntke, J.
        Wandinger, U.
        Mattis, I.
        Mueller, D.
        Attention
        2299/11261
        Abstract
        The formation of the ice phase in tropical altocumulus has been studied with multiwavelength aerosol-cloud Raman lidar, wind Doppler lidar, and radiosonde, providing information on geometrical and optical properties, cloud phase, cloud top temperature, updraft and downdraft velocity, and fall speed of ice crystals. The observations were conducted at Praia (15 degrees N, 23.5 degrees W), Cape Verde, in the tropical North Atlantic in the framework of the Saharan Mineral Dust Experiment (SAMUM) project in January and February 2008. More than 200 different altocumulus layers were analyzed. The coldest liquid cloud had a temperature of -36 degrees C and appeared at a height of 9800 m. Tropical altocumulus is found to be geometrically (262 +/- 137 m) and optically thin (0.69 +/- 0.61), mostly short-lived, and horizontally small with extents of less than 50 km in 80% of the cases. A clear relationship between the occurrence of the ice phase in altocumulus and cloud top temperature is observed, even more clear after the removal of effects of cloud seeding, which is found to be an important process of ice production in lower layers of multilayer altocumulus systems. Because almost all altocumulus layers (99%) showed a liquid cloud top (region in which ice nucleation begins), we conclude that deposition and condensation ice nucleation are unimportant processes during the initial phase of altocumulus glaciation. A pronounced impact of aerosols such as mineral particles known to be favorable ice nuclei is not found in this region with strong dust-smoke outbreaks from Africa. The different phases of an almost complete life cycle of an altocumulus were monitored over 5 hours. The observed processes of droplet and ice formation are discussed based on height-resolved depolarization-ratio (cloud phase) and vertical-velocity time series.
        Publication date
        2009-09-16
        Published in
        Journal of Geophysical Research: Atmospheres
        Published version
        https://doi.org/10.1029/2008JD011659
        Other links
        http://hdl.handle.net/2299/11261
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