Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 4 2008 10.5194/acpd-8-15901-2008 http://www.atmos-chem-phys-discuss.net/8/15901/2008/ http://www.atmos-chem-phys-discuss.net/8/15901/2008/acpd-8-15901-2008.html http://www.atmos-chem-phys-discuss.net/8/15901/2008/acpd-8-15901-2008.pdf 15901 15939 2008-08-20 Cloud phase identification of low-level Arctic clouds from airborne spectral radiation measurements: test of three approaches A. Ehrlich ehrlicha@uni-mainz.de E. Bierwirth M. Wendisch J.-F. Gayet G. Mioche A. Lampert J. Heintzenberg Johannes Gutenberg-University Mainz, Institute for Atmospheric Physics, Mainz, Germany Laboratoire de Météorologie Physique (LAMP), Univ. Blaise Pascal, Aubière Cedex, France Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany Leibniz-Institute for Tropospheric Research, Leipzig, Germany Boundary layer clouds were investigated with a complementary set of remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds that formed in a cold air outbreak over the open Greenland sea showed a variety in their thermodynamic state. Beside the predominant mixed-phase clouds pure liquid and ice clouds were observed. Utilizing the measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud were defined and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) characterized by ice and water absorption. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of cloud albedo and reflectance. Radiative transfer simulations showed that IS, IP and IA range between 5 to 80, 0 to 20 and 1 to 1.25, respectively, with lowest values indicating pure liquid water clouds and highest values pure ice clouds. IS and IP were found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. IA has the disadvantage that this index is mainly dominated by the uppermost cloud layer (τ<1.5). Typical boundary layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. 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