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Arctic boundary-layer clouds were investigated with remote sensing and insitu instruments during the Arctic Study of Tropospheric Aerosol, Clouds andRadiation (ASTAR) campaign in March and April 2007. The clouds formedin a cold air outbreak over the open Greenland Sea. Beside the predominantmixed-phase clouds pure liquid water and ice clouds were observed. Utilizingmeasurements of solar radiation reflected by the clouds three methods toretrieve the thermodynamic phase of the cloud are introduced and compared.Two ice indices IS and IP were obtained byanalyzing the spectral pattern of the cloud top reflectance in the nearinfrared (1500–1800 nm wavelength) spectral range which is characterized byice and water absorption. While IS analyzes the spectral slopeof the reflectance in this wavelength range, IS utilizes aprinciple component analysis (PCA) of the spectral reflectance. A third iceindex IA is based on the different side scattering of sphericalliquid water particles and nonspherical ice crystals which was recorded insimultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that IS, IPand IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectivelywith lowest values indicating pure liquid water clouds and highest valuespure ice clouds. The spectral slope ice index IS and the PCAice index IP are found to be strongly sensitive to theeffective diameter of the ice crystals present in the cloud. Therefore, theidentification of mixed-phase clouds requires a priori knowledge of the icecrystal dimension. The reflectance-albedo ice index IA ismainly dominated by the uppermost cloud layer (τ<1.5). Therefore,typical boundary-layer mixed-phase clouds with a liquid cloud top layer willbe identified as pure liquid water clouds. All three methods were applied tomeasurements above a cloud field observed during ASTAR 2007. Thecomparison with independent in situ microphysical measurements shows theability of the three approaches to identify the ice phase in Arcticboundary-layer clouds

Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: test of three approaches