The Cloud, Aerosol Polarization and Backscatter Lidar at Summit, Greenland

Signatures of climate change are known to be most evident in the polar regions. Thus, it is not surprising that concurrent with the dramatic sea-ice losses, the Greenland Ice Sheet is experiencing similar rapid melting. Detailed information on cloud amount and type is needed to accurately determine the effect of climate change on snowmelt by using energy-balance in global climate models. Precise and accurate measurements of cloud properties over Greenland are necessary to document the full range of cloud conditions and characteristics throughout the Arctic. The Cloud, Aerosol Polarization and Backscatter Lidar (CAPABL) has been developed to address this need by measuring depolarization, particle orientation and the backscatter of clouds and aerosols. The lidar is located at Summit, Greenland (72.6º N, 38.5º W; 3200 AMSL) as part of the NSF's Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit (ICECAPS) project. Here the instrument is described with particular emphasis placed upon the implementation of new polarization methods developed to measure particle orientation and improve the overall accuracy of lidar depolarization measurements. Initial results from the lidar are also shown to demonstrate the ability of the lidar to observe cloud properties. The data shown demonstrate CAPABL's ability to detect polarization signatures that may be used to assess the occurrence of horizontally oriented ice crystals (HOIC). HOIC lead to increased cloud albedo, which leads to a proportional reduction in the surface solar flux. Thus, an accurate long-term record of the occurrence of HOIC, in conjunction with the full array of cloud parameters collected by ICECAPS, is needed to understand the consequences orientation may have for the heating of the atmosphere and the surface. Also, HOIC, when scattering normal to its surface, can exhibit low linear depolarization ratios that can result in erroneous classification of thermodynamic phase. Diattenuation measurement enables lidar systems to detect oriented scatterers within the same dynamic range as other cloud signals. This new observational method therefore allows for an easier and more certain means of collecting comprehensive observations of clouds and oriented particles.