Physical properties of High Arctic tropospheric particles during winter

L. Bourdages, T. J. Duck, G. Lesins, J. R. Drummond, and E. Eloranta, Atmospheric Chemistry and Physics, 9, 6881 - 6897, 2009.

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Abstract. A climatology of particle scattering properties in the wintertime High Arctic troposphere, including vertical distributions and effective radii, is presented. The measurements were obtained using a lidar and cloud radar located at Eureka, Nunavut Territory (80N, 86W). Four different particle groupings are considered: boundary-layer ice crystals, ice clouds, mixed-phase clouds, and aerosols. Two-dimensional histograms of occurrence probabilities against depolarization, radar/lidar colour ratio and height are given. Colour ratios are related to particle minimum dimensions (i.e., widths rather than lengths) using a Mie scattering model. Ice cloud crystals have effective radii spanning 25-220 microns, with larger particles observed at lower altitudes. Topographic blowing snow residuals in the boundary layer have the smallest crystals at 15-75 microns. Mixed-phase clouds have water droplets and ice crystal precipitation in the 5-40 micron and 40-200 micron ranges, respectively. Ice cloud crystals have depolarization decreasing with height. The depolarization trend is associated with the large ice crystal sub-population. Small crystals depolarize more than large ones in ice clouds at a given altitude, and show constant modal depolarization with height. Ice clouds in the mid-troposphere are sometimes observed to precipitate to the ground. Water clouds are constrained to the lower troposphere (0.5-3.5 km altitude). Aerosols are most abundant near the ground and are frequently mixed with the other particle types. The data are used to construct a classification chart for particle scattering in wintertime Arctic conditions.



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