Ice properties of single-layer stratocumulus during the Mixed-Phase Arctic Cloud Experiment: 1. Observations

During the Department of Energy’s Atmospheric Radiation Measurement Program’s Mixed-Phase Arctic Cloud Experiment (M-PACE) in fall 2004, the University of North Dakota Citation measured 53 profiles within single-layer stratus clouds by executing spiral ascents and descents over Barrow and Oliktok Point, Alaska, and by flying ramped ascents and descents between. Cloud phase was identified from an algorithm that uses voltage change from the Rosemount ice detector, the size distribution (SD) shape measured by the Forward Scattering Spectrometer Probe (FSSP), and manual identification of particles imaged by the Cloud Particle Imager, the two-dimensional cloud probe (2DC) and the high-volume precipitation sampler (HVPS). Size and mass distribution functions were derived using data from the FSSP, one-dimensional cloud probe, 2DC and HVPS in conjunction with total water content (TWC) measured by the Counterflow Virtual Impactor. With clouds defined as locations where TWC > 0.001 g m-3, there were a total of 513 30-s averaged SDs in single-layer clouds, of which 71% were in mixed-phase parcels, 23% in ice-phase and 6% in liquid-phase. The mixed-phase parcels were dominated by contributions from liquid drops, with the liquid mass fraction fl having averages and standard deviations of 0.89 ± 0.18 with 75% of cases having fl > 0.9. For these single-layer clouds, fl increased with normalized cloud altitude zn, defined as linearly increasing from 0 at cloud base to 1 at cloud top with fl averaging 0.96 ± 0.13 near zn = 1 and 0.70 ± 0.30 near zn = 0. The effective radius of water droplets rew increased with zn, from an average of 6.9 ± 1.8 mm near zn = 0 to 11.4 ± 2.4 mm near zn = 1, whereas the effective radius of ice crystals rei (25.2 ± 3.9 mm) was nearly independent of zn. The averaged cloud droplet number concentration and concentrations of ice crystals with maximum dimensions greater than 53 mm were 43.6 ± 30.5 - 103 L-1 and 2.8 ± 6.9 L-1, respectively, and nearly independent of zn. In contrast to past measurements in mixedphase clouds combined from many geographical locations where fl increased with temperature, fl decreased from -12° to -3°C as clouds typically consisted of a liquid topped layer with precipitating ice below.