Small cloud particle shapes in mixed-Phase clouds

The shapes of cloud particles with maximum dimensions D_max between 35 and 60 mm in mixed-phase clouds were studied using high-resolution particle images collected by a cloud particle imager (CPI) during the Mixed-Phase Arctic Cloud Experiment (M-PACE) and the Indirect and Semi-Direct Aerosol Campaign (ISDAC). The area ratio a, the projected area of a particle divided by the area of a circle with diameter D_max, quantified particle shape. The differing optical characteristics of CPIs used in M-PACE and ISDAC had no effect on derived a provided that D_max > 35 μm and CPI focus > 45. The fraction of particles with 35 < D_max < 60 μm with α > 0.8 increased with the ratio of liquid water content (LWC) to total water content (TWC). The average α_mean of small particles in each 10-s interval in mixed-phase clouds was correlated with LWC/TWC with a correlation coefficient of 0.60 for M-PACE and 0.43 for ISDAC. The stronger correlation seen during M-PACE was most likely associated with the presence of more liquid droplets that were larger than the CPI detection threshold contributing to amean; the modal effective radius was larger (11 vs 6 μm), and drops with D>35 μm had concentrations during M-PACE that were 6 times as large as those of ISDAC. This study hence suggests that area ratio can be used to identify the phase of particles with 35< D_max <60 mmand questions the assumption used in previous studies that all particles in this size range are supercooled droplets.