Small cloud particle shapes in mixed-phase clouds

 
Poster PDF

Authors

Greg McFarquhar — University of Oklahoma
Junshik Um — University of Oklahoma
Robert Jackson — Argonne National Laboratory

Category

Infrastructure & Outreach

Description

The shapes of cloud particles with maximum dimensions (Dmax) between 35 and 60 micrometers 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 Indirect and Semi-Direct Aerosol Campaign (ISDAC). The area ratio (alpha), the projected area of a particle divided by the area of a circle with diameter Dmax, quantified particle shape. The differing optical characteristics of CPIs used in M-PACE and ISDAC had no effect on derived alpha provided Dmax > 35 micrometers and CPI focus > 45. The fraction of particles with 35 < Dmax < 60 micrometers with alpha > 0.8 increased with the ratio of liquid water content (LWC) to total water content (TWC). The average alpha_mean of small particles in each 10-second 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 larger than the CPI detection threshold contributing to alpha_mean; the modal effective radius was larger (11 micrometers compared to 6 micrometers), and drops with D > 35 micrometers had 6 times larger concentrations during M-PACE compared to ISDAC. This study hence suggests that area ratio can be used to identify the phase of particles with 35 < Dmax < 60 micrometers and questions the assumption used in previous studies that all particles in this size range are supercooled droplets. The impacts on the representation of mixed-phase clouds in numerical models are discussed.