Small Particles in Mixed-Phase Clouds: Ice or Water?
Mixed-phase clouds, in which super-cooled water droplets and ice crystals coexist in the same volume of air, persist for long time periods over the Arctic due to a delicate balance between cloud-top radiative cooling, microphysical heating, ice sedimentation, and large-scale forcing. Because mixed-phase clouds are radiatively significant and thermodynamic phase affects cloud radiative properties, knowledge of phase distribution is critical for understanding the role of mixed-phase clouds in the climate system. The phases of small particles are especially important because they can contribute to more than half of the total extinction in the clouds and are also important for understanding nucleation processes occurring in the clouds. Typically, small particles in mixed-phase clouds are assumed to be liquid, while larger particles are assumed to be ice. Atmospheric System Research program researchers have used in situ aircraft measurements from two recent Atmospheric Radiation Measurement (ARM) field campaigns in the Arctic to challenge those assumptions. They performed detailed image analysis of particles with maximum diameters of less than 60 microns taken during the two campaigns. The researchers were able to identify particle sizes and probe focusing conditions under which reliable information about such small particles could be obtained. For each image, they calculated the area ratio and projected area of a particle divided by a circle with diameter equal to the maximum particle diameter, showing that the average area ratio of the small cloud particles was correlated with the ratio of liquid water content to total water content. A stronger correlation was found when large cloud droplets were present. This analysis indicated that a large average area ratio could be used to discriminate liquid cloud droplets from small ice crystals. The study’s most important finding was that the assumption that all small particles in mixed-phase clouds are super-cooled water droplets does not hold true. This finding may have important ramifications for developing parameterizations of single scattering and sedimentation properties in mixed-phase clouds and retrieving cloud properties from ground- and satellite-based remote sensors.
McFarquhar, G. M., J. Um, and R. Jackson. 2013. “Small Cloud Particle Shapes in Mixed-Phase Clouds,” Journal of Applied Meteorology and Climatology 52, 1277–93. DOI: 10.1175/JAMC-D-12-0114.1.