Roundness and Aspect Ratio of Particles in Ice Clouds

Abstract

The frequency of occurrence of the aspect ratio and roundness of particles in ice clouds from aircraft observations have been examined. Images of cloud particles were measured by a cloud particle imager (CPI) at 2.3-μm resolution, installed on the National Research Council (NRC) of Canada Convair-580. Data were collected in winter midlatitude and polar stratiform clouds associated with frontal systems during three field projects in the Canadian and U.S. Arctic and over the Great Lakes. Approximately 106 images of particles measured in ice clouds were included in the statistics. The frequency of occurrence of the aspect ratio and roundness were calculated in eight 5° temperature intervals from −40°C to 0°C. In each temperature interval, the distributions were calculated for 12 size intervals in the range from 20 μm to 1 mm. It was found that the roundness is a function of particle size and within each size interval it does not depend significantly on temperature. However, the aspect ratio of particles with 60 μm < D < 1000 μm is mainly a function of temperature and does not depend on size. The fraction of spherical particles in ice clouds rapidly decreases with particle size. The fraction of spherical particles in the size range 20 μm < Dmax < 30 μm on average does not exceed 50%. Ice clouds do not contain significant numbers of spherical particles larger than 60 μm. The information on the habits of small ice particles obtained here gives an insight on the mechanisms of ice formation in clouds. The results can be used for parameterization of particle habits in radiation transfer, weather and climate models, and in remote sensing retrievals. It may also be of interest for calculations of collision efficiency in modeling of interaction of cloud particles with moving platforms related to in-flight icing.