Small cloud particle shape and its phase determination in mixed-phase clouds

This study focuses on better defining the shapes of particles with maximum dimensions between 15 µm and 50 µm in mixed-phase clouds. In-situ measurements in stratiform mixed-phase clouds were obtained during the Alliance Icing Research Study II (AIRS II) held during November and December of 2003 in the Great Lakes Region. An algorithm applied to each 10 second sample of in-situ data identifies the cloud phase as either ice, mixed, or liquid by determining if the voltage change of the Rosemount icing detector exceeds a given threshold, by examining the spectral shape of the small particle size distribution measured by the forward scattering spectrometer probe and by inspecting cloud particle images from the two-dimensional cloud probe, high volume precipitation sampler and Cloud Particle Imager (CPI). For mixed-phase clouds liquid water content (LWC) from the King probe and total water content (TWC) from Counterflow Virtual Impactor were used to calculate the ratio of LWC to TWC. A criterion called roundness, calculated using various measures of particle morphology, is suggested to determine the shape of cloud particles between 15 µm and 50 µm. Using images from the CPI it is found that average roundness of cloud particles between 15 µm and 50 µm is linearly correlated to the ratio of LWC to TWC in the mixed-phase clouds.