Ice particle habits in stratiform clouds

Ice crystals in clouds in the atmosphere have shapes that relate to their density, terminal fall velocity growth rate and radiative properties. In calculations for climate-change predictions, forecasting of precipitation, and remote-sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are often assumed. The objective of this work is to study the frequency of occurrence of different habits of ice particles in natural clouds from aircraft observations. Images of cloud particles were measured by a PMS Optical Array Probe-2DC at 25 mum resolution installed on the National Research Council Convair-580. The processing of particle images was conducted with a newly developed algorithm for pattern recognition. Data were collected during four field projects in the Canadian and US Arctic over the North Atlantic near Newfoundland, and over the Great Lakes. Approximately 5 x 10(6) images of cloud particles having a size larger than 125 mum were analysed. The cloud particles were classified into four categories: spheres, irregulars, needles/columns and dendrites. The habit classification of particles was done for three different size ranges: >125 mum, >250 mum, and >500 mum. The frequency of occurrence of different habits was found for each 5 degC temperature interval in the range -45 degreesC < T < 0 degreesC. It was concluded that the majority of ice particles in natural clouds were of irregular shape. The frequency of occurrence of irregular ice decreases with increasing particle size. On average, the concentration of particles larger than 125 mum was approximately constant down to -35 degreesC, whereas the concentration of particles larger than 500 mum decreased at temperatures below -15 OC. Since the data were collected in different climatic zones within many cloud types, and covered a significant cloud path length (3.6 x 10(4) km), the conclusions are applicable to most stratiform clouds containing ice.