Initial stages in the morphological evolution of vapour-grown ice crystals: A laboratory investigation

We describe experiments to investigate the first stages in the evolution of small (100-200 mum) ice crystals levitated in air at temperatures and humidities characteristic of fully glaciated stratiform clouds. We find that in these conditions particle morphology is not uniquely determined by environmental temperature and humidity as has commonly been assumed; other parameters, such as the mode of ice initiation, appear to have important effects on particle shape. Crystals grown from frozen droplets at vapour excesses of less than about 0.05 g m(-3) adopt isometric compact habits at temperatures above -22degreesC, while polycrystals dominate at lower temperatures. In contrast, crystals grown under similar conditions from frost 'seeds' usually develop into pristine hexagonal prisms with varying aspect ratios. An increasing fraction of frozen droplets develop into 'florid' crystals, containing thin side-planes, at higher supersaturations. This mix of particle types is similar to that found in field studies of cold clouds. The change in morphology from compact hexagon to 'florid' with increasing growth rate defines a facet instability transition, from which we infer a lower limit for the critical supersaturation for layer nucleation on the prism facet to be 2.4% at -15degreesC and 3.1% at -25degreesC. We develop a simple analytic model for the size dependence of facet stability and show its predictions compare well with the data.