Jerry Harrington, Penn State University

Abstract:

The vapor growth of ice crystals at cirrus temperatures is complex and few growth time-series measurements have been achieved. At present, even the primary growth mode of crystals at low temperatures is not  known. I will present ice crystal growth measurements at temperatures below -40C taken by our group with in diffusion chambers. Measurements show that the growth of small (10 to 50 micron) ice crystals is often, but not always, limited by attachment kinetics at low supersaturation. Once a threshold vapor excess is achieved, morphological transformations occur amplifying the mass growth rate. This amplification is increased substantially if ice crystals form from solutions rather than pure water, indicating a different morphological transformation. Larger ice crystals are often hollowed columnar crystals at temperatures below -45C. The aspect ratio (major to minor dimension) of these crystals approaches a constant value over time. This result is predicted to occur if crystals grow by steps forming near crystal corners and edges, providing some of the first evidence of the primary growth mode of low temperature ice. The capacitance-based theories used in forecast models cannot reproduce the measurements even if the models are supplied with the measured aspect ratios. In contrast, theoretical models of near-edge growth can reproduce the data thus providing information on the surface kinetics that control growth, and a path forward for numerical model development.