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SoMAS - Ice Crystal Growth at Low Temperatures: Measurements, Theories, and Cloud-Scale Impact

Jerry Harrington from Penn State speaks to SoMAS at the Topics in Atmospheric and Oceanic Sciences seminar on September 18, 2024 on the topic "Ice Crystal Growth at Low Temperatures: Measurements, Theories, and Cloud-Scale Impact." Jerry is interested in all aspects of cloud processes. He is involved in field, laboratory, and numerical modeling studies. The range of topics cover the growth of individual drops and ice crystals, the structure, microphysics, and dynamical evolution of cloud systems, and the radiative, turbulent, and microphysical processes within clouds. For more information about Jerry's many interests, please visit: https://www.met.psu.edu/directory/jer... 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 a levitation diffusion chamber and a new ice crystal imaging chamber. Measurements from the levitation chamber 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 on small crystals 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 grown in the imaging chamber 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. Theoretical models of near-edge growth can reproduce the data thus providing information on the surface kinetics that control growth. Neither the capacitance model nor the adaptive habit model can reproduce the measurements even if the models are supplied with the measured aspect ratios. Lagrangian microphysics simulations using the laboratory data indicate that the diversity of crystals in cirrus is controlled largely by the local thermodynamics paths of the crystals, and not by updraft variability as Nelson demonstrated occurs for lower altitude mixed-phase cloud systems. For more information about the School of Marine and Atmospheric Sciences, please visit https://somas.stonybrook.edu