Effective Cirrus Ice Crystal Densities: A General Criterion for The Morphological Instability that Causes Hollowing and Crystal Complexity

Description

Results from the recent DOE-ASR and DOE-ARM Ice Cryo-Encapsulation Balloon (ICEBall) borne field campaign showed a surprising degree of hollowing on crystals ranging in maximum dimension of 40 to 300 micrometers. Hollowing on small particles is significant, indicating that hollowing probably started when the crystals were very small. This result stands in stark contrast to theoretical results that suggest hollowing begins once crystals reach a threshold size (Mason, 1993), and it contradicts the size threshold used in some bulk model for reductions in crystal effective density (e.g. Jensen et al., 2017). The observations also conflict with measurements at higher temperatures that suggest effective density reductions occur above a specific supersaturation or vapor excess (generally about 0.05 g m^-3). When hollowing occurs is important because it causes increases in the vapor growth rate. Analysis of the ICE-Ball observations in combination with laboratory measurements of growing ice crystals at low temperatures indicates that compact crystals transition to complex hollowed and branched crystals above a relative vapor excess threshold. We show that this threshold approximately delineates compact crystals from complex crystals, and that the results also explain observations at higher temperatures (above -30C). The faceting instability that causes hollowing is directly related to the critical supersaturations that determine the deposition coefficients, allowing us to estimate the critical supersaturations at temperatures below -40C were no data currently exist. Simulations of cirrus using the new criterion combined with a measurement-based budding rosette model produce cloud with much more horizontal and vertical heterogeneity than current bulk schemes