Response of Simulated Mixed-Phase Arctic Stratus Clouds to Slowly Activated Ice Nuclei

Tuesday, 16 December 2014: 5:10 PM
Ann M Fridlind1, Alexander Avramov2, Andrew S Ackerman1, Peter Aaron Alpert3,4 and Daniel Alexander Knopf5, (1)NASA Goddard Institute for Space Studies, New York, NY, United States, (2)MIT, Cambridge, MA, United States, (3)Stony Brook University, Stony Brook, NY, United States, (4)Centre National de la Recherche Scientifique, IRCELyon, CNRS, University Claude Bernard, Lyon, France, (5)Stony Brook University, Institute for Terrestrial and Planetary Atmospheres / School of Marine and Atmospheric Sciences, Stony Brook, NY, United States
Supercooled mixed-phase cloud decks are common in the Arctic, often persisting for days. Individual ice crystals in such clouds have relatively short lifetimes, typically an hour or less. Thus new ice crystals must be generated continuously in such long-lived cloud layers. Field campaigns investigating the microphysics of the simplest such clouds—unseeded single-layer cases in coupled or decoupled boundary layers—have aimed to measure the background ice nuclei (IN) required to initiate ice formation processes, specifically by measuring the concentration of IN above cloud top that are active at water saturation at cloud-top temperature. In previous detailed simulations of observed case studies, we demonstrated that if all ambient IN are assumed to be activated rapidly, and if there is no surface source of IN over pack ice or efficient multiplication process in the absence of riming, as commonly assumed, then overlying IN concentrations must exceed those of in-cloud ice crystals by a factor of order 10–100 or more, generally much higher than measured. However, under such conditions, entrainment and rapid activation quickly achieve a long-lived quasi-steady cloud microphysical state in simulations that seems consistent with that commonly observed. These previous studies made the assumption that all relevant IN have a lifetime of roughly one second at water saturation under cloud-top conditions, using a singular ice nucleation scheme. Here we investigate the behavior of the same cloud systems in the presence of IN with longer activation time scales, including those only available in the contact mode and those with a wider range of lifetimes under in-cloud conditions. We make a range of assumptions about IN properties to constrain ice nucleation schemes to the degree possible using field data. When ice crystals are primarily sustained by slowly activated IN, we find that the relative depletion rate of the boundary-layer reservoir of IN impacts the degree of quasi-steady behavior.