Abstract

Low level mixed-phase clouds in the Arctic, that is boundary layer clouds consisting of both ice crystals and liquid drops, are often found to persist during several days (especially in autumn) despite being naturally unstable. As a matter of fact, water saturation over ice occurring typically at temperatures lower than saturation over liquid water, when both phases coexist in a single cloud, the ice crystals have the tendency to grow at the expense of liquid droplets, therefore removing all the liquid water until complete cloud glaciation. The conditions required for mixed-phase clouds to persist over long periods of time involve complex dynamical-microphysical interactions specific to the Arctic environment (low insulation, weak surface fluxes, specific aerosol composition…). Although much has yet to be discovered, I will try in this talk to describe some of the most important atmospheric phenomena playing a role in the persistence of Arctic mixed-phase clouds. The results presented rely mostly on small-scale/high-resolution numerical simulations of clouds observed during a measurement campaign which took place in northern Alaska in April 2008.