Julien Savre
Department of Meteorology, Stockholm University, Sweden

Ice formation in cold clouds: Parameterization and model evaluation based
on an Arctic mixed-phase boundary layer cloud case study

Time and place
Thu 13 June 2013, 14.15
Room A601, Arrhenius Laboratory, 6th floor

(This event has taken place)


Understanding the effects of anthropogenic aerosols on all-ice and
mixed-phase clouds requires a detailed knowledge of the mechanisms leading to primary ice crystal formation. Unfortunately, despite the recent
theoretical progresses made based on more numerous dedicated laboratory studies, our understanding of ice nucleation in the atmosphere and our capacity to model it adequately still suffers from several limitations.

It is widely admitted that ice crystals can be formed through four major
processes: deposition of water vapor onto dry aerosol particles, freezing of cloud droplets after collision with a particle, nucleation of ice germs on aerosol particles immersed into liquid drops and freezing after condensation of water on the particle's surface. The rates of ice production in each of these freezing modes will then depend mostly on thermodynamic conditions, aerosol size, aerosol composition and drop size. Due to the great complexity of the physical phenomena involved, most existing parameterizations rely on empirical formula derived from bulk cloud and aerosol properties. Recently, advanced models have appeared where nucleation theory was introduced to represent the nucleation ability of the most abundant atmospheric aerosol species. The approach is in theory very attractive, but in practice important uncertainties on the characterization of aerosol properties remain leading to large modeling errors. Some properties required to develop these models can only be derived from laboratory experiments but the reported results often show large discrepancies due to differing experimental procedures, aerosol nature, or overall thermodynamics conditions.

This seminar aims at introducing ice nucleation processes in the atmosphere and the theoretical basis necessary to apprehend the related scientific issues. An advanced ice nucleation parameterization considering several possible ice nuclei species (dust, soot, organics) has been implemented in MISU's new Large-Eddy Simulation model, which helped identify the main uncertainties associated with ice nucleation modeling. Some examples of ice nucleation in an Arctic mixed-phase cloud (ISDAC) as simulated by the LES constrained by observed aerosol composition and concentrations will be given.