A Re-analysis of AIDA Cloud Simulation Experiments for Homogeneous Freezing Rates of Water and Solution Droplets

A more detailed understanding and formulation of cloud microphysical processes is still one of the major challenges for developing improved versions of atmospheric cloud, weather, and climate models. Reliable weather predictions and climate change projections require a sound understanding and appropriate model parameterization of the ice formation processes in tropospheric clouds. This includes homogeneous freezing of water and solution droplets, though this part is considered to be well understood and formulated for models. However, recent laboratory experiments conducted for typical cloud droplet sizes of a few ten µm and typical convective cloud glaciation temperatures around -35°C to -38°C revealed up to one or two orders of magnitude lower nucleation rates for pure water droplets compared to previous literature results. Concerning the homogeneous freezing of solution droplets at lower temperatures, current freezing rate formulations used in global climate models seem to overestimate freezing rates measured at temperatures below about -50°C in AIDA cirrus cloud expansion experiment series with aqueous sulphuric acid and other solution particles. Furthermore, the ice formation onset was shifted to higher ice supersaturations at low temperatures. This could have important consequences for the occurrence of ice supersaturated regions in the upper troposphere and the microphysical properties of cirrus clouds. This contribution will include a comprehensive reanalysis of AIDA experiments with pure water and solution droplets and discuss atmospheric implications of the results.