Simulations of ice clouds during FIRE ACE using the CCCMA single-column model

The single-column model (SCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA) solves prognostic equations for the number concentration and mass mixing ratios of ice crystals. Ice crystal formation is specified via different ice nucleation mechanisms. The CCCMA SCM was used to simulate the evolution of ice clouds for three different flights during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) in April 1998, where measurements of cloud droplets, ice crystals, and aerosols > 0.08 Pm in radius were conducted. The CCCMA SCM in its original setup predicts larger ice crystal concentrations than measured with the 2DC probe. The agreement with observations of ice crystals detectable with the 2DC probe was improved if a condensation freezing parameterization depending on supersaturation with respect to ice was used. The best correlation between observed and simulated ice crystal number, which still showed a lot of scatter, was obtained if an empirical linear relationship between the number of aerosols and the number of ice crystals deduced from FIRE ACE was used.