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André Tremblay, Paul A. Vaillancourt, Stewart G. Cober, Anna Glazer, and George A. Isaac


To improve the quality of forecasts of mixed-phase clouds in winter storms, some aspects of a cloud scheme are examined in detail. Modifications to the basic formalism and specification of selected parameters of the cloud model are studied, and simulation results are compared with aircraft observations and satellite data. In particular, a sensitivity study to the parameterization of the ice particle size distribution is presented. A special technique allowing the reconstruction of any model variable along a virtual aircraft trajectory is used to compare model results with aircraft observations. It has been possible from these comparisons to optimize the scheme and improve the quality of forecasts.

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Paul A. Vaillancourt, André Tremblay, Stewart G. Cober, and George A. Isaac


In order to provide guidance for the further improvement of a mixed-phase cloud scheme being developed for use in an NWP model, comparisons of dynamical, thermodynamical, and microphysical variables between in situ aircraft data and model data were made. A total of 21 flights (∼88 h of data) from the First and Third Canadian Freezing Drizzle Experiments were selected and simulated. The basis of the evaluation of the model performance is a point-by-point comparison of each pertinent variable along the real and “virtual” aircraft trajectories. The virtual aircraft trajectory is constructed by choosing, for every observed data point, the closest available model data point in terms of time, pressure level, and latitude–longitude position. Observed and model data were used to calculate simple descriptive statistics to evaluate the ability of the forecast system to predict the presence of clouds, their phase, and water content.

Even though a point-by-point comparison of the aircraft and model data is a very severe test given the errors in the initial conditions and the disparity in temporal and spatial resolution, the results were encouraging for about half the flights simulated. It was found that, in general, the model predicts ice clouds better than water clouds. The model generally overpredicts (underpredicts) both the presence and the quantities of ice water content (supercooled liquid water content). Furthermore, where mixed-phase clouds are present in the model, the ice phase represents a large fraction of the total water content, contrary to the observations. These conclusions suggest that the parameterization of the ice particle size distribution is an important aspect of the mixed-phase cloud scheme that should be optimized.

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