Application of a mixed-phase microphysics scheme to predict aircraft icing

The most sophisticated microphysics option within the PSU/NCAR research mesoscale numerical model (MM5) has undergone extensive testing and modification. The bulk microphysical parameterization allows for the prediction of mixing ratios of cloud water, rain, cloud ice, snow, and graupel as well as the number concentration of ice. The impetus for this work is improving forecasts of aircraft icing under sponsorship by the Federal Aviation Administration. Such improvement implies correct forecasts of supercooled liquid water which, in turn, implies proper handling of ice initiation and growth.

The main focus of this study is numerical simulations of clouds and precipitation as they relate to aircraft icing and, of particular importance, freezing drizzle/rain (or supercooled large drops, SLD). Many tests were conducted in two-dimensions using idealized flow over a barrier before continuing to fully three-dimensional simulations involving case studies of freezing drizzle/rain events. While precipitation amount and type reaching the ground is important, we also examine research aircraft data to compare against the model simulations of supercooled liquid water.

Observational data suggests that many freezing drizzle/rain cases are non-classical in the sense that collision/ coalescence is responsible for cloud droplet growth to drizzle/rain sizes. On the other hand, the classical mechanism consists of ice initiation, growth to snow, and subsequent melting through a warm layer (temperature greater than 0C) before passing back into a sub-freezing layer and supercooling to freezing drizzle/rain. In this paper, both classical and non-classical freezing drizzle/ rain cases are investigated using numerical simulations.