The Effects of Multiple Free Ice-Phase Categories in the P3 Microphysics Scheme

Microphysics schemes have traditionally represented ice-phase hydrometeors by partitioning them into predefined categories with prescribed bulk physical quantities, usually with fixed parameters. This approach has several limitations and inherent weakness such as the need to include conversion between categories, a purely artificial process which depends on arbitrary and non-physical thresholds. In recent years, scheme developers have been moving away from the pre-defined category approach and have placed more emphasis on the prediction of ice particle properties. One of the conclusions from the 2016 International Cloud Modeling Workshop (in Exeter, UK) that was explicitly made was that in order to make advances in the ability to represent ice microphysics in atmospheric models, schemes must abandon the traditional pre-defined category paradigm adopt the particle property approach.

At the 2014 AMS Cloud Physics conference, the Predicted Particle Properties (P3) bulk microphysics scheme was introduced. It was shown that with a single “free” ice-phase category, a wide range of ice particle types can be simulated. However, the 1-category scheme has the important limitation that it cannot represent populations of different ice particle types at the same point in time and space. Thus, attempts to include processes such as ice multiplication resulted in a “dilution” of the physical properties. P3 has since been generalized to allow for a user-defined number of free categories. An overview of the multiple free category approach will be provided and the effects on different simulated weather systems will be presented.