The Impacts of Liquid Fraction and Multiple Free Ice-Phase Categories on the Simulation of Hail

In 2015 the Predicted Particle Properties (P3) bulk microphysics scheme was introduced, based on the concept that ice-phase hydrometeors can be represented a single category, whose physical properties evolve continuously in time and space, rather than by several predefined categories with prescribed parameters. Since its inception, P3 has undergone several major further developments. The ice phase is now represented by a user-specified number of freely evolving categories, each with prognostic spectral dispersion (triple-moment), density, rime fraction, liquid fraction, and other properties. As such, P3 is a unique microphysics scheme which can, in principle, simulate a wide range of microphysical conditions. However, the impacts of certain aspect of the scheme on the simulation of deep convection have not been fully explored.

The study examines the impacts of the prognostic liquid fraction and the use of multiple ice categories on the simulation of hail in the context of near convection-resolving (250-m grid spacing) quasi-idealized simulations of two hail-producing supercell storms in different environments. All simulations use the triple-moment-ice configuration but varying the prognostic liquid fraction (on or off) and the number of ice categories (from 1 to 4). The sensitivity to the conditions for initiation of new ice into specific categories and to merge categories with similar physical properties is also examined. The inclusion of prognostic liquid fraction significantly modifies the rates of melting and shedding of hail and ultimately reduces the amount of ice reaching the surface. This has important implications for assumptions commonly made in conventional 2-moment schemes. The increasing number of ice categories reduces the amount of “property dilution”, which occurs when two or more populations of particles with different properties are represented by a single category (and thus a single size distribution and set of properties). This impacts the amount of ice reaching the surface and the maximum hail size.