Prediction of Hail in a 2.5-km NWP System with Detailed Microphysics

The Meteorological Services of Canada runs several numerical weather prediction (NWP) systems using the Global Environmental Multiscale (GEM) model, including a high-resolution deterministic system with a horizontal grid-spacing of 2.5 km. In this configuration, nearly all clouds and precipitation are represented by the model's bulk microphysics parameterization. The GEM microphysics is a detailed 2-moment scheme with six distinct hydrometeor types, making it versatile and capable of representing a broad range of types of clouds and precipitation. This is an important aspect for NWP in Canada which experiences wide range of weather systems, including hail-producing convective storms during the summer season.

A new development to the microphysics has been made which has important implications to the prediction of hail. In most schemes, hydrometeor categories are defined primarily by the choice of values for various fixed parameters, including particle density and terminal fall speed parameters. Graupel is typically defined to have a fixed density of 400 kg m-3 with fall speeds ranging from 1-3 m s-1. In the modified microphysics, the graupel density now varies as a predicted variable and the fall speeds depend on both density and size. This results in realistically broad range of graupel characteristics and the simulation of a more realistic transition from graupel to hail.

An overview of the modified GEM microphysics scheme will be presented with emphasis on the improvements to the representation of graupel and hail. Case examples will be shown to illustrate the capacity of the modeling system to explicitly predict hail.