Why the Hail? The Benefits of a 'True' Bulk Hail Category for Simulating Severe Storms

A number of bulk cloud microphysics schemes predict at least two integral moments (e.g., mass and number) of a rimed-ice (graupel) species, perhaps with additional characteristics such as bulk density, water fraction, or a third moment (e.g., reflectivity). Although multiple moments and other variables improve physical realism (e.g., prediction of mean particle diameter), a single rimed-ice species cannot realistically span the range of small low-density graupel to large damaging hail. Hail-like sizes can arise in 2-moment graupel when size-sorting is insufficiently controlled and produces fictitiously large particles. Excessive size sorting leads to unrealistic differential reflectivity (Z_DR), although reflectivity (Z_H) itself may not look excessive (e.g., not exceed 55dBZ) upon casual inspection. In this study, the NSSL microphysics scheme is run with various levels of 2-moment size-sorting constraints in high-shear storm environment (i.e., supercell) and also compared with 3-moment results. Comparisons are also made with either graupel-only, separate graupel and hail species, or separate graupel, frozen drops, and hail. Additionally, hail can be sourced by particle density (frozen drops) or by process (riming of large graupel into ‘true’ hail). Treating hail as just high-density ice leads to high number concentration and smaller sizes, whereas hail from large graupel provides lower concentrations, larger diameters, appreciable surface accumulations, improved radar reflectivity structure, and more realistic simulated polarimetric signatures.