Analyzing the Sensitivity of Hail Prediction to Model Grid Spacing

At grid spacings of 1 kilometer or less, there has been little to no research on possible sensitivities of hail prediction to model gird spacing using ensemble modeling. Research on sensitivities of hail at fine grid spacings (500 meters or less) will be important for future operational products, as one of Warn on Forecast’s goals is short term prediction of hail using ensemble modeling. In this study, three ensemble runs of ARPS (The Advanced Regional Prediction System) using horizontal grid spacings of 500, 250, and 100 meters were analyzed to find systematic differences in explicitly predicted model hail using a Milbrandt and Yau double moment bulk microphysical scheme. Hail and hail related fields were investigated to identify sensitivities as horizontal gird spacing was changed and whether hail related fields such as distribution of updrafts and graupel could help to explain potential sensitivities. Systematic differences were identified in the mass of hail field, with increased spatial coverage of large hail mass values at select vertical levels as grid spacing was decreased. Particle size distributions indicate the increase in spatial distribution of high hail mass is accompanied with an increase in concentration of large diameter hailstones. Differences in updraft distribution and near storm environment did not prove to account for differences found in the mass of hail field. However, distributions of mass of graupel suggest that a microphysical budget analysis is needed to explain sensitivities in the mass of hail field as horizontal grid spacing is decreased.