Impact of Increased Resolution on Ensemble Forecasts of Thunderstorm Objects in the US Southeast

The VORTEX-SE program aims to reduce loss of life and property from tornado damage. Amongst the challenges of risk communication and warning systems, there is evidence that increased resolution of a limited-area numerical weather prediction model will better represent the updraughts associated with tornadogenesis in the southeastern US. These may occur in both cellular and linear convective modes. However, a threefold increase in grid resolution requires more than a ninefold increase in computational demand; as such, given a finite computational resource, the number of ensemble members must be balanced with grid spacing. This presentation details results comparing 3-km to 1-km ensemble forecasts of severe thunderstorm outbreaks to assess resolution impact on skill and spread. Evaluation is performed with a range of scoring techniques, including object-based and scale-aware methods, reflecting the multi-faceted challenge of storm-scale verification. We also address whether the majority of the ensemble envelope can be captured by a limited subset of ensemble members, which would free up computational power for use in resolution improvements. Furthermore, we assess the impact of a scale-aware MYNN (boundary layer) parameterisation, particularly on the nature of storm objects in the forecast field.