Evaluating the Impact of Stochastic Physics in a Convection-Allowing Ensemble for Severe Weather Forecasting

During the past three years of the Hazardous Weather Testbed (HWT) Spring Forecasting Experiment (SFE), community collaborators have contributed a group of similarly configured convection-allowing models (CAMs) to make up the Community-Leveraged Unified Ensemble (CLUE). These similar configurations of members within the CLUE allow for controlled experiments that test various aspects of convection-allowing ensemble design. One such experiment during the 2018 HWT SFE involved using different physics configuration methods to address model-related uncertainty within three ensembles, one using single physics, one using mixed physics , and one using single physics with stochastic perturbations. Each ensemble otherwise used the same perturbed initial and lateral boundary conditions to generate ensemble forecasts comprised of 8 members.

During previous experiments, mixed physics approaches have shown potential for providing spread within a convection-allowing ensemble. These mixed physics approaches, however, are more difficult and costly to maintain in an operational setting. The stochastic physics approach, in which stochastic parameter perturbations (SPPs) are applied within physics parameterization schemes, is one potential method to provide ensemble spread while being part of a more sustainable unified system. The single physics approach will provide a performance baseline, which the mixed and stochastic physics approaches can be compared against.

Objective verification metrics detailing the utility of each ensemble for severe weather forecasting will be explored by generating surrogate severe probability forecasts (SSPFs) from updraft helicity (UH) fields, and verifying against observed severe probabilistic fields (OSPFs) generated from storm reports. Additionally, daily subjective evaluations from experiment participants for each ensemble will be presented, along with an examination of differences in the ensemble guidance for days in which noteworthy features are indicated by the subjective evaluations.