Challenges to ensemble system design for Warn-on-Forecast

Warn-on-Forecast is a ten-year National Oceanic and Atmospheric Administration project to extend severe weather warning lead times by incorporating forecasts from a convection-permitting ensemble modeling system into the warning decision process. Initial results with convection-permitting ensembles that assimilate radar observations have shown that while reasonable thunderstorm analyses are produced, obtaining accurate very short-range thunderstorm forecasts is more challenging. Model error is often blamed as the dominant source in the loss of forecast skill, especially inaccuracies in the model microphysical parameterizations. Observational studies indicate that the densities and intercept parameters of hydrometeor distributions can vary widely among storms and even within a single storm. Therefore, assuming a fixed set of microphysical parameters in a convection-permitting numerical model can lead to significant errors in not only the forecasts but also the analyses of severe storm events. To explore the impact of variations in parameters within the same microphysics scheme, several observing system simulation experiments (OSSEs) are conducted using a range of different realizations of the hydrometeor intercept and density parameters. Results highlight the potential for using a variety of realistic microphysical parameters across the ensemble members in improving the analyses and very short-range forecasts of severe weather events. Other challenges to ensemble design for convection-permitting resolutions will also be discussed.