Impact of Horizontal Resolution on Next-Day CAM-Derived Probabilistic Severe Weather Forecasts

Numerical weather prediction (NWP) models with horizontal grid spacing of 4 km or less are commonly referred to as Convection-allowing Models (CAMs). Previous research has generally agreed that CAMs provide forecasts of significantly greater quality and value relative to non-CAMs; however, previous work offers mixed results on the benefits of further reducing horizontal grid spacing beyond 4 km. Given the current trend toward increasing computing power, an important question is whether additional computational resources would be better spent on further decreasing the horizontal grid spacing of CAMs or on adding members to form CAM ensembles.

The present study analyzes Advanced Research Weather Research and Forecasting (ARW-WRF) model data from 63 days of the 2010 and 2011 NOAA Hazardous Weather Testbed Spring Forecasting Experiments (HWT SFEs) in order to investigate the above question as it pertains to next-day probabilistic severe weather forecasts. Next-day probabilistic severe weather forecasts, which are created by spatially smoothing large values of forecast updraft helicity, are produced and analyzed from a deterministic 1-km CAM, a deterministic 4-km CAM, and an 11-member 4-km CAM ensemble. Forecast quality is assessed using relative operating characteristic (ROC) curves, performance diagrams, reliability diagrams, and area under the ROC curve (AUC). A 2-sided resampling hypothesis test is used to test for significant differences in AUC between the three forecasts. Finally, forecasts from select individual days are evaluated in order to assess the three forecasts' relative value.

Results indicate that, while no significant differences in AUC exist between the 4-km and 1-km deterministic forecasts, the 4-km ensemble forecast has a significantly greater AUC than the 4-km deterministic forecast when calculated over the entire 63-day analysis period. Moreover, the individual day analysis indicates that, in general, the 4-km ensemble forecasts provide greater value relative to both of the deterministic forecasts. Collectively, these results suggest that, for purposes of improving next-day CAM-derived probabilistic severe weather forecasts, additional computational resources may be better spent on adding members to form CAM ensembles than on further reducing the horizontal grid spacing beyond 4 km.