The Vice and Virtue of Increased Horizontal Resolution in Ensemble Forecasts of Tornadic Thunderstorms in Low-CAPE, High-Shear Environments

• Published in: Monthly weather review Vol. 149; no. 4; pp. 921 - 944
• Main Authors: Lawson, John R., Potvin, Corey K., Skinner, Patrick S., Reinhart, Anthony E.
• Format: Journal Article
• Language: English
• Published: Washington American Meteorological Society 01.04.2021
• Subjects: Aggregation; Convection; Ensemble forecasting; Information theory; Lead time; Reflectance; Shear; Storm forecasting; Thunderstorms; Tornadoes; Verification
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-20-0281.1

Abstract Tornadoes have Lorenzian predictability horizons O (10) min, and convection-allowing ensemble prediction systems (EPSs) often provide probabilistic guidance of such events to forecasters. Given the O (0.1)-km length scale of tornadoes and O (1)-km scale of mesocyclones, operational models running at horizontal grid spacings (Δ x ) of 3 km may not capture narrower mesocyclones (typical of the southeastern United States) and certainly do not resolve most tornadoes per se. In any case, it requires O (50) times more computer power to reduce Δ x by a factor of 3. Herein, to determine value in such an investment, we compare two EPSs, differing only in Δ x (3 vs 1 km), for four low-CAPE, high-shear cases. Verification was grouped as 1) deterministic, traditional methods using pointwise evaluation, 2) a scale-aware probabilistic metric, and 3) a novel method via object identification and information theory. Results suggest 1-km forecasts better detect storms and any associated rapid low- and midlevel rotation, but at the cost of weak–moderate reflectivity forecast skill. The nature of improvement was sensitive to the case, variable, forecast lead time, and magnitude, precluding a straightforward aggregation of results. However, the distribution of object-specific information gain over all cases consistently shows greater average benefit from the 1-km EPS. We also reiterate the importance of verification methodology appropriate for the hazard of interest.