Next-day convection-allowing WRF model guidance: A second look at 2- vs. 4-km grid spacing

During the 2007 NOAA Hazardous Weather Testbed (HWT) Spring Experiment, the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma produced forecasts from a single deterministic 2-km model and a 10-member 4-km resolution ensemble. In this study, the 2-km deterministic output is compared with forecasts from the 4-km ensemble control member. Other than the difference in horizontal resolution, the two sets of forecasts featured identical WRF-ARW configurations, including vertical resolution, forecast domain, initial and lateral boundary conditions, and physical parameterizations. Therefore, forecast disparities are attributed solely to differences in horizontal resolution.

This study is a follow-up to similar work that was based on results from the 2005 Spring Experiment. Unlike the 2005 Experiment, however, model configurations are more rigorously controlled in the present study, providing a more robust dataset and a cleaner isolation of the dependence on horizontal resolution. It is important to emphasize that both the 2005 and 2007 datasets are based on “cold-start” initializations from the NAM (no convective-scale features in initial conditions) and comparisons focus on verification of next-day (~24h) model forecasts.

Model forecasts are analyzed using objective verification of mean hourly precipitation during the 21- to 33-hour forecast period in addition to visual comparison of individual events. Although the 2-km forecasts provide more detailed structures on the smallest resolvable scales, patterns of convective initiation, evolution, and organization are remarkably similar. Moreover, on average, metrics such as equitable threat score, frequency bias, and fractions skill score reveal no statistical improvement of the 2-km forecasts compared to the 4-km forecasts. These results, based on the 2007 dataset, corroborate previous findings, suggesting that decreasing horizontal grid spacing from 4 to 2 km provides little added value as next-day guidance for severe convective storm forecasters in the U. S.