Next-Day Convection-Allowing WRF Model Guidance: A Second Look at 2-km versus 4-km Grid Spacing

Abstract

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 convection-allowing forecasts from a single deterministic 2-km model and a 10-member 4-km-resolution ensemble. In this study, the 2-km deterministic output was compared with forecasts from the 4-km ensemble control member. Other than the difference in horizontal resolution, the two sets of forecasts featured identical Advanced Research Weather Research and Forecasting model (ARW-WRF) configurations, including vertical resolution, forecast domain, initial and lateral boundary conditions, and physical parameterizations. Therefore, forecast disparities were attributed solely to differences in horizontal grid spacing. 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 were more rigorously controlled in the present study, providing a more robust dataset and a cleaner isolation of the dependence on horizontal resolution. Additionally, in this study, the 2- and 4-km outputs were compared with 12-km forecasts from the North American Mesoscale (NAM) model. Model forecasts were analyzed using objective verification of mean hourly precipitation and visual comparison of individual events, primarily during the 21- to 33-h forecast period to examine the utility of the models as next-day guidance. On average, both the 2- and 4-km model forecasts showed substantial improvement over the 12-km NAM. However, although the 2-km forecasts produced more-detailed structures on the smallest resolvable scales, the patterns of convective initiation, evolution, and organization were remarkably similar to the 4-km output. Moreover, on average, metrics such as equitable threat score, frequency bias, and fractions skill score revealed 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 and heavy rain forecasters in the United States.

Benefit of convection permitting climate model simulations in the representation of convective precipitation

“…In addition, a higher resolution model allows a better representation of orography and surface fields which are crucial for the initiation of convection in complex terrain (Hohenegger et al 2008). Copious literature proves the added value of this spatial resolution in the representation of the precipitation field especially in cases of moist convection and/or in regions with strong orography (Langhans et al 2013;Mass et al 2002;Miura et al 2007;Grell et al 2000;Richard et al 2007; Lean et al 2008;Schwartz et al 2009;Weusthoff et al 2010;Baldauf et al 2011;Hohenegger et al 2008;Prein et al 2013). Most of these studies are related to numerical weather prediction (NWP) or limited to few summers.…”

Added value of convection permitting seasonal simulations

“…In numerical weather prediction (NWP) convection resolving models are already widely used for operational forecasts and research purposes (e.g., Mass et al 2002;Kain et al 2006;Schwartz et al 2009;Gebhardt et al 2011). According to Weisman et al (1997) the critical horizontal grid spacing for CPCSs is *4 km.…”

Multiple-scale error growth in a convection-resolving model

“…Some attention has been devoted to evaluating whether a higher resolution and a more realistic description of convective events are in fact associated with a quantitative improvement in forecasting their location, timing and intensity (e.g. Weisman et al, 2008;Schwartz et al, 2009).…”