Object-based evaluation of the impact of horizontal grid spacing on convection-allowing forecasts

Forecasts generated with 1 and 4 km grid spacing using the WRF-ARW model (ARW1 and ARW4, respectively) during the 2009-2011 NOAA Hazadous Weather Testbed Spring Experiments by the Center for Analysis and Prediction of Storms are compared and verified using object-based measures, including average values of object attributes, the Object-based Threat Score (OTS) and the Median of Maximum Interest (MMI). Verification was first performed against observations at scales resolvable by each forecast model and then performed at scales resolvable by both models by remapping ARW1 to the ARW4 grid (ARW1to4). Thirty-hour forecasts of one-hour accumulated precipitation initialized at 0000 UTC on 22, 36 and 33 days during the spring of 2009, 2010 and 2011, respectively, are evaluated over a domain covering most of the central and eastern United States. ARW1, ARW1to4 and ARW4 all significantly over-forecasted the number of objects during diurnal convection maxima. The over-forecasts by ARW1 and ARW1to4 were more pronounced than ARW4 during the first convection maximum at 1-h lead time. The average object area and aspect ratio were closer to observations for ARW1 and ARW1to4 than for ARW4. None of the models showed a significant advantage over the others for average orientation angle and centroid location. Increased accuracy for ARW1, compared to ARW4, was statistically significant for the MMI but not the OTS. However, ARW1to4 had similar MMI and OTS as ARW4 at most lead times. These results are consistent with subjective evaluations that the greatest impact of grid spacing is on the smallest resolvable objects.