Multiresolution Ensemble Forecasts of an Observed Tornadic Thunderstorm System. Part I: Comparsion of Coarse- and Fine-Grid Experiments

Using a nonhydrostatic numerical model with horizontal grid spacing of 24 km and nested grids of 6- and 3-km spacing, the authors employ the scaled lagged average forecasting (SLAF) technique, developed originally for global and synoptic-scale prediction, to generate ensemble forecasts of a tornadic thunderstorm complex that occurred in north-central Texas on 28–29 March 2000. This is the first attempt, to their knowledge, in applying ensemble techniques to a cloud-resolving model using radar and other observations assimilated within nonhorizontally uniform initial conditions and full model physics. The principal goal of this study is to investigate the viability of ensemble forecasting in the context of explicitly resolved deep convective storms, with particular emphasis on the potential value added by fine grid spacing and probabilistic versus deterministic forecasts. Further, the authors focus on the structure and growth of errors as well as the application of suitable quantitative metrics to assess forecast skill for highly intermittent phenomena at fine scales. Because numerous strategies exist for linking multiple nested grids in an ensemble framework with none obviously superior, several are examined, particularly in light of how they impact the structure and growth of perturbations. Not surprisingly, forecast results are sensitive to the strategy chosen, and owing to the rapid growth of errors on the convective scale, the traditional SLAF methodology of age-based scaling is replaced by scaling predicated solely upon error magnitude. This modification improves forecast spread and skill, though the authors believe errors grow more slowly than is desirable. For all three horizontal grid spacings utilized, ensembles show both qualitative and quantitative improvement relative to their respective deterministic control forecasts. Nonetheless, the evolution of convection at 24- and 6-km spacings is vastly different from, and arguably inferior to, that at 3 km because at 24-km spacing, the model cannot explicitly resolve deep convection while at 6 km, the deep convection closure problem is ill posed and clouds are neither implicitly nor explicitly represented (even at 3-km spacing, updrafts and downdrafts only are marginally resolved). Despite their greater spatial fidelity, the 3-km grid spacing experiments are limited in that the ensemble mean reflectivity tends to be much weaker in intensity, and much broader in aerial extent, than that of any single 3-km spacing forecast owing to amplitude reduction and spatial smearing that occur when averaging is applied to spatially intermittent phenomena. The ensemble means of accumulated precipitation, on the other hand, preserve peak intensity quite well. Although a single case study obviously does not provide sufficient information with which to draw general conclusions, the results presented here, as well as those in Part II (which focuses solely on 3-km grid spacing experiments), suggest that even a small ensemble of cloud-resolving forecasts may provide greater skill, and greater practical value, than a single deterministic forecast using either the same or coarser grid spacing.