Predictability Experiments Using a High-Resolution Limited-Area Model

Recently reported results indicate that limited-area mesoscale models with prescribed lateral boundaries do not exhibit the same predictability error growth as observed in large-scale (global) models. These results have been reanalyzed in greater detail. New methods of limited-area initialization and spectral analysis have been used. The new analyses indicate that several model properties act to restrict the growth of perturbation. These include: the Projection of initial perturbations onto gravity waves which interact only weakly with other, more significant motions; the “sweeping out” of errors by correct or perfect lateral boundaries; and the reduction of differences by subgrid dissipation. This last property suggests that there is a strong dynamical forcing of small scales by much larger scales, so that this forcing is only weakly affected by typical, small perturbations in this model. New experiments suggest that some quasi-geostrophic components of the forecasts, away from the inflow boundaries, exhibit local error doubling times of approximately one day within active baroclinic regions. Such doubling is not observed in a lower-resolution, global forecast model.