Evaluation of a short-range multimodel ensemble system

Forecasts from the National Centers for Environmental Prediction's experime ntal short-range ensemble system are examined and compared with a single ru n from a higher-resolution model using similar computational resources. The ensemble consists of five members from the Regional Spectral Model and 10 members from the 80-km Eta Model, with both in-house analyses and bred pert urbations used as initial conditions. This configuration allows for a compa rison of the two models and the two perturbation strategies, as well as a p reliminary investigation of the relative merits of mixed-model, mixed-pertu rbation ensemble systems. The ensemble is also used to estimate the short-r ange predictability limits of forecasts of precipitation and fields relevan t to the forecast of precipitation. Whereas error growth curves for the ensemble and its subgroups are in relat ive agreement with previous work for large-scale fields such as 500-mb heig hts, little or no error growth is found for fields of mesoscale interest, s uch as convective indices and precipitation. The difference in growth rates among the ensemble subgroups illustrates the role of both initial perturba tion strategy and model formulation in creating ensemble dispersion. Howeve r, increase spread per se is not necessarily beneficial, as is indicated by the fact that the ensemble subgroup with the greatest spread is less skill ful than the subgroup with the least spread. Further examination into the skill of the ensemble system for forecasts of precipitation shows the advantage gained from a mixed-model strategy, such that even the inclusion of the less skillful Regional Spectral Model member s improves ensemble performance. For some aspects of forecast performance, even ensemble configurations with as few as five members are shown to signi ficantly outperform the 29-km Meso-Eta Model.