Initial Weather Regimes as Predictors of Numerical 30-Day Mean Forecast Accuracy

Abstract

A set of thirty 30-day mean 500-mb height anomaly forecasts run from National Meteorological Center initial analyses by the NCAR Community Climate Model is examined in order to learn if the forecast accuracy can be estimated with the initial conditions. Defining initial weather regimes by a 500-mb geostrophic zonal index anomaly difference between 50°W and 10°E discriminates between the best and worst 30-day mean forecasts in the sample. Initial regimes characterized by anomalously high zonal index (500-mb geostrophic westerlies) at 50°W and low index at 10°E yield on average lower 30-day mean forecast-observed anomaly correlation than initial regimes with opposite conditions (anomalously low zonal index at 50°N and high index at 10°E). It is suggested that initial regimes with abnormally fast geostrophic 500-mb westerlies at 50°W are followed in time by intense and poorly forecast synoptic-scale cyclones over the Atlantic Ocean. It is shown in a case study that the local synoptic- to planetary-scale interaction, as measured by its contribution to quasigeo-strophic 500-mb height tendencies, is misforecast early in the forecast cycle following these initial conditions. Early rapid synoptic-scale error growth, in this case, is followed by rapid planetary-scale error growth (incorrectly forecast demise of a blocking pattern), deterioration of forecast accuracy, and an unskillful 30-day mean forecast. It is further suggested that initial regimes with abnormally weak 500-mb geostrophic westerlies at 50°W are followed in time by less intense, but better forecast, cyclone waves over the Atlantic Ocean. In a representative case study, the local synoptic- to planetary-scale interaction is well forecast early in the forecast cycle. The local synoptic- and planetary-scale error growths are more restrained (a blocking pattern is correctly maintained), deterioration of forecast accuracy is postponed, and a skillful 30-day mean forecast results. It is hypothesized on the basis of this work that the accuracy of numerical 30-day mean forecasts may depend upon the accuracy with which the cyclones and their interactions with the planetary scale are predicted early in the forecast cycle, and that this accuracy in turn may depend upon the initial conditions.