Dynamics of Systematic Errors in the NMC Medium Range Forecast Model

Abstract

A simple error vorticity model is used to study processes contributing to the evolution of the 300-hPa systematic nondivergent flow errors in the National Meteorological Center Medium Range Forecast model (MRF) during the 1992–93 winter season. The error model is forced by two source terms representing, respectively, systematic errors in the irrotational flow and transient eddy vorticity fluxes. The results indicate that the model simulates reasonably well the development of many of the large-scale features of the zonally asymmetric part of the MRF systematic nondivergent flow errors, but it fails to simulate the zonally symmetric portion unless an extra forcing term representing systematic errors in the irrotational flow analyses is included. Two independent methods are employed to estimate the systematic errors in the irrotational flow analyses. The two estimates are highly correlated, and both indicate that the analyzed irrotational flow in the Tropics is too weak. The magnitude of the estimated analysis errors is of the same order as the difference between the divergence in the 10-day forecasts and the analysis. Globally, systematic errors in the irrotational flow dominate the evolution of nondivergent flow errors during the first few days of the model integration. Beyond 5–6 days, the extratropical error evolution is determined mainly by the integrated effects of systematic errors in the transient eddy vorticity fluxes. In the extratropics, transients eddy vorticity flux errors appear to be the major factor in producing systematic errors in the zonal mean nondivergent flow. In the Tropics, the rapid development of the zonal mean easterly wind bias is directly related to systematic irrotational flow errors. The authors postulate that early in the forecasts systematic errors in the irrotational flow associated with deficiencies in parameterized tropical convection force extratropical stationary wave errors, which in turn lead to systematic changes in the midlatitude storm tracks. The transient eddy flux errors associated with the altered storm tracks then feedback positively on the initial stationary wave errors and, after several days, become the dominant source of systematic rotational flow errors.