Response of the NMC MRF Model to Systematic-Error Correction within Integration

Abstract

We describe an extensive nudging (within-integration correction) experiment with a large and sophisticated atmospheric model. The model is an R30 version of the National Meteorological Center (NMC) T80 operational global medium-range forecast model. The purpose is to combat the systematic-error growth right from the start of the integration process by adding artificial sources and sinks (the corrections) of heat, momentum, and mass. The corrections derived from 30 antecedent 24-h integrations (by subtracting the forecasts from their verifying initial conditions) are applied to 30 subsequent independent 5-day forecasts from 1 July 1988 to 30 July 1988. Verification statistics over these 30 5-day forecasts are computed for the control cases, the nudged cases, and for forecasts corrected after the fact. The main results show that the nudging process, when carefully designed, does not lead to any technical problems and the model accepts the applied corrections quite faithfully. Both nudging and after-the-fact corrected forecasts have greatly reduced systematic errors. In terms of forecast accuracy, nudging is, on the whole, not better than after-the-fact correction. However, for forecast lead times beyond 10 days, where after-the-fact corrections are currently not possible, nudging is an attractive alternative. The physical process most affected by the nudging process is precipitation. In the nudged model atmosphere without the traditional ?cold bias,? both large-scale and convective precipitation is reduced detrimentally relative to the control runs, possibly due to tuning of the model.