FORECAST SKILL AND LOW-FREQUENCY VARIABILITY IN NMC DERF90 EXPERIMENTS

A series of 90-day integrations by a low-resolution version (T40) of t he National Meteorological Center's global spectral model was analyzed for its performance as well as its low-frequency variability behavior . In particular, 5-day mean 500-mb forecasts with leads up to 88 days were examined and compared with the observations. The forecast mean he ight decreased rapidly as forecast lead increased. A severe negative b ias of the mean height in the Tropics was caused by a negative tempera ture bias and a drop of the surface pressure of about 2 mb. The foreca st variance also dropped rapidly to a minimum of 75% of the atmospheri c standard deviation before being stabilized at day 18. The model coul d not maintain large anomalous Bows from the atmospheric initial condi tions. However, it is quite capable of generating and maintaining larg e anomalies after drifting to its own climatology and temporal variabi lity. At extended ranges, the model showed better skill over the North Pacific than North Atlantic when the season advanced to the colder pe riod of the DERF90 (dynamical extended-range forecasts 1990) experimen ts. The model also displayed dependence on circulation regimes, althou gh the skill fluctuated widely from day to day in general. Blocking ho ws in the forecast were found to systematically retrogress to the Baff in Island area from the North Atlantic. Therefore, improvements of the model's systematic errors, including its drift, appear to be essentia l in order to achieve a higher level of forecast performance. However, no generalization can be made due to the usage of a low-resolution mo del and the experiments being carried out over a rather short time spa n, from only 3 May to 6 December 1990.