Performance of the ECMWF Model in Predicting the Movement of Typhoon Wayne (1986)

Typhoon ``Wayne'' (1986) was one of the most unusual typhoons ever recorded over the western North Pacific. During its life span of >20 days, it made four directional reversals, all of which took place over the northern part of the South China Sea and the coastal waters near Taiwan. Such a track resulted in some rather large forecast errors. Three of these directional reversals were due to a change in the environmental flow. Interaction between ``Wayne'' and another typhoon, ``Vera,'' was the cause of the other directional change. As the European Centre for Medium-Range Weather Forecasting (ECMWF) model is known to produce rather good predictions of the large-scale flow, its performance in forecasting the movement of ``Wayne'' was examined. It was found that, when the environmental flow around ``Wayne'' was well defined, the model was able to predict the directional change rather well. On the other hand, during the period when the interaction between ``Wayne'' and ``Vera'' took place, the predictions were not satisfactory. The model predictions were also wrong when ``Wayne'' was in a situation when the environmental flow was weak. These results suggest that the ECMWF model can provide a rather accurate prediction of tropical cyclone movement if the environmental flow is well defined. However, the model performance is degraded in situations in which the vortex structure is important. This, therefore, points to the need to insert a bogus vortex in the model analysis to simulate the interaction between the tropical cyclone and its environment.