Performance of Global and Regional NWP Models in Their Prediction of Typhoon Nat (1991)

Abstract

In 1991, Typhoon Nat over the western North Pacific made four directional reversals due to its interactions with two other tropical cyclones (TCs), Luke and Mireille. This paper analyzes the performance of three global and two regional models in predicting the movement of Nat to determine the extent to which each of the models was capable of correctly simulating such binary interactions. The global models include those of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the U.K. Meteorological Office (UKMO) and the U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS). The regional models studied are the Typhoon Model (TYM) of the Japan Meteorological Agency and the One-Way Tropical Cyclone Model (OTCM) of the U.S. Navy. It was found that in general the global models made better predictions than the regional ones, especially when the large-scale flow was well defined. During the interaction periods, the UKMO model and the TYM were the best. The ECMWF model was also quite good in capturing the latter part of the Nat-Mireille interaction when Mireille had a large circulation. Although NOGAPS had a bogus vortex in the model, it did not predict the interactions very well. The OTCM was the worst of the models, possibly because of the steering flow imposed onto the model vortex. The main conclusions from this study are that a bogus vortex representative of the actual TC appears to be necessary for properly simulating the interaction between TCs. An increase in resolution may also help in this respect. However, imposing a persistence vector into a model to simulate steering may prove detrimental in predicting binary interactions.