TROPICAL CYCLONE TRACK PREDICTABILITY

In this study an attempt is made to estimate the inherent limits to tr opical cyclone mean absolute track position errors out to 72 hours ahe ad and to compare these estimates with the position errors currently b eing obtained in practice at weather centres around the world. A knowl edge of the magnitude of the difference between the lower limit to pre dictability and that being achieved with state-of-the-art numerical we ather prediction (NWP) models is of vital importance. A small differen ce would indicate that there is little further need for continued init iatives in the prediction of tropical cyclone tracks. On the other han d, a large difference would imply either that the problem requires con tinued emphasis or if there has been no significant trend towards redu cing the forecast track errors, that present research and development techniques need to be extended or new procedures developed. It was fou nd that the difference between the inherent and practical limits of tr opical cyclone track position errors is presently about 35 to 40 per c ent for advanced baroclinic NWP systems, a moderate to large differenc e, and one that is almost invariant between tropical cyclone basins. F or simpler models, such as barotropic models, the difference is closer to 45 per cent but is again almost invariant. As far as the authors a re aware, these are the first estimates of the lower bounds of tropica l cyclone track predictability. Finally, very recent research studies with emerging range of high quality data, high density data sources, i mproved models and new data assimilation techniques suggest that the d ifference possibly is now down to about 30 to 35 per cent. This value is encouragingly small but still large enough to continue active resea rch programs in improving tropical cyclone motion prediction. Much of the forecast track errors now come from major forecast errors associat ed with tropical cyclones that follow erratic tracks.