Some aspects of the improvement in skill of numerical weather prediction

Abstract Recent verification statistics show a considerable improvement in the accuracy of forecasts from three global numerical weather prediction systems. The improvement amounts to about a 1‐day gain in predictability of mean‐sea‐level pressure and 500 hPa height over the last decade in the northern hemisphere, with a similar gain over the last 3 years in the southern hemisphere. Differences between the initial analyses from the three systems have been substantially reduced. Detailed study of the European Centre for Medium‐Range Weather Forecasts verifications shows that identifiable improvements in the data assimilation, model and observing systems have significantly increased the accuracy of both short‐ and medium‐range forecasts, although interannual (flow‐dependent) variations in error‐growth characteristics complicate the picture. The implied r.m.s. error of 500 hPa height analyses has fallen well below the 10 m level typical of radiosonde measurement error. Intrinsic error‐doubling times, computed from the divergence of northern hemisphere forecasts started 1 day apart, exhibit a small overall reduction over the past 10 years at day two and beyond, and a much larger reduction at day one. Error‐doubling times for the southern hemisphere have become generally shorter and are now similar to those for the northern hemisphere. One‐day forecast errors have been reduced so much in the southern hemisphere that medium‐range forecasts for the region have become almost as skilful as those for the northern hemisphere. The approach to saturation of forecast error beyond the 10‐day range has been examined for sets of 21‐day forecasts. When the systematic (sample‐mean) component of the error is subtracted, forecast errors and the differences between successive forecasts both appear to level out near the end of the 21‐day range at values close to the limit set by the natural level of variance of the atmosphere for the northern hemisphere. A number of features of the model 500 hPa height fields remain quite realistic at the three‐week range. The most obvious discrepancy in mean climate is in the Pacific/North‐American sector, and variance is too high in the southern hemisphere. Copyright © 2002 Royal Meteorological Society