Some aspects of the improvement in skill of numerical weather prediction

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 in level typical of radiosonde measurement error. Intrinsic error-doubling times, computed from the divergence of northern hemisphere forecasts started I 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 2 1-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.