ECMWF Forecasting System Research Articles

Error growth and estimates of predictability from the ECMWF forecasting system

Error growth and estimates of predictability from the ECMWF forecasting system

Error growth and estimates of predictability from the ECMWF forecasting system

AbstractExamination has been made of the skill of ECMWF forecasts of the 500 hPa height field produced daily out to ten days ahead, verifying in the period from 1 December 1980 to 31 May 1994. Over this time accuracy has been improved substantially over the first half of the forecast range. the systematic (seasonal‐mean) component of the error has been greatly reduced at all forecast times, but there has been little reduction in the non‐systematic (transient) component later in the range.The simple model proposed by Lorenz for the intrinsic growth of forecast error has been applied to the evolution of differences between consecutive forecasts. the implied growth‐rates of small forecast errors have increased significantly since 1981. They do not show much variation with season, and are a little lower in the southern than in the northern hemisphere. the most recent error‐doubling times are around 1.5 days for the northern hemisphere and 1.7 days for the southern hemisphere. Error saturation levels are at present similar to or greater than those of the 1981 version of the model, having been significantly lower in intermediate years. the accuracy of recent short‐ and early medium‐range forecasts and realism of the climatology of the forecast model support the view that estimates of intrinsic error‐growth parameters from the current forecasting system are more reliable than those obtained earlier. Forecast accuracy later in the medium range may thus not have benefited fully from improvements earlier in the range because of the faster error‐growth associated with a more active, though more realistic, forecast model. Overprediction of variance may nevertheless detrimentally affect present levels of skill and estimates of predictability in all seasons other than summer.The error‐growth model currently indicates that it is possible, in principle, to make deterministic mediumrange forecasts for the extratropical 500 hPa height field of the northern hemisphere that are as accurate five days ahead as present forecasts are three days ahead, provided the one‐day forecast error can be reduced by the same factor in the future as has actually been achieved in the years since 1981. the level of error currently reached at day seven would then be reached at around day ten. the scope for improvement of forecasts for the southern hemisphere appears to be rather larger. Improvements seem to be possible throughout the spectral range studied, up to total wave‐number 40. This is found also for the rotational and divergent wind components at 850 and 200 hPa. For these components, particularly the divergent component, there is a quite pronounced error in the representation of the largest scales.

Systematic errors of the ecmwf operational forecasting model in tropical regions

AbstractThe performance of the ECMWF forecasting system in tropical prediction is discussed in terms of objective scores and systematic errors. the temperature, humidity and wind errors appear to be mutually consistent. the forecasts exhibit a cooling in the tropical stratosphere, warming at and below the tropical tropopause and a cooling in the mid and lower tropical troposphere. the analyses appear too humid in the lower troposphere, while the forecasts show a drying. Precipitation and evaporation are both too weak; a new energy balance within the model is slowly established through cooling, this new balance having a reduced total energy (Lq + cqT). the associated wind errors are seen to have a highly baroclinic structure and a meridional extent such as to affect significantly the baroclinicity of the mid‐latitude flow, particularly over the North and South Atlantic. Systematic errors are large compared to the variability of the fields themselves and show a remarkable temporal consistency from month to month.