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It has become necessary to diagnose model systematic errors over a longer forecast range and dependence upon season and model resolution. Zonal mean and mid-latitude errors are dependent strongly upon the lower end of the resolution spectrum. Tropospheric errors can be described as a poleward displacement of the main jet maxima with equivalent barotropic structure. In the tropics, a progressive loss of the vertical shear in zonal wind occurs. The 30-day mean forecast fields are almost consistently better than persistence.

A set of 16 ensembles of time-lagged extended-range forecasts have been run at different times of year using the T63 version of the ECMWF operational model. Each ensemble was composed of 9 integrations from consecutive 6-hourly analyses. Theoretical properties of ensemble-mean skill and ensemble spread are studied using a simple model of error growth with a parametrization of the ECMWF model error. The impact of systematic error on the potential improvement in skill of the ensemble-mean forecast is discussed. The presence of model errors considerably reduces the gain from ensemble averaging. In practice, about a third of the ensemble-mean forecasts, at forecast days 11-20, were more skilful than both persistence and climate, and, in addition, were more skilful than the latest member of the ensemble. At days 21-30. only one of the ensemble-mean forecasts was similarly skilful. Whilst there is an overall hemispheric scale correlation between ensemble spread and skill, a substantial part of this reflects the impact of the annual cycle on both quantities. In the winter period, however, no clear spread/skill correlation was found. Within the winter period, there was considerable case-to-case variability in forecast skill. Of all the ensembles that of January 1986 was poorest, whereas that of February 1986 was one of the best. The different character of these two ensembles was shown by considering phase-space trajectories of the ensemble forecasts in the plane spanned by the two principal forecast EOFs of 500mb height. During the first 15 days, the trajectories of the January ensemble forecasts were consistent with each other, but not with the observed atmospheric trajectory (which was associated with the onset of blocking over Europe). During the last 15 days, as the January ensemble forecasts migrated from positive to negative PNA index, the trajectories dispersed quite strongly, becoming disordered. By contrast, the trajectories of the February forecasts remained mutually consistent and in agreement with the real atmosphere’s trajectory throughout most ot the forecast period.

A set of 16 ensembles of time-lagged extended-range forecasts have been run at different times of year using the T63 version of the ECMWF operational model.

The dataset of 30-day integrations with various model resolutions was used to evaluate model climate drift as a function of resolution. The zonal mean and eddy systematic errors show a strong dependence upon resolution. Some information about the initial conditions is retained almost throughout the integration time. There is a progressive loss of stationary and transient eddy activity. Both, the time-mean circulation and eddy activity are much weaker than observed in the tropics.

The performance of a number of time-lagged extended-range ensembles of forecasts with ECMWF operational model at T63 resolution was evaluated. Ensemble averaging reduces the RMS error. There is positive correlation between RMS spread and skill, however, much of this could be explained by the annual cycle. During winter spread and skill tend to be anticorrelated, which may indicate a systematic error in the model.

The current ECMWF operational forecasting system is insufficient to support monthly deterministic forecasts. Climate drift can be identified as one of the main reasons for this drawback. It causes the disruption of the main steering flow that occurs in the low frequency timescales. The variance in the model is underestimated.

It has become necessary to diagnose model systematic errors over a longer forecast range and dependence upon season and model resolution. Zonal mean and mid-latitude errors are dependent strongly upon the lower end of the resolution spectrum. Tropospheric errors can be described as a poleward displacement of the main jet maxima with equivalent barotropic structure. In the tropics, a progressive loss of the vertical shear in zonal wind occurs. The 30-day mean forecast fields are almost consistently better than persistence.