Naslov
Extended-range predictions with ECMWF models. III. Time-lagged ensemble forecasting.

Autori
Branković, Č. ; Palmer, T.N. ; Molteni, F. ; Tibaldi, S. ; Cubasch, U.

Izvornik
ECMWF Technical Memorandum 153

Vrsta, podvrsta
Ostale vrste radova, izvještaj

Godina
1989

Ključne riječi
time-lagged ensembles; extended-range; ECMWF

Sažetak
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.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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