Aj. Simmons et al., ERROR GROWTH AND ESTIMATES OF PREDICTABILITY FROM THE ECMWF FORECASTING SYSTEM, Quarterly Journal of the Royal Meteorological Society, 121(527), 1995, pp. 1739-1771
Examination has been made of the skill of ECMWF forecasts of the 500 h
Pa 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 h
as been improved substantially over the first half of the forecast ran
ge. The systematic (seasonal-mean) component of the error has been gre
atly reduced at all forecast times, but there has been little reductio
n in the non-systematic (transient) component later in the range. The
simple model proposed by Lorenz for the intrinsic growth of forecast e
rror has been applied to the evolution of differences between consecut
ive forecasts. The implied growth-rates of small forecast errors have
increased significantly since 1981. They do not show much variation wi
th 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 thos
e of the 1981 version of the model, having been significantly lower in
intermediate years. The accuracy of recent short- and early medium-ra
nge forecasts and realism of the climatology of the forecast model sup
port 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 realis
tic, forecast model. Overprediction of variance may nevertheless detri
mentally affect present levels of skill and estimates of predictabilit
y in all seasons other than summer. The error-growth model currently i
ndicates that it is possible, in principle, to make deterministic medi
um-range forecasts for the extratropical 500 hPa height field of the n
orthern hemisphere that are as accurate five days ahead as present for
ecasts are three days ahead, provided the one-day forecast error can b
e reduced by the same factor in the future as has actually been achiev
ed in the years since 1981. The level of error currently reached at da
y seven would then be reached at around day ten. The scope for improve
ment of forecasts for the southern hemisphere appears to be rather lar
ger. Improvements seem to be possible throughout the spectral range st
udied, up to total wave-number 40. This is found also for the rotation
al and divergent wind components at 850 and 200 hPa. For these compone
nts, particularly the divergent component, there is a quite pronounced
error in the representation of the largest scales.