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Abstract

The prediction of extratropical cyclones by the European Centre for
Medium-Range Weather Forecasts (ECMWF) and the National Centers for
Environmental Prediction (NCEP) ensemble prediction systems (EPSs) has
been investigated using an objective feature tracking methodology to
identify and track the cyclones along the forecast trajectories. Overall
the results show that the ECMWF EPS has a slightly higher level of skill
than the NCEP EPS in the Northern Hemisphere (NH). However in the
Southern Hemisphere (SH), NCEP has higher predictive skill than ECMWF
for the intensity of the cyclones. The results from both EPSs indicate a
higher level of predictive skill for the position of extratropical
cyclones than their intensity and show that there is a larger spread in
intensity than position. Further analysis shows that the predicted
propagation speed of cyclones is generally too slow for the ECMWF EPS
and shows a slight bias for the intensity of the cyclones to be
overpredicted. This is also true for the NCEP EPS in the SH. For the
NCEP EPS in the NH the intensity of the cyclones is underpredicted.
There is small bias in both the EPS for the cyclones to be displaced
toward the poles. For each ensemble forecast of each cyclone, the
predictive skill of the ensemble member that best predicts the cyclone's
position and intensity was computed. The results are very encouraging
showing that the predictive skill of the best ensemble member is
significantly higher than that of the control forecast in terms of both
the position and intensity of the cyclones. The prediction of cyclones
before they are identified as 850-hPa vorticity centers in the analysis
cycle was also considered. It is shown that an indication of
extratropical cyclones can be given by at least 1 ensemble member 7 days
before they are identified in the analysis. Further analysis of the
ECMWF EPS shows that the ensemble mean has a higher level of skill than
the control forecast, particularly for the intensity of the cyclones,
from day 3 of the forecast. There is a higher level of skill in the NH
than the SH and the spread in the SH is correspondingly larger. The
difference between the ensemble mean error and spread is very small for
the position of the cyclones, but the spread of the ensemble is smaller
than the ensemble mean error for the intensity of the cyclones in both
hemispheres. Results also show that the ECMWF control forecast has 1/2
to 1 day more skill than the perturbed members, for both the position
and intensity of the cyclones, throughout the forecast.