Description of System 3

The resolution of the atmosphere model is increased from TL95L40 to TL159L62. The resolution of the ocean model is unchanged.

The ensemble sizes are changed. The real-time forecasts will now have 41 members (previously 40). The re-forecasts will have 11 members uniformly for all start months (previously 5 members, with 40 members for May and November starts only).

The dates of the re-forecasts have been extended, and will cover the period 1981-2005, i.e. a 25 year period. This is significantly longer than the 1987-2001 15 year period of the previous system, and should allow better calibration of the forecasts and better assessment of their skill.

The forecast length (for both forecasts and back integrations) is increased from 6 to 7 months. This gives a forecast range of 6.5 months from the release date of the forecasts.

Four times a year, from the Feb, May, Aug and Nov starts, 11 members of the forecast ensemble will run to 13 months. This will allow an "ENSO outlook" to be given. The re-forecasts for these start months have 5 ensemble members that extend to 13 months. (The November starts are actually 14 months long in all cases, so they extend to the end of the following calendar year). Note that, at least initially, these extended runs will be classed as experimental rather than operational, and data from them will not be available via dissemination.

Scientifically, there have been many enhancements to the ocean analysis system used to prepare ocean initial conditions. The atmosphere model has been upgraded to IFS cycle 31r1, which is the latest version for medium-range forecasting at ECMWF (introduced on 12 September 2006). There have also been significant revisions to the perturbation methods used to generate the forecast ensemble.

Tests show that the prediction skill for Pacific equatorial SSTs (i.e. El Nino variability) is measurably improved in the new system. Changes in skill for atmospheric variables are less clear cut with the limited test sample presently available. Overall, improvements in predictive skill are expected to be more evident during the Northern Hemisphere summer season, and in tropical regions throughout the year. Full information on the forecast skill, including the WMO SVS scores, will be made available at a later date.