12 September 2000 The data assimilation procedure moved from 6-hour to 12-hour cycling. 4D-Var now processes the observations in 12-hour sets, spanning 03 -15 UTC for the 12 UTC analysis, and 15 - 03 UTC for the 00 UTC analysis. Surface analyses still run every six hours. Analysis fields are still archived every six hours. Other changes included in this model version are :
- use of more accurate background trajectory in 4D-Var, thanks to an improved interpolation procedure, the use of the prognostic cloud scheme and first-guess cloud fields;
- change to the 4D-Var incremental formulation by which the low-resolution increment is added to the high-resolution trajectory at analysis time (00 UTC and 12 UTC), instead of at the start of the 4D-Var window;
- new quality-control step that prevents the use of observations that the incremental formulation of 4D-Var cannot handle correctly;
- resetting of the stratospheric ozone and switching off of the multivariate coupling between ozone and vorticity;
- monitoring of TOVS radiances in cloudy areas.