Revisions during 1996

30 January 1996 ECMWF introduced a 3-dimensional variational (3D-Var) analysis scheme. 3D-Var is a new code for the analysis of model-level values of temperature, vorticity, divergence and specific humidity, and surface pressure. Minor changes to the forecast model were implemented at the same time.

On average, forecasts from 3D-Var for the Northern Hemisphere are of similar quality as forecasts from the previous Optimum Interpolation system, while forecasts for the southern hemisphere tend to exhibit higher skill. In addition, 3D-Var gives generally better temperature verification results, especially at low levels and in the stratosphere, and better wind scores at 200 hPa and above.

The analysis and prediction of tropical cyclones appear to have improved with 3D-Var.

An article from the ECMWF Newsletter describing the changes is given in pdf(232KB) file.

4 March 1996 The generation of initial perturbations for the Ensemble Prediction System (EPS) was changed to be based on singular vectors in the southern hemisphere as well as the northern hemisphere.

23 April 1996 A new definition of the sea surface temperature and sea ice was introduced. The input to the scheme is the 1 degree SST analysis from NCEP Washington and the gridded ice fields derived from SSM/I data from NESDIS.

31 May 1996 A technical error affecting the computation of sub-grid orography processes in the forecast model which was introduced with the change to sea surface temperature and sea ice on 23 April 1996

18 September 1996 The complete operational suite was implemented on the Fujitsu VPP700

1. Main T213 ten-day forecast suite



2. Optional project wave forecast suite



3. 00 UTC optional project forecast suite



4. Ensemble prediction suite.

Together with the move to the VPP700 a new model cycle (cy15r5) and changes to the 3D-Var code were introduced:

model cycle cy15r5, introduced on 18 September, had the following changes:

1. revision of boundary layer diffusion and introduction of soil moisture freezing. The effect is a reduction of the near-surface temperature errors in stable situations. It implies a reduction of the night time temperature errors over land in summer and a significant reduction of the winter cold bias of day and night time forecasts;

An article from the ECMWF Newsletter describing these changes is given in pdf(124KB) file.

2. a revised semi-Lagrangian treatment of the thermodynamic equation leading to smoother meteorological fields over steep orography.

This cycle also contained changes necessary for the migration to the Fujitsu, none of which have any detectable meteorological impact, at the same time.

The 3D-Var code has been generalised to allow it to be run on parallel, distributed-memory computers using message passing. A new parallel code is used for observation pre-processing and screening. Quality control and the calculation of background error variances are now based on the variational analysis rather than the earlier OI approach. Objective verification indicates little overall sensitivity of forecast performance to these changes.

5 December 1996 A high resolution version of the optional project global wave forecast model was introduced. The new model has a resolution of approximately 50km using a quasi-regular latitude/longitude grid with 0.5 degree intervals between latitude rows and varying numbers of points along latitudes.

10 December 1996 Model cycle 15r7 was introduced in both the T213 L31 10-day forecast and the EPS

The main features of the new cycle are:

1. a two-time level semi-Langrangian scheme



2. advection cloud variables



3. changes in the snow albedo, expected to reduce the cold bias in 2m temperature in spring



4. change to the humidity analysis to eliminate unrealistic drying of the extra-tropical lower stratosphere



5. change to the use of SSM/1 sea ice data to eliminate erroneous sea points over permanent ice



6. minor change to the model orography and slight adjustments to other fixed surface fields.