The ECMWF implementation of three-dimensional variational assimilation (3D-Var). III: Experimental results
Article first published online: 15 DEC 2006
Copyright © 1998 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society
Volume 124, Issue 550, pages 1831–1860, July 1998 Part B
How to Cite
Andersson, E., Haseler, J., Undén, P., Courtier, P., Kelly, G., Vasiljevic, D., Brankovic, C., Gaffard, C., Hollingsworth, A., Jakob, C., Janssen, P., Klinker, E., Lanzinger, A., Miller, M., Rabier, F., Simmons, A., Strauss, B., Viterbo, P., Cardinali, C. and Thépaut, J.-N. (1998), The ECMWF implementation of three-dimensional variational assimilation (3D-Var). III: Experimental results. Q.J.R. Meteorol. Soc., 124: 1831–1860. doi: 10.1002/qj.49712455004
- Issue published online: 15 DEC 2006
- Article first published online: 15 DEC 2006
- Manuscript Revised: 27 AUG 1997
- Manuscript Received: 20 DEC 1996
- Data assimilation;
- Numerical weather prediction;
- Objective analysis
In this third and final paper of a series, we assess the performance of the three-dimensional variational data assimilation scheme, in the light of the results from the extensive pre-operational programme of numerical experimentation. Its performance is compared with that of the previous operational scheme at the European Centre for Medium-Range Weather Forecasts, which was based on Optimal Interpolation. The main features of the new scheme are illustrated, in particular the effects of non-separable structure functions and the improved data usage. TIROS-N Operational Vertical Sounder cloud-cleared radiances, for example, are used directly without a separate retrieval step. Scatterometer data are assimilated in the form of ambiguous winds with the ambiguity removal taking place within the analysis itself. Problems encountered during the tests are discussed and the solutions implemented are explained.
The overall impact on forecast accuracy in the troposphere of the northern hemisphere extratropics is neutral for geopotential and positive for wind and temperature. The impact is neutral in the tropics, and significantly positive in the southern hemisphere. Analyses and forecasts for the stratosphere have improved in all regions. Other positive results include a clear improvement in analyses of near-surface winds over oceans, particularly in the vicinity of tropical storms. This is predominantly because of the assimilation of scatterometer wind-data.