1. Bannister, R. N., 2017: A review of operational methods of variational and ensemble-variational data assimilation. Quart. J. Roy. Meteor. Soc., 143, 607–633, doi: 10.1002/qj.2982.
2. Clayton, A. M., A. C. Lorenc, and D. M. Barker, 2013: Operational implementation of a hybrid ensemble/4D-Var global data assimilation system at the Met Office. Quart. J. Roy. Meteor. Soc., 139, 1445–1461, doi: 10.1002/qj.2054.
3. Courtier, P., J. N. Thépaut, and A. Hollingsworth, 1994: A strategy for operational implementation of 4D-Var, using an incremental approach. Quart. J. Roy. Meteor. Soc., 120, 1367–1387, doi: 10.1002/qj.49712051912.
4. Courtier, P., E. Andersson, W. Heckley, et al., 1998: The ECMDECEMBERWF implementation of three-dimensional variational assimilation (3D-Var). I: Formulation. Quart. J. Roy. Meteor. Soc., 124, 1783–1807, doi: 10.1002/qj.49712455002.
5. Fisher, M., 1998: Minimization algorithms for variational data assimilation. Annual Seminar on Recent Developments in Numerical Methods for Atmospheric Modelling, Shinfield Park, Reading, 7–11 September 1998, ECMWF, 364–385.