Symposium: 20 years of 4D-Var at ECMWF

ECMWF | Reading | 26 January 2018


One of ECMWF’s biggest-ever projects was the development of the variational data assimilation system. The project culminated with the operational implementation of 4D-Var on 25 November 1997, less than two years after the introduction of 3D-Var.

4D-Var is a four-dimensional variational data assimilation technique. It performs a statistical interpolation in space and time between a distribution of meteorological observations and an estimate of the model state (the background). This is done in such a way that account is taken of the dynamics and physics of the forecast model to ensure the observations are used in a meteorologically consistent way.

4D-Var development continued after initial implementation with a longer window (12 hour instead of 6 hour, in September 2001), improved physics and multiple outer loops (e.g. in June 2007), and an ensemble of lower-resolution 4D-Var analyses to provide background uncertainty information (in May 2011). This development path continues with 4D-Var now being extended to include atmosphere-ocean coupling in the outer loop and a hybrid extended control vector. A notable success of 4D-Var has been the all-sky assimilation system, which has allowed dynamical information to be extracted from rain- and cloud-affected radiances through the 4D-Var tracer mechanism.

In parallel, ECMWF is working on a major project to provide an Object Oriented Programming System. This system is being developed and interfaced to the Integrated Forecasting System (IFS) to allow a more flexible calling of the data assimilation algorithm, allowing alternative solvers and time parallel weak-constraint approaches such as the saddle point algorithm to be tested.

This symposium is being organised to mark 20 years of 4D-Var at ECMWF.


Olivier Talagrand
Massimo Bonavita

Mark Buehner

Philippe Courtier

John Derber
Ko Koizumi
Andrew Lorenc
Met Office
Jean-Noël Thépaut


The symposium will start at 09:00 and finish at 13:00.


This symposium is open to anyone interested in data assimilation. To register, please complete the online registration form by 22 December 2017.

To register, you will need an ECMWF username. If you already have one and are not logged in you can use the link at the top of this page to do so. If you do not have an ECMWF username, you can register for one using the log in link at the top of the page.