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 was organised to mark 20 years of 4D-Var at ECMWF.


Olivier Talagrand (LMD/ENS)
A few reminiscences about the genesis of 4D-Var, and what followed
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Massimo Bonavita (ECMWF)
Advancing Data Assimilation in Global NWP: the ECMWF Perspective
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Mark Buehner (ECCC)
The big leap: Replacing 4D-Var with 4D-EnVar and life ever since

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Philippe Courtier (UTC)
20 years of 4D-Var, the decision making process

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John Derber (NCEP)
The early years of variational data assimilation: A perspective from NCEP
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Ko Koizumi (JMA)
4D-Var data assimilation system for a limited-area model in JMA and the assimilation of precipitation amounts
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Andrew Lorenc (Met Office)
A comparison of hybrid variational data assimilation methods in the Met Office global NWP system
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Jean-Noël Thépaut (ECMWF)
4D-Var: From early results to operational implementation
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