Linearised physics: the heart of ECMWF's 4D-Var
Physique linéarisée : le coeur du système 4D-Var du CEPMMT
Janisková, Marta ; Lopez, Philippe
In 1997, the incremental four-dimensional variational (4D-Var) data assimilation system became operational at ECMWF. 4D-Var is the system used at ECMWF to combine a short-range forecast with observations to produce the best possible estimate of the current state of the atmosphere. This then provides the initial conditions for a new forecast. The system works by minimising a cost function, which measures the departure of the analysis from observations and the short-range forecast. In this minimisation, linearised versions of the forecast model are used to efficiently determine the optimal 3D representation of the atmospheric state at a given time. From the very beginning, it was recognised that it is crucial to represent physical processes in the linearised model. Parametrization schemes used in linearised models have gradually become more and more complex. Nowadays, comprehensive schemes are included in ECMWF's 4D-Var system. Recent data assimilation tests using the latest version of the linearised physics have again demonstrated a systematic and significant improvement for all parameters, levels, and regions.</p>
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