Newtonian Relaxation

OpenIFS uses initial and boundary conditions to calculate its own model dynamics, i.e. meteorological variables that are resolved on the grid scale. It is however possible to constrain the model dynamics with external data. Newtonian relaxation, here sometimes referred to as "nudging", is a simple form of data assimilation which allows to constrain or force the model's meteorological fields with reanalysis data. This is sometimes referred to as running the model in "offline" mode and . In nudged configuration the model's dynamics is continually specified by the meteorological reanalysis independent of the run length of the experiment. 

This method can be useful, for instance, in sensitivity studies which aim to isolate the model physics or chemistry while preventing feedbacks to the model dynamics. In nudged configuration the model's dynamics remains specified by the meteorological reanalysis independent of the run length of the experimentAnother example for its use is to align a climate model simulation closer to historic meteorology for comparison with measurements. 

In this article we explain how to prepare and configure the model OpenIFS for a nudged simulation. Therein the model will read in meteorological parameters at the grid scale from pre-computed external forcing files. These forcing files need to be created prior to the nudged OpenIFS model experiment which is also described here.