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. Newtonian relaxation, here 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 "offline" mode and 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 experiment. 

In this article we explain how to prepare and configure the model for a nudged simulation. Therein the model will read in meteorological parameters at the grid scale from pre-computed external forcing files.