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In this article we explain how to prepare and configure OpenIFS 43r3 for a nudged simulation. Therein the model needs to read meteorological parameters at the grid scale from pre-computed external forcing files. These forcing files have to be created prior to the nudged OpenIFS model experiment and this process is also described here. | Info |
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Please note that nudging in OpenIFS is an experimental research tool and therefore may change between model versions. For further assistance on nudging and configuring OpenIFS please post your question in the OpenIFS User Forums or alternatively email openifs-support@ecmwf.int |
Newtonian RelaxationOpenIFS 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, sometimes referred to as "nudging", is a simple form of data assimilation which allows the user to constrain or "force" the model's meteorological fields with reanalysis data. This is sometimes referred to as running the model in "offline" mode. In nudged configuration the model's dynamics is continually nudged towards the meteorological reanalysis independent of the run length of the experiment. This method relaxes the model state towards gridded re-analysis data (or towards output from another atmospheric model, or gridded observational data) by adding a non-physical relaxation term to the model equations (Jeuken et al., 1996). In the equation below X represents a prognostic model variable and Fmodel the model forcing which determines the evolution of X. The relaxation term G (Xobs - X) includes the relaxation coefficient G (in s-1) which determines the "tightness" of the nudging. | Mathdisplay |
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\frac{\partial X}{\partial t} = F_{model}(X) + G(X_{obs} - X) |
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. Another example for its use is to align a climate model simulation closer to historic meteorology for comparison with measurements. |
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For the preparation of a nudged model experiment the following points should be given considerationconsidered:
Nudged variables
The current setup model version permits the nudging of the following 9 prognostic variables:
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The decision on which variable requires nudging will depend on the scientific objectives of the model experiment. Often it is not necessary to nudge all variables, however as a minimum it is recommended we recommend to constrain vorticity, divergence and temperature.
In this context disk space usage may also be will become a consideration as an increasing number of nudged variables will result in larger forcing files. At T255L60, for instance, forcing files that contain all 9 variables require 70 MB for each time step (grid point file 47 MB, spectral file 23 MB).
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As a standard all relaxation coefficients are initially set to 0.5 which results in a relatively "tight" nudging to the external data. These values should be adjusted for each experiment according to the its objectives of the experiment. Frequently the best results are obtained with when different relaxation coefficients are used, specific to each nudged variable. The external analysis data is updated every six hours and the model linearly interpolates in time between these data points. Too tight nudging can result in unrealistic behaviour in the freely calculated model variables.
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