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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

Please note that nudging in OpenIFS is an experimental research tool and therefore may change between model versions.

For further assistance with configurations of OpenIFS, please contact: openifs-support@ecmwf.int.

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, 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. 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.

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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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Planning the Experiment

For the preparation of a nudged model experiment the following points should be given consideration:

  • Nudged variables: The current setup permits the nudging of the following 9 prognostic variables:  Vorticity, divergence, temperature, surface pressure, specific humidity, ice water contents, liquid water contents, cloud fraction , surface pressure and stratospheric ozone. Depending Dependent on the scientific objectives of the model experiment not all variables will require nudging. As a minimum it is recommended to use vorticity, divergence and temperature.  Further, In this context disk space usage is should also be a consideration as forcing files increase in size when they contain more 3D fields. At T255L60 an increasing number of nudged variables will result in larger forcing files. At T255L60 forcing files that contain all 9 variables require 70 MB for each time step the corresponding spectral and gridpoint forcing files require 70 MB.  
  • Tightness of the nudging
  • Grid resolution: data source, interpolation issues
  • (grid point file 47 MB, spectral file 23 MB).
      
  • Relaxation coefficient: The standard value for all relaxation coefficients is 0.5 which corresponds to a relatively "tight" nudging to the analysis. These values should be modified according to the objectives of the experiment and not all variables require the same relaxation coefficient. The external analysis data is updated every six hours and the model linearly interpolates in time between these data points. A too tight nudging can result in unrealistic behaviour in the freely calculated model variables. 

  • Grid resolution:  One of the strengths of OpenIFS is its ability to operate with a wide range of grid resolutions. The model is however not capable of spatial interpolation of the gridded forcing file data. Therefore the forcing files need to be prepared at the horizontal and vertical resolution of the model experiment. ERA-Interim re-analyses have a horizontal resolution of T255 (79 km globally) with 60 levels and ERA-5 at T639 (31 km globally) with 137 levels and the IFS experiment described below is able to generate forcing files at these resolutions. In order to nudge the model at other resolutions the forcing files need to be first interpolated to the target grid. 

  • Data storage:  The spectral and gridpoint forcing files need to be readable from a file system that is accessible to the model. Depending on the length of the model experiment (e.g. for seasonal forecasts or in climate model configuration) it is conceivable that several months or years of 6-hourly forcing files need to be prepared which will have implications for disk space availability. At T255L60 one year of forcing files requires up to 100 GB depending on the number of variables. Once these forcing files have been created their long-term storage may need to be considered.Data storage: length of the experiment

Preparation of Forcing Files

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