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- 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 and stratospheric ozone. 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. In this context disk space usage should also be a consideration as 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 (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.
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