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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 As a standard value for all relaxation coefficients is are initially set to 0.5 which corresponds to represents a relatively "tight" nudging to the analysisexternal data. These values should be modified adjusted according to the objectives of the experiment and not all variables require the same relaxation coefficient. Frequently the best results are obtained with different relaxation coefficients 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. 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 re-analyses are at T639 (31 km globally) with 137 levels. The IFS experiment described below is able to generate forcing files at either of these two resolutions. In order to nudge the model when using other resolutions the forcing files need to be first interpolated to the model grid.
- Data storage: The spectral and gridpoint forcing files need to be accessible to the model at runtime, i.e they must not be read from an offline tape archive. Depending on the length of the model experiment (e.g. for a seasonal forecasts or in climate model configuration) it is conceivable that several months or even years of 6-hourly forcing files need to be prepared. This will have implications for the available disk space. At T255 with 60 levels one year of forcing files can require up to 100 GB disp disk space (depending on the number of variables). Once the forcing files have been created their long-term storage may need to be considered.
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