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Grid size restrictions
To be able to use spectral nudging in CRCM5, the core grid needs to match the FFT criteria in x- and y-direction. You can use 'findfft' to get the possible values to set 'Grd_ni' and 'Grd_nj' in your gemclim_settings.nml. (You need to set the model environment first to have access to 'find'fft'.)
For example:
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where the key '-cfl' needs to be set to value of 'Pil_maxcfl' from your gemclim_settings.nml!!!!!
Your 'Grd_ni' and 'Grd_nj' will then have to be set to one of the values findfft is listing.
If your grid is larger than 500 points, just set '-gnimax' to a number large enough.
Activate spectral nudging in CRCM5
To activate spectral nudging in CRCM5 one has to set several parameters in the file 'gemclim_settings.nml' in the namelist 'gem_cfgs'.
Fields to nudge
In CRCM5 one has the option of nudging u-wind, v-wind, and/or temperature.
Tests have shown that it is sufficient to nudge u-wind and v-wind and that nudging the temperature as well does not change much the results. Therefore I suggest to set:
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One needs to set this parameter to switch the spectral nudging on!
Control the strength of the spectral nudging
There are 3 aspects controlling the strength of the spectral nudging which will all get applied at each time step, one after the other:
- a filter in spectral space (horizontal)
- the shape of the vertical profile
- the intensity of nudging applied at each time step
1) Half response wavelength
This is the wavelength at which equal weight is given to the driving data and the RCM solution. For longer wavelengths more weight is given to the driving data, for shorter wavelengths more weight is given to the RCM. A recommanded value is about 10 or 15 times the grid point size of the driving data. The half response wavelength needs to be given in kilometers.
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Lam_spn_half_respon = 1000. ,
The default is 1177.
2) Controlling the vertical profile
There are 3 parameters to control the nudging profile:
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Lam_spn_up_const_lev = 0. ,
Lam_spn_start_lev = 0.5 ,
Lam_spn_trans_shape_S = 'LINEAR' ,
3) Controlling the strength
The spectral nudging gets applied at each time step. The difference between RCM and driving data gets calculated (for different wave lengths) and with that an increment, by which the model results would need to get nudged to match the driving data. But this increment is usually not fully applied, but only a fraction of it. Since the idea is to "nudge" the large scales of the model towards the ones of the driving data and not to impose them.
To control how strongly the model is nudged, one does not say "I want to apply n% of this increment", but one rather says "I want this increment to be applied over h hours". So if, for example, one wants to apply only 5% of the increment and the model time step is 30 minutes, the time needed to apply this increment fully (if applied at each time step) would be 10 hours. So in this case one would set:
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10 is also the default value.
Modifications to your executable
Unfortunately, the spectral nudging in GEMCLIM v_3.3.3 and v_3.3.3.1 is not exactly doing what it is supposed to do.
You therefore need a patch.
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