Gemclim 3.3.3.1 nudging

Spectral nudging

Spectral nudging is a way to force (nudge) the larger scales of an RCM towards the ones of its driving GCM resp. RCM.

The information below is for CRCM5/GEMCLIM v_3.3.3 and v_3.3.3.1 only!
Older version of CRCM5/GEMCLIM do not use the same spectral nudging, but a veeeery costly one which is controlled differently and might or might not work. - Sorry.

References:

• Šeparović L, de Elía R, Laprise R (2012) Impact of spectral nudging and domain size in studies of RCM response to parameter modification. Clim Dyn 38(7-8): 1325-1343 http://link.springer.com/article/10.1007%2Fs00382-011-1072-7
  Exerpt:
  "Nested limited-area Regional Climate Models (RCMs) are models that dynamically downscale global General Circulation Model (GCM) simulations or objective analyses to high-resolution computational grids, using a high-resolution representation of the surface forcing and model dynamics. RCMs require the information on some prognostic variables as their lateral boundary conditions (LBC). The choices of integration domains and nesting techniques are free parameters of RCMs. The optimal integration domain depends on the particular situation, although there are some general recommendations that can facilitate user’s judgment (e.g., Laprise et al. 2008). For example, Leduc and Laprise (2008) showed that the use of a too small domain could result in the simulations being deficient in fine-scale variance. It has been also noted that in large continental-scale domains RCM large-scale variables can considerably drift from the driving fields, which can then result in appearance of large spurious gradients in the vicinity of the outflow boundaries. Spectral nudging (SN; von Storch et al. 2000; Biner et al. 2000) has been employed to ensure that the model solution remains close to the large-scale components of the driving fields over the entire domain. However, the use of spectral nudging remains an open issue. …"

• von Storch H, Langenberg H, Feser F (2000) A spectral nudging technique for dynamical downscaling purposes. Mon Weather Rev 128: 3664–3673
• Biner S, Caya D, Laprise R, Spacek L (2000) Nesting of RCMs by imposing large scales. Research activities in Atmospheric and Oceanic Modelling, WMO/TD 987, Report 30: 7.3-7.4

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:

    findfft -gnimax 500 -cfl Pil_maxcfl

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:

    Lam_spn_nudging_S = 'UV'     ,

One needs to set this parameter to switch the spectral nudging on!

Half responds 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 size of the driving data. The half responds wavelength needs to be given in kilometers. For example set:

    Lam_spn_half_respon = 1000.  ,

The default is 1177.

Controlling the vertical profile

There are 3 parameters to control the nudging profile:

    Lam_spn_up_const_lev    : above this level (in hybrid values) the profile is set to 1, to full strength nudging (default 0., model top).
    Lam_spn_start_lev           : below this level (in hybrid values) the profile is set to 0, no nudging (default 1., surface).
    Lam_spn_trans_shape_S : Between these two levels the profile goes from 0 to 1. Either linearly ('LINEAR') (default) or with a cos2 ('COS2') function.

For example set:

    Lam_spn_up_const_lev   = 0.           ,
    Lam_spn_start_lev         = 0.5         ,
    Lam_spn_trans_shape_S = 'LINEAR' ,

Controlling the strength

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. To control how much is applied, 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:

    Lam_spn_relax_hours = 10. ,

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.

If you are running v_3.3.3, please talk to me (winger.katja@uqam.ca).

If you are running v_3.3.3.1, please follow the instructions below:

1. Set the model environment (with 3331   resp.   . s.ssmuse.dot gemclim_3.3.3.1-dev )
   
2. Go into the directory in which you create your executables
3. Get a copy of the corrected function:
       cp ${gemclim}/Sources/Patches/spn_calfiltre.ftn .
4. Compile the corrected function:
       make spn_calfiltre.o
5. Link and load the model executable:
       make gemclimdm
   

Then launch your simulation as usual.