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.

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

Set the key '-cfl' 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'     ,

Half responds wavelength

This is the wavelength at which equal weight is given to the driving data and the RCM solution. For longer wavelength more weight is given to the driving data, for shorter wavelength more weight is given to the RCM. Therefore, this is one way to control the strength of the nudging. Typical wavelengths are between 1000km and 2000km. For example set:

    Lam_spn_half_respon = 1000.  ,

Controlling the 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.).
    Lam_spn_start_lev           : below this level (in hybrid values) the profile is set to 0, no nudging (default 1.).
    Lam_spn_trans_shape_S : Between these two levels the profile goes from 0 to 1. Either linearly ('LINEAR') 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' ,