Re: [ferret_users] Telling Ferret How to Convert Units of "sigma_level"

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi John,
If you look at the dataset you'll see that "sigma" is not a variable:

   yes? use "http://205.156.4.56/cgi-bin/nph-dods/dgom/ngom311_05Nov043D.nc"
    *** NOTE: Units on axis "sigma" are not recognized: sigma_level
    *** NOTE: They will not be convertible:
  
   yes? show data
        currently SET data sets:
       1> http://205.156.4.56/cgi-bin/nph-dods/dgom/ngom311_05Nov043D.nc  (default)
    name     title                             I         J         K         L
    LON      Longitude                        1:217     1:160     ...       ...
    LAT      Latitude                         1:217     1:160     ...       ...
    MASK     Land Mask                        1:217     1:160     ...       ...
    DEPTH    Bathymetry                       1:217     1:160     ...       ...
    ZETA     Water Surface Elevation          1:217     1:160     ...       1:15
    U        Eastward Water Velocity          1:217     1:160     1:37      1:15
    V        Northward Water Velocity         1:217     1:160     1:37      1:15
    TEMP     Temperature                      1:217     1:160     1:37      1:15
    SALT     Salinity                         1:217     1:160     1:37      1:15
   
So, check the grid of the variables:

   yes? show grid temp
  
       GRID GQD3
    name       axis              # pts   start                end
    NX        X                  217 r   1                    217
    NY        Y                  160 r   1                    160
    SIGMA     Z (sigma_level      37 i-  0                    1
    TIME      TIME    

Sigma is a coordinate axis, so you need to define a variable with the values of sigma, and then you can proceed with your calculation.

  yes? let sig = z[gz=temp]
  yes? let depthValues = sig * ( depth + zeta )
  yes? define axis/z=1:37:1/units=meters/depth zdepth
  yes? let saltondepth = zaxreplace(salt,depthValues,z[gz=zdepth])

Note also that the result is still a 4-dimensional variable, so to PLOT it, choose a 1-dimensional subset

  yes? PLOT/i=100/j=100/L=1 saltondepth



John Schattel wrote:

Hello All,

I'm trying to use ZAXREPLACE to regrid from sigma to depth using the commands listed below.  But Ferret complains about an unknown variable SIGMA.  Is this an issue with Ferret not being able to convert the sigma axis because it uses units of "sigma_level"?  If so, can anyone tell me how to let Ferret know how to do the conversion?

Thanks,

John

use "http://205.156.4.56/cgi-bin/nph-dods/dgom/ngom311_05Nov043D.nc"
*** NOTE: Units on axis "sigma" are not recognized: sigma_level
*** NOTE: They will not be convertible:
let depthValues = sigma * ( depth + zeta )
define axis/z=1:37:1/units=meters/depth zdepth
let saltondepth = zaxreplace(salt,depthValues,z[gz=zdepth])
plot saltondepth
**ERROR: variable unknown or not in data set: SIGMA