3.10 Using Ferret

Here are some useful things to keep in mind when using Ferret to analyze diagnostic output in Netcdf format.

• Ferret recognized files as being Netcdf format by the ``use'' command. For example, to analyze the diagnostic file ``snapshots.dat.nc'', try

  
  

   use  snapshots.dat.nc
  

  
  

  If a message about negative values at the start of the time axis appears, it just means that the time stamp in the file is before year 1900.

  
  If a message appears complaining that ``evenly spaced axis has edges definition: xt_i - ignored'', it just means that the grid has constant resolution and edges specifications which have been added to the NetCDF file to account for non-uniform grid resolution is being ignored. Nothing to worry about.

• All data on land points are currently set to a flag value of -1.E34 in MOM_3. In early versions, the flag value was set to zero. Ferret can use either of these values to mask out land points for plots. For example, if a flag value of zero was used, the temperature (variable ``temp'') at level k=1 from a snapshot file ``snapshots.yyyyyy.mm.dd.dta.nc'' can be plotted using

  
  

  shade if temp ne 0 then temp
  

  
  

  If the flag value is -1.E34, the following command will produce the same plot

  
  

  shade temp
  

  
  

  because Ferret interprets -1.E34 as missing data.

• When analyzing data from global models where option cyclic has been enabled, it is sometimes useful to move the Greenwich Meridian to the middle of a plot. Otherwise, the Atlantic ocean will be split at the Greenwich Meridian between the eastern and western sides of the plot. As an example, consider the file ``snapshots.yyyyyy.mm.dd.dta.nc'' from the test case. Moving the Greenwich Meridian can be done in Ferret by defining a new longitude axis ``xnew'' by cloning a portion of the original x-axis without the extra longitudes (i=1 and i=92), and using option ``modulo''. The Ferret command is

  
  

  define axis/from_data/name=xnew/x/units=degrees x=[g=temp,i=2:91]
  set axis/modulo xnew
  

  
  

  The following Ferret commands will contour the stream function from file ``snapshots.yyyyyy.mm.dd.dta.nc'' with the Greenwich with a longitudinal region specified from $20^\circ W$ to $20^\circ E$.

  
  

  set reg/x=20w:20e
  fill psi[gx=xnew]
  

  
  

• Some NetCDF datasets such as Hellerman windstress which has been interpolated to model resolution by script run_sbc or Levitus data which has been interpolated to model resolution by script run_ic do not have land values flagged. The interpolated data from which NetCDF formatted data is constructed contains linearly extrapolated values over land (there is no information on which cells are land and which are ocean). The reason for this is so that if changes are made to topography the datasets don't need to be re-generated. However, the un-interpolated Levitus NetCDF dataset produced by script run_levitus_netcdf) has land values flagged. When comparing interpolated datasets (without flagged land values) with model generated data, the land flags can easily be generated for plotting purposes. For instance, suppose model generated temperature (variable ``temp'' from ``snapshots.yyyyyy.mm.dd.dta.nc'' which is assumed to be the first dataset [d=1]) is to be compared with interpolated Levitus temperature (variable ``t_lev'' from ``levitus.dta.nc'') for March at level k=2 (the second dataset [d=2]). The following commands will show the interpolated Levitus data with land masked out

  
  

  shade if temp[d=1] ne -1.E34 then t_lev[k=2,l=3,d=2,j=2:60]
  

  
  

  If the flag value of ``-1.E34'' does not work then use a ``0'' instead. The l=3 signifies the month of March and the range on ``j'' is to make the latitude range match the range from the file snapshots.yyyyyy.mm.dd.dta.nc for the test case resolution.