Re: [ferret_users] Store an array of georeferenced values for climate model : Masking

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

Hi Eowyn,
You can set up a variable in Ferret and use it to define a set of masks 
that can be combined to form the mask you want. Use the same grid as the 
climate model.  It's a little tedious, but not too hard, really.

You could use the grid from your climate model, or just define axes 
having the same grid. For example,

yes? define axis/x=1:360:1/units=degrees/modulo xlon
yes? define axis/y=-90:90:1/units=degrees ylat

Then define a set of masks.  I'll define them to have values of 1 or 0, 
and then at the end these can be used to define your regions with the 
values N=300 or N=100.  Remember that adding two masks gets us an OR 
operation (mask1 or mask2) , and multiplying gets an AND (mask1 and 
mask2).  Try this and you'll see how it goes. 

define axis/x=1:360:1/units=degrees/modulo xlon
define axis/y=-90:90:1/units=degrees ylat
let testvar = 1 + 0*x[gx=xlon] + 0*y[gy=ylat]   ! apply the mask to this

(Or start with a file:
  use my_data.nc
  let testvar = 1+0*var_from_file 
  using x[gx=testvar] etc below)

let Alon1 = if x[gx=xlon] gt 20 then 1 else 0
let Alon2 = if x[gx=xlon] lt 120 then 1  else 0
let Alat1 = if y[gy=ylat] gt -60 then 1 else 0
let Alat2 = if y[gy=ylat] lt -20 then 1 else 0
let Alonmask = Alon1 * Alon2
let Alatmask = Alat1 * Alat2
let Alonlatmask = Alonmask* Alatmask  ! creates a 2-D mask

shade Alonlatmask * testvar; pause  ! to check what you've done

let Blon1 = if x[gx=xlon] gt 120 then 1 else 0
let Blon2 = if x[gx=xlon] lt 145 then 1  else 0
let Blat1 = if y[gy=ylat] gt -10 then 1 else 0
let Blat2 = if y[gy=ylat] lt 20 then 1 else 0
let Blonmask = Blon1 * Blon2
let Blatmask = Blat1 * Blat2
let Blonlatmask = Blonmask* Blatmask

shade Blonlatmask * testvar; pause

shade (Alonlatmask+Blonlatmask) * testvar; pause  ! both rectangles

let Clon1 = ...

! And so forth. When all done, add them all up, and where the
! sum > 0 the mask is your value of 300, else 100. This works
! even if some of the rectangles overlap each other.

LET all_mask = if (Alonlatmask+Blonlatmask+...) GT 0 then 300 else 100


! Multiply by test_var to define the final mask over the whole grid:
shade all_mask * testvar; pause


LET n = all_mask * testvar
save/file=mask_file.nc n


Eowyn.Connolly-Brown@xxxxxxxx wrote:
> Greetings!
> I have searched the mail archives, but have been unable to find a solution to my problem. 
>
> I want to create a netCDF file that contains an array of values that are referenced by latitude and longitude. Specifically, I have 4 regions in which I want to set N=300 and set N=100 otherwise. This array will then be supplied to a climate model. 
>
> The mask=if ... else command would be perfect, except it requires me to already have a data set from which I take a variable to analyze. The set region/x=/y= command would be perfect, except you can't "add" nonadjacent regions the way you can add masks. 
>
> Has anyone done this before? 
>
> Thank you very much!!
>