Re: masking the CCSM T42 output with a shape file

I need to point something out here. The lat/lon arrays I'm
generating get large
in a hurry, because there are 88565 points in the MRB outline. This
may cause
memory problems if your data array is large to start with. I'm only
using one that is
16 x 32.

You may have to modify the script to loop across each lat/lon point
in your data
grid and call gc_inout for each one, rather than calling gc_inout
with big data arrays.

This will slow your script down considerably, but it won't require as
much memory.

You would remove the lines:

> mrb_lat3d = conform_dims((/nlat,nlon,nmrb/),mrb_lat,2)
> mrb_lon3d = conform_dims((/nlat,nlon,nmrb/),mrb_lon,2)
> lat2d = conform_dims((/nlat,nlon/),lat1d,0)
> lon2d = conform_dims((/nlat,nlon/),lon1d,1)

and do this instead:

;---Create masked data array.
   data_mask = data

   do ilt=0,nlat-1
     xlat = lat1d(ilt)
     do iln=0,nlon-1
       xlon = lon1d(iln)
       if(.not.gc_inout(xlat,xlon,mrb_lat,mrb_lon)) then
         data_mask(ilt,iln) = data_mask@_FillValue
       end if
     end do
   end do

This can be slow!

--Mary

On Mar 26, 2010, at 3:27 PM, Mary Haley wrote:

> Sanjiv,
>
> I'm not sure what you mean by "mask the CCSM3 T43 outputs". Do you
> mean you only want to
> do *calculations* over areas of your grid that are not part of the
> Missippippi River Basin, or do
> you want to *graphically* mask out the area that is the Missippippi
> River Basin?
>
> There's no way in NCL to provide a lat/lon boundary and say "do the
> calculations only on the part of
> the grid that falls within this boundary". In a situation like
> this, you would generally need a separate
> "mask" array that conrtains 0s and 1s indicating where your data is
> inside or outside a particular area.
>
> One thing people have done is to set the areas in the grid that
> they are not interested in to
> data@_FillValue (missing value). This will only work if the
> functions you plan to use know
> how to handle missing data.
>
> To get your data grid filled with missing values in the area you're
> not interested in, see the
> "gc_inout" function:
>
> http://www.ncl.ucar.edu/Document/Functions/Built-in/gc_inout.shtml
>
> Note that this can be slow if you have a lot of points in your
> basin, which you do. And,
> this will provide an approximate area only.
>
> I've attached a sample NCL script that creates a dummy (small)
> array. I then use gc_inout
> to mask out the dummy array that is *not* the MRB (maybe you want
> the opposite of this).
>
> I draw the full data contours on the first frame, the masked data
> contours on the second,
> and the lat/lon grid over the masked area in the third.
>
> [Note: I'm used the new "png" option that will be available in
> V5.2.0 to gsn_open_wks to create these png images.]
> <mrb.000001.png>
> <mrb.000002.png>
> <mrb.000003.png>
> <mrb2.ncl>
> --Mary
>
>
> On Mar 26, 2010, at 1:35 PM, Sanjiv Kumar wrote:
>
>> Mary,
>>
>> Thank you for the reply.
>>
>> I did go through that example. This example does not seem
>> sufficient for my problem.
>>
>> I will try to make my question more clear:
>>
>> I want to mask the CCSM3 T42 outputs with the Mississippi River
>> Basin shape (MRB) file. I am attaching the MRB shape file here.
>>
>> So the question is: processing the climate model output for the
>> given shape.
>>
>> Sanjiv
>>
>>
>>
>> On Thu, Mar 25, 2010 at 10:13 AM, Mary Haley <haley@ucar.edu> wrote:
>> Sanjiv,
>>
>> If you have V5.1.1, we have support for reading shapefiles via
>> "addfile".
>>
>> See:
>>
>> http://www.ncl.ucar.edu/Applications/shapefiles.shtml
>>
>> for some examples.
>>
>> Let me know if this is not enough information.
>>
>> --Mary
>>
>>
>> On Mar 25, 2010, at 7:16 AM, Sanjiv Kumar wrote:
>>
>> Hello,
>>
>> I want to process the output from CCSM T42 runs for the
>> Mississippi River Basin (MRB). I have a shape file for the MRB.
>>
>> I was wondering whether this can be done with ncl or not.
>>
>> Thanks
>>
>> Sanjiv
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>>
>>
>> <mrb.zip>
>
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Received on Fri Mar 26 15:45:26 2010