[ncl-talk] test if lat-lon point is within gridded model domain

[Mary Haley]

Hi Jared,

It would help if I could see a picture of your WRF grid. You said that it
is polar stereographic, yet it crosses the dateline.

I think that gc_inout might still work for you, but I agree there could be
issues if it crosses the dateline. However, it's possible we could break
this up into two grids, but I first need to see what it looks like.

Can you take the attached script, plug in your WRF file, run it, and send
me the mapgrid.png file?

Or, even better, if have the WRF and MADIS files on the NCAR supers, can
you point me to them?

Thanks,

--Mary




On Mon, Nov 20, 2017 at 6:17 PM, Jared Lee <jaredlee at ucar.edu> wrote:

> Hi Dave,
>
> Thanks for the suggestion, but I'm not optimistic that that will work for
> my problem. I think finding the boundaries of an arbitrary WRF grid as a
> spherical polygon would be a non-trivial task; I don't know how I would
> even go about doing that.
>
> Thinking about it, I can probably approximately solve this problem in a
> few steps with a combination of wrf_user_ll_to_ij and then gc_latlon (by
> calculating the distance between obs and "nearest grid points" along the
> edge of the WRF domain and then applying a distance threshold). I'll try
> that tomorrow. I think I've found a bug with wrf_user_ll_to_ij, though (it
> gives me non-existent (i,j) values for (lon,lat) points outside the
> domain), and I'll submit a separate ticket on that.
>
> Jared
>
> On Mon, Nov 20, 2017 at 5:39 PM, Dave Allured - NOAA Affiliate <
> dave.allured at noaa.gov> wrote:
>
>> Jared,
>>
>> See whether the function gc_inout will work in your case.  You would need
>> to get the boundaries of the WRF grid as a spherical polygon.  There may
>> already be published WRF polygons as shape files; I am not familiar with
>> that.  Then use the function gc_inout to determine inside or outside for
>> each obs location.
>>
>> In essence you would be applying two mask tests to each obs point,
>> because you also say you want to select within a lat/lon box.  Use gc_inout
>> to test against the larger WRF grid, and use simple arithmetic comparisons
>> for the lat/lon box.
>>
>> Caution, there is a little trap here.  Do not use gc_inout for testing
>> simple four-sided lat/lon bounding boxes.  You would probably get something
>> unexpected.  gc_inout uses spherical polygons on the earth's surface.  Such
>> polygons have curved edges on the simple lat/lon plane.
>>
>> --Dave
>>
>>
>> On Mon, Nov 20, 2017 at 4:26 PM, Jared Lee <jaredlee at ucar.edu> wrote:
>>
>>> Hi, I have a WRF domain on a polar stereographic grid (which also
>>> happens to span the dateline), and I want to interpolate model data to
>>> MADIS observation locations that are within my domain. Is there a
>>> straightforward way to evaluate which observation lat/lon points are
>>> outside the WRF grid?
>>>
>>> Because it's a polar stereographic grid, doing a first pass to eliminate
>>> MADIS stations by comparing to the min/max latitude and longitude of the
>>> WRF grid still leaves a ton of geographic area that would be inside that
>>> lat/lon box but outside the WRF grid (and that's complicated further by the
>>> WRF domain spanning the dateline).
>>>
>>> I tried using rcm2points to do horizontal interpolation, but that
>>> function is still interpolating to numerous grid points that are thousands
>>> of km outside my WRF domain. I also tried feeding wrf_user_ll_to_ij some
>>> lat/lon values for points well outside my domain, and it gives me nonsense
>>> (but non-missing) values for nearest grid points.
>>>
>>> Anyone have any ideas? Or is there a function that already exists to do
>>> this that I'm apparently not seeing? It seems like a function to return the
>>> (i,j) values of the four surrounding grid points would be an ideal way to
>>> accomplish this and be useful in additional contexts and applications.
>>>
>>> Jared
>>>
>>
>
>
> --
> ===============================
> Jared A. Lee, Ph.D.
> Project Scientist I
> Research Applications Laboratory
> National Center for Atmospheric Research
> Boulder, Colorado, USA
>
> Member, AMS Planning Commission
>
> Email: jaredlee at ucar.edu (w)
> Phone: 303.497.8485 <(303)%20497-8485> (w)
> Web: https://staff.ucar.edu/users/jaredlee
> ===============================
>
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