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

[Jared Lee]

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 (w)
Web: https://staff.ucar.edu/users/jaredlee
===============================
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