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

Tue Nov 21 11:34:13 MST 2017

Hi Mary,

I'm attaching both the mapgrid.png and another map of the domain I made
earlier from my own script. (The landmask plot was made from a wrfout file
that contained all the static fields, such as LANDMASK and HGT; the wrfout
files I'm reading in are subsetted with only a small number of variables to
minimize storage requirements.)

I have also written a demo script to show what I believe are bugs both in
the wrf_user_ll_to_ij and rcm2points_Wrap functions. That script points to
a sample wrfout and madis file on glade. I've attached that script here as
well. The wrf_user_ll_to_ij function returns nonsense (i,j) values for
(lon,lat) points outside the WRF domain. rcm2points also interpolates WRF
data to many of these points outside the WRF domain. I set up some print
statements in stages of thinning the data to progressively highlight the
problems with both functions.

Preferred behavior (from my perspective) would be to return missing values
for (lat,lon) points outside the WRF domain, both for wrf_user_ll_to_ij and
rcm2points. At the very least, wrf_user_ll_to_ij should not give
non-existing (i,j) coordinates. And rcm2points should only be an
interpolation function, not an extrapolation function.

Jared

On Mon, Nov 20, 2017 at 9:59 PM, Mary Haley <haley at ucar.edu> wrote:

> 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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>>
>

-- 
===============================
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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