NCL wrf_ll_to_ij / wrf_user_ll_to_ij error?

Hi,

I have a 25km/5km/1km nested WRF model with a Lambert conformal
mapping projection that I am working with.  I am trying to find the
nearest grid point to a met tower that I have access to in order to
validate the model.  I am using the NCL function wrf_user_ll_to_ij but
I am getting a result that is significantly different than I expect.
Am I doing something wrong or is there a bug?

(Note that I have also tried this with the wrf_ll_to_ij function but
get the same result).

--------------------
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRF_contributed.ncl"

begin

path_to_domain_3_output_file = ".........."

a = addfile(path_to_domain_3_output_file,"r")

lat2d = a->XLAT(0,:,:)
lon2d = a->XLONG(0,:,:)

opt = True
opt@returnInt = True

print(opt)

pointLon = -87.9949
pointLat =  41.7012

loc = wrf_user_ll_to_ij (a, pointLon, pointLat, opt )

ival = loc(0)
jval = loc(1)

print("i: " + ival + ", jval: " + jval)

print("Requested: (" + pointLat + "," + pointLon + "), got (" +
lat2d(ival,jval) + "," + lon2d(ival,jval) + ")")

ival2 = ival-6
jval2 = jval+4

print("Compare:   (" + lat2d(ival2,jval2) + "," + lon2d(ival2,jval2) + ")")

end
--------------------

The result I get is:

--------------------
(0)    i: 192, jval: 187
(0)    Requested: (41.7012,-87.9949), got (41.76,-88.0333)
(0) Compare: (41.7014,-87.9899)
--------------------

Note that the first distance between the point I requested and the
point I received is is 7.2814 km while the distance between the
requested and comparison points is only 0.4162 km according to the
distance calculator found here:

http://jan.ucc.nau.edu/~cvm/latlongdist.html

I would expect to get the comparison values instead of the values I
received. Am I doing something wrong?

Best wishes,

Jeff
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Received on Wed Mar 3 16:07:15 2010