[ncl-talk] lat,lon not accurately generated by NCL

[Guoqing Ge - NOAA Affiliate]

Dear NCL talk,

Recently, I found an issue on the generation of grid meshes using a
prescribed map projection information. The detail of the problem is as
follows:

When I read HRRR grib files to generate plots, I get lat,lon information
form these two NCL-provided variables:
mylat = t_file->*gridlat_0*
mylon = t_file->*gridlon_0*

The variable summary is as follows:

*Variable: mylat*
*Type: float*
*Total Size: 7620564 bytes*
*            1905141 values*
*Number of Dimensions: 2*
*Dimensions and sizes:   [ygrid_0 | 1059] x [xgrid_0 | 1799]*
*Coordinates:*
*Number Of Attributes: 11*
*  corners :     ( 21.13812, 21.14055, 47.84219, 47.83862 )*
*  long_name :   latitude*
*  grid_type :   Lambert Conformal (secant, tangent, conical or bipolar)*
*  units :       degrees_north*
*  Latin2 :      38.5*
*  Latin1 :      38.5*
*  Dy :   3*
*  Dx :   3*
*  Lov : 262.5*
*  Lo1 : 237.2805*
*  La1 : 21.13812*

As show above, the variable contains the prescribed HRRR model map
projection information. I believe this information is used by NCL
internally to generate lat,lon values.

The problem is that these re-generated lat,lon values are not consistent
with the original HRRR model lat,lon values.
I printed out lat,lon values for the botten-left corner, center and its
surrounding four points, and the upper-right corner as below:

*For original HRRR lat,lon values:*
(1,1):    21.13812, -122.7195
c_left:   38.49999,-97.53448
center:  38.5, -97.5
c_right: 38.49999,-97.46552

c_down:  38.47303, -97.5
center:  38.5, -97.5
c_up:    38.52699, -97.5

(nx,ny): 47.84364, -60.90137

*For NCL re-genrated lat,lon values based on prescribed HRRR map projection
info:*
(1,1) :  21.13812, -122.7195

c_left:  38.49724, -97.54045
center:  38.49725, -97.50597
c_right: 38.49724, -97.4715

c_down:  38.47027, -97.50597
center:  38.49725, -97.50597
c_up:    38.52422, -97.50598

(nx,ny): 47.84219, -60.91719

You can see that for the original HRRR lat,lon mesh, the center is exactly
(38.5, -97.5) as defined by the map projection info, while the NCL
re-gererated lat,lon mesh is *not* so. Except the bottom-left point, all
other lat,lon values from NCL shift from the original HRRR values.

This NCL behavior caused a slight shift in my physical variable plots
(based on GRIB files) as compared to original ones. Could you advise me how
NCL generate these lat,lon values? Can you help me examine and fix this
issue?

Let me know if you need more information.

Thanks,

Guoqing



--
Guoqing Ge, PhD
CIRES Research Scientist
at NOAA/ESRL/GSD (Rm 2B502)
325 Broadway, Boulder, CO 80305
(303)497-4163
&
Thursdays at
Developmental Testbed Center (FL3-1039)
3450 Mitchell Ln, Boulder, CO 80301
(303)497-8445
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