[ncl-talk] Problem aligning data with map

[Herbster, Christopher G.]

Hi folks,

Here is a sequence of events that we are using to download some NARR data from NOMADS at NCDC:
We use the get-httpsubset.pl script from
https://nomads.ncdc.noaa.gov/guide/index.php?name=advanced#adv-httpsubset and the Perl scripts from
http://www.cpc.ncep.noaa.gov/products/wesley/fast_downloading_grib.html to grab a handful of parameters from the NARR dataset.

This gives us GRIB files like:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/narr-a_221_20101201_1200_000.sub.grb

I then used: ncl_convert2nc narr-a_221_20101201_1200_000.sub.grb -nc4c -l
To make
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/narr-a_221_20101201_1200_000.sub.nc4

I did this for 30 years of data ending 12-31-2010.

We then used an NCL script to do some calculations on a month of data, and write the results into a new file.
Here is the script:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/read_narr_ncar-help.ncl
My apologies to those of you who actually know how to code in NCL.  I am strictly an amateur at this.

Here is the file we created:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/D-Value_Monthly_Stats.2010.12.averages.1200.nc

Our problem seems to be aligning the data to the map background, as I can show with the following images:

The grey area in this image is supposed to be where the terrain is higher than the calculated value:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/narr-paper.000001.png

The data mask area is too far to the west, even extending out over the ocean.

When we plot it in the Unidata IDV we get what we expect:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/IDV_4K_Mask_D-Val.png

Then we decided to try plotting it in Matlab as another test.  We got:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/Matlab_Plot_NARR_Mask.jpg


While I suspect that I am not doing something properly with the mapping of the data, I am also wondering about the results of making the NetCDF file from the ncl_convert command.

First, is the compression lossy?  I realized that the default compression is zero, so this step did not do much for disk space!  (Actually made bigger files, but never mind.)

When I load the original GRIB file into the IDV it displays the region properly without any problem.  When I try to load the original NetCDF file, the IDV does not seem to know the geographic region of the data.

This can be seen in the following images:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/load-both-idv.png shows the Mean Sea Level Pressure data from both files.  They appear to match.  However, when I added the second panel (NetCDF file), the IDV did not display the data properly, and did so only after I selected a predefined CONUS projection (as shown).

When I open the NetCDF file and plotted MSLP in a window by itself, I got this:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/load-netcdf-idv.png

I had to zoom way out to display this data.  It knows where to put the data, but the IDV thinks the data should be in the white box area, I think.  (I'm not really sure about this interpretation.)

The questions I have are:

1)      Why does this happen?

2)      Is this related to my data masking and mapping problem, or are they separate issues?

Thanks in advance for any help on any of these problems/questions.

Cheers,

Chris

PS  If you want to test our script, there is a month of data in this tar file:
http://wx.erau.edu/faculty/herbster/demo/.hidden/NCL/sample-narr.tgz
--

Dr. Christopher G. Herbster
Associate Professor
Director of Science and Technology
for the ERAU Weather Center
Applied Aviation Sciences
Embry-Riddle Aeronautical Univ.
600 S. Clyde Morris Blvd.
Daytona Beach, FL 32114-3900
 386.226.6444 Office
386.226.6446 Weather Center
http://wx.erau.edu/

Schedule at:  http://wx.erau.edu/faculty/herbster/Schedules/

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