<div dir="ltr">Ivan,<div><br></div><div>I am not familiar enough with wgrib2 to answer your question regarding wgrib2 interpolation. Perhaps another user can chime in on that. Perhaps doing the "gr2stn" interpolation in GrADS will yield useful results for you. Good luck.</div><div><br></div><div>Stephen</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Aug 25, 2020 at 12:27 PM Ivan Toman <<a href="mailto:ivtoman@inet.hr">ivtoman@inet.hr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<p>Hello,</p>
<p>No the 273.15 is not factor because I use same conversion in both
methods and also difference is not constant.</p>
<p>My lat/lon points do not correspond to the exact grid point
locations. However, I am aware that GrADS will select nearest grid
point, but, doesn't wgrib2 also do the same? Do you think that
wgrib2 (and tslist output) does interpolation automatically?</p>
<p>My wgrib2 method is this:</p>
<p>wgrib2 $gribfile -rpn "273:-" -match "(TMP:2 m)" -lola "$lon:1:1"
"$lat:1:1" grid.grb grib >/dev/null</p>
<p>wgrib2 grid.grb -csv grid.csv >/dev/null<br>
</p>
<p>where $lon and $lat are exactly same figures as are used in GrADS
script.</p>
<p>Thank you for your idea of interpolation with gr2stn, I will try
it and see what do I get in that case.</p>
<p> Ivan<br>
</p>
<p><br>
</p>
<p><br>
</p>
<div>On 8/24/20 3:14 PM, Stephen McMillan
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Ivan,
<div><br>
</div>
<div>In addition to Brian's response, which would account for a
difference of 0.15 deg only, please consider this...</div>
<div><br>
</div>
<div>You said nothing about your grid data resolution so, in the
example you provided, did you set the lat and lon to a precise
grid point or to a point in between grid points? When setting
lat or lon, GrADS will automatically set to the nearest grid
point, which may be different than what you intended.</div>
<div><br>
</div>
<div>Have you considered interpolating instead to your desired
point? E.g., using gr2stn or lterp. Here's an example using
gr2stn:</div>
<div><br>
</div>
<div>'d gr2stn(tmp2m-273.15,16.034,45.822)'</div>
<div><br>
</div>
<div>Stephen Mc</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sun, Aug 23, 2020 at 6:08
AM Ivan Toman <<a href="mailto:ivtoman@inet.hr" target="_blank">ivtoman@inet.hr</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hello
dear GrADS users,<br>
<br>
I have a problem. If I extract 2m temperature for a single
lat/lon point <br>
using GrADS, it is in slight disagreement compared to same
record <br>
obtained from grib2 file using wgrib2.<br>
<br>
The difference is usually around 1C but can be larger from
time to time. <br>
I'm almost 100% sure I did not overlook anything obvious like
selecting <br>
different time or diffrent lat/lon coordinates, but I leave
something <br>
like that as a theoretical possibility if there is no better <br>
explanation. Below is an example how I extract temperatures
using GrADS. <br>
Grib2 is WRF file, Lambert projection, and ctl/idx files are
created <br>
using latest g2ctl and gribmap utilities. I note this because
it might <br>
be that projection gets somehow skewed and that GrADS takes
data from <br>
incorrect positions. Is that possible at all?<br>
<br>
It seems very important to add that wgrib2 method is in exact
agreement <br>
to the tslist output from WRF, so for now I take that one as
"correct" <br>
one and GrADS figures as "incorrect".<br>
<br>
Simple extraction method in GrADS:<br>
<br>
'set lat 45.822'<br>
'set lon 16.034'<br>
'set string 1 bl 3 0'<br>
'set t 13'<br>
'd tmp2m-273'<br>
ans = sublin(result,2)<br>
temp = subwrd(ans,4)<br>
<br>
<br>
Is this difference normal/expected? If it is, what's the
reason for <br>
that? If not, is there anything else where I could search for
user error?<br>
<br>
Thank you,<br>
<br>
Best regards,<br>
<br>
Ivan Toman<br>
<br>
<br>
<br>
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