[gradsusr] GrADS and wgrib2 temperatures in disagreement

Stephen McMillan smcmillan at planalytics.com
Mon Aug 24 09:14:12 EDT 2020


Ivan,

In addition to Brian's response, which would account for a difference of
0.15 deg only, please consider this...

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.

Have you considered interpolating instead to your desired point?  E.g.,
using gr2stn or lterp.  Here's an example using gr2stn:

'd gr2stn(tmp2m-273.15,16.034,45.822)'

Stephen Mc

On Sun, Aug 23, 2020 at 6:08 AM Ivan Toman <ivtoman at inet.hr> wrote:

> Hello dear GrADS users,
>
> I have a problem. If I extract 2m temperature for a single lat/lon point
> using GrADS, it is in slight disagreement compared to same record
> obtained from grib2 file using wgrib2.
>
> The difference is usually around 1C but can be larger from time to time.
> I'm almost 100% sure I did not overlook anything obvious like selecting
> different time or diffrent lat/lon coordinates, but I leave something
> like that as a theoretical possibility if there is no better
> explanation. Below is an example how I extract temperatures using GrADS.
> Grib2 is WRF file, Lambert projection, and ctl/idx files are created
> using latest g2ctl and gribmap utilities. I note this because it might
> be that projection gets somehow skewed and that GrADS takes data from
> incorrect positions. Is that possible at all?
>
> It seems very important to add that wgrib2 method is in exact agreement
> to the tslist output from WRF, so for now I take that one as "correct"
> one and GrADS figures as "incorrect".
>
> Simple extraction method in GrADS:
>
> 'set lat 45.822'
> 'set lon 16.034'
> 'set string 1 bl 3 0'
> 'set t 13'
> 'd tmp2m-273'
> ans = sublin(result,2)
> temp = subwrd(ans,4)
>
>
> Is this difference normal/expected? If it is, what's the reason for
> that? If not, is there anything else where I could search for user error?
>
> Thank you,
>
> Best regards,
>
> Ivan Toman
>
>
>
> _______________________________________________
> gradsusr mailing list
> gradsusr at gradsusr.org
> http://gradsusr.org/mailman/listinfo/gradsusr
>

-- 
***************************************************The information 
contained in this e-mail message is intended only for the use of the 
recipient(s) named above and may contain information that is privileged, 
confidential, and/or proprietary. If you are not the intended recipient, 
you may notreview, copy or distribute this message. If you havereceived 
this communication in error, please notify the sender immediately by 
e-mail, and delete the original 
message.***************************************************
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://gradsusr.org/pipermail/gradsusr/attachments/20200824/442bf4f4/attachment-0001.html>


More information about the gradsusr mailing list