[gradsusr] gradsusr Digest, Vol 126, Issue 20

[Wesley Ebisuzaki - NOAA Federal]

Ivan,

GraDS using a ctl file created by g2ctl is
    (1) bllinear interpolating from the raw grid (probably rotated lat-lon)
           to the GrADS display grid as defined by XDEF and YDEF in the ctl
file.
    (2) bilinear interpolatiing from the GrADS display grid to the  point
that you selected.
                lat 45.822 lon 16.034

Wgrib2 using the -lon option is using a nearest neighbor interpolation
    from the raw grid to the point that you selected.

The next release of wgrib2 will have bilinear interpolation to a set of
points.  A user reminded me that the interpolation library had that
facility.

Wesley



On Sun, Aug 23, 2020 at 12:57 PM Brian Gaze <brianw.gaze at googlemail.com>
wrote:

> Ivan,
>
> Not sure if it would account for the difference but when converting from K
> to C it should be -273.15.
>
> So in your example:
>
>  'd tmp2m-273.15'
>
> Brian
>
> On Sun, 23 Aug 2020 at 17:01, <gradsusr-request at gradsusr.org> wrote:
>
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>> Today's Topics:
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>>    1. GrADS and wgrib2 temperatures in disagreement (Ivan Toman)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Sun, 23 Aug 2020 12:01:51 +0200
>> From: Ivan Toman <ivtoman at inet.hr>
>> To: GrADS Users Forum <gradsusr at gradsusr.org>
>> Subject: [gradsusr] GrADS and wgrib2 temperatures in disagreement
>> Message-ID: <80efbf07-0e18-4a6f-b073-651ffe3f0cf4 at inet.hr>
>> Content-Type: text/plain; charset=utf-8; format=flowed
>>
>> 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
>>
>>
>>
>>
>>
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>> End of gradsusr Digest, Vol 126, Issue 20
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