[ncl-talk] Read correctely WAFS_blended GRIB2 file

[Rabah Hachelaf]

Hi Dave and all,

Thanks for your answers,
When do you plan the next NCL release, Dave ?

Rabah


2014-08-21 19:19 GMT-04:00 David Brown <dbrown at ucar.edu>:

> I have determined that there is indeed a bug in the NCL GRIB2 module. I am
> creating a ticket and expect to have a fix in time for the next release.
> (6.2.1) Sorry for the inconvenience.
>  -dave
>
>
> On Thu, Aug 21, 2014 at 1:57 PM, David Brown <dbrown at ucar.edu> wrote:
>
>> I am currently trying to figure out where wgrib2 finds the level numbers.
>> The NCEP g2clib (the low-level GRIB2 library that NCL relies on) reports
>> all the fields with multiple levels as having level type 255 (which means
>> undefined or missing), and the level values as 0. So I am trying to go
>> through the wgrib2 code to figure out where it is getting the various level
>> numbers (and the units of "mb").
>>  -dave
>>
>>
>> On Thu, Aug 21, 2014 at 1:30 PM, Dennis Shea <shea at ucar.edu> wrote:
>>
>>> wgrib2 -netcdf ....    works.  The generated nc file is 'technically
>>> correct'.   However, it is not the way most nc files are constructed.
>>>
>>> Going back a bit, GRIB is a record format *not* a file format like, say,
>>>  netCDF or HDF. Each GRIB record is for one level only.
>>> There are *no rules* for how a collection of GRIB records should be
>>> structured
>>> within a file.
>>>
>>> Hence, the GRIB records could be in random order and placed within one
>>> file.
>>> The great feature of NCL is that it reorders the GRIB records according
>>> to time and, if multiple level, collects the levels, etc. It does this 'on
>>> the fly'. The output looks like the variable came from a netCDF file.
>>>
>>>
>>> It looks like  'wgrib2 -netcdf ...' took each grib record and created a
>>> different variable for one time step.
>>>
>>>       float CTP_700mb ( time, latitude, longitude )
>>>          _FillValue :    9.999e+20
>>>          short_name :    CTP_700mb
>>>          long_name :    In-Cloud Turbulence
>>>          level :    700 mb
>>>          units :    percent
>>>
>>>       float CTP_600mb ( time, latitude, longitude )
>>>          _FillValue :    9.999e+20
>>>          short_name :    CTP_600mb
>>>          long_name :    In-Cloud Turbulence
>>>          level :    600 mb
>>>          units :    percent
>>>
>>> [snip]
>>>
>>> A common netCDF file might have
>>>
>>>      level = 5
>>>
>>>      float CTP (time, level, latitude, longitude)
>>>
>>>      level(level) ....
>>>
>>>
>>>
>>>
>>>
>>> On Thu, Aug 21, 2014 at 12:35 PM, HAVENER, KEVIN F GS-12 USAF AFWA 14
>>> WS/WXED <kevin.havener at us.af.mil> wrote:
>>>
>>>> Rabah,
>>>>
>>>> I can't say with too much authority what is going on here, but I think
>>>> the grib2 file was created by concatenating a bunch of single-level grib
>>>> files together and ncl_filedump sees the two differently named fields and
>>>> the first two or so of identically named fields (that have some different
>>>> metadata) and that’s all.
>>>>
>>>> I found the same problem you did when you view the file with
>>>> ncl_filedump and not surprisingly ncl_convert2nc gave the same result.
>>>> However, I used wgrib2's -netcdf option to convert the file to netcdf and
>>>> got a netcdf file that looks much more like the original grib (gzip'ed
>>>> netcdf attached) and probably what you expected to see.
>>>>
>>>> I've attached the ncl_filedump of my converted file to this email and
>>>> will mail you a copy of the converted file (3 mb) directly to you rather
>>>> than the list.
>>>>
>>>> Kevin Havener
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: ncl-talk-bounces at ucar.edu [mailto:ncl-talk-bounces at ucar.edu] On
>>>> Behalf Of Rabah Hachelaf
>>>> Sent: Thursday, August 21, 2014 1:34 PM
>>>> To: ncl-talk
>>>> Subject: [ncl-talk] Read correctely WAFS_blended GRIB2 file
>>>>
>>>>
>>>> Hi all,
>>>>
>>>> I am trying to plot icing, turbulence for several pressure levels from
>>>> WAFS_blended GRIB2 files, which can be find attached.
>>>>
>>>> The inventory of this file type is in this link:
>>>> http://www.nco.ncep.noaa.gov/pmb/products/gfs/WAFS_blended_2012010606f06.grib2.shtml
>>>>
>>>> When i use ncl_filedump i can't find any information about pressure
>>>> levels, it seems like there only one level although in the inventory file
>>>> there is several levels.
>>>>
>>>> Thanks to help me about this.
>>>>
>>>> Rabah
>>>> --
>>>>
>>>> ------------------------------
>>>> Cordialement,
>>>> Best regards,
>>>> Rabah Hachelaf
>>>>
>>>>
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>>
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-- 
------------------------------
Cordialement,
Best regards,
Rabah Hachelaf
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