[ncl-talk] Read correctely WAFS_blended GRIB2 file

[David Brown]

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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