[ncl-talk] grib2 file read

Tue Jul 26 16:12:30 MDT 2016

I am afraid there have been some misunderstandings.
First of all the file "ngac.t00z.aerf24.grib2" (actually called
ngac.t00z.aerf24 on the ftp site if anyone else wants to download it)
is
not a GRIB file at all. A simple test if you suspect something is not GRIB:

strings <filename> | grep GRIB

If this produces nothing then it's definitely not a GRIB file (of
course the presence of "GRIB" does not prove it is a GRIB file).
To digress for a moment, if it is a GRIB file then the command,

strings <filename> | egrep 'GRIB|7777'

will produce alternating instances of 'GRIB' and '7777'. These strings
mark the beginning and end of each GRIB record. This is true for both
GRIB 1 and 2.

strings ngac.t00z.aerf24 |grep GRIB
produces nothing so it is not GRIB.

Second, the warning messages that are emitted when an ngac file that
is valid GRIB is read by NCL such as:

warning: Entry (62006) not found in code table file
/Users/shea/devel/lib/ncarg/grib2_codetables/ncep/4/4.3.table

refer to entries not found in table 4.3 which is "type of generating
process". This is not the same as table 4.233 "Aerosol Type"
and so the number 62006 in this context has nothing to do with 62006
("Sulphate Dry") in the Aerosol Type table

62006 as a type of generating processing is not documented in the 4.3
table and hence the message. Since type of generating process is
usually of less concern to users I am considering eliminating the
error message in this case.

It is true that NCL does not yet make use of the 4.233 table. That is
because it is an auxiliary table that needs to be used in the context
of certain GRIB2 product templates that address aerosols. It does not
contain actual parameters. Rather a parameter might be named, e.g.
"Aerosol optical thickness" and based on the template there is another
piece of metadata called "Aerosol Type" which is as documented in the
"Aerosol Types" table. A small extension to our GRIB2 data model is
required to handle these templates. We hope to implement it in time
for 6.4.0 but that is not yet certain.
 -dave

On Tue, Jul 26, 2016 at 12:23 PM, Debasish Hazra
<debasish.hazra5 at gmail.com> wrote:
> Hi Dennis,
>
> The variables in the range  62000 to 62010 in the updated grib2 table
> "http://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_table4-233.shtml" are
> "optical depth" and has no units associated with them.
>
> 62000 Total Aerosol
> 62001 Dust Dry
> 62006 Sulphate Dry
> 62008 Sea Salt Dry
> 62009 Black Carbon Dry
> 62010 Particulate Organic Matter Dry
>
> 62010-62016 are mixing ratios in kg/kg
>
> Debasish
>
>
>
> On Tue, Jul 26, 2016 at 2:13 PM, Dennis Shea <shea at ucar.edu> wrote:
>>
>> Again, I will forward this. please send to ncl-talk. Otherwise, I am just
>> a middleman forwarding the emails.
>>
>> Usually the GRIB tables have units:
>>
>> Examples:
>> http://www.ncl.ucar.edu/Document/Manuals/Ref_Manual/ncep_141.htm
>> http://www.ncl.ucar.edu/Document/Manuals/Ref_Manual/ecmwf_132.htm
>>
>> NCL ... see GRIB section
>> See:
>> http://www.ncl.ucar.edu/Document/Manuals/Ref_Manual/NclFormatSupport.shtml
>>
>> On Tue, Jul 26, 2016 at 12:05 PM, Debasish Hazra
>> <debasish.hazra5 at gmail.com> wrote:
>>>
>>> The link updated grib2 table is here :
>>>
>>> http://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_table4-233.shtml
>>>
>>> Thanks
>>> Debasish
>>>
>>> On Tue, Jul 26, 2016 at 1:33 PM, Dennis Shea <shea at ucar.edu> wrote:
>>>>
>>>> The issue is that the NGAC grib tables are not publicly available.
>>>> If you know of where they are located, they can be included within NCL
>>>>
>>>> In fact, there is a JIRA ticket on this:
>>>>
>>>> [JIRA] (NCL-2442) GRIB-2: warning: Entry (not found in code table file
>>>> .../grib2_codetables/ncep/4/4.3.table
>>>>
>>>> Just yesterday, a comment was made on this ticket:
>>>>
>>>> ---------
>>>>
>>>> Unfortunately the "not found in code table file" errors
>>>> that Dennis reported in this ticket cannot be corrected
>>>> because the entries for that were not found are not documented
>>>> by the NCEP GRIB2 web pages. Table 4.3 contains the generating process
>>>> mapping. The values found in the file, all in the range 62000 to 62016,
>>>> probably refer to local "generating processes" related to the NGAC
>>>> model.
>>>> If documentation can be found to describe these I would be happy to add
>>>> them to the tables.
>>>>
>>>> The other option to get rid of these admittedly annoying
>>>> messages might be to just not print the warnings for this piece of
>>>> metadata
>>>> since it is not usually of that much interest to the data consumer. I
>>>> would
>>>> be happy to consider this.
>>>>
>>>> ---------
>>>> Note: the *values* returned by NCL are correct. The meta data is not
>>>> because the
>>>> NCEP GRIB2 web pages are not available.
>>>>
>>>> --------
>>>>
>>>> Again, if you know where the appropriate tables are located, please send
>>>> to ncl-talk.
>>>>
>>>> THX
>>>>
>>>> On Tue, Jul 26, 2016 at 11:21 AM, Debasish Hazra
>>>> <debasish.hazra5 at gmail.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> I am trying to read grib2 file with ncl (v6.3) and it shows the
>>>>> following error while trying to open it:
>>>>>
>>>>> fatal:An internal error has occurred. The file format requested does
>>>>> not appear to be supported, could not open (ngac.t00z.aerf24.grib2)
>>>>> Variable: f
>>>>> Type: file
>>>>> (0) File Missing Value : -1
>>>>>
>>>>> I kept the data file in ftp. Any help on this is appreciated.
>>>>>
>>>>> Thanks.
>>>>> Debasish
>>>>>
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>>>>
>>>
>>>
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>
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