[Met_help] The GRIB file

John Halley Gotway johnhg at rap.ucar.edu
Wed May 6 14:40:32 MDT 2009

I don't have much advice to offer about how to convert a network of point
observations into a gridded dataset.  I would guess that you could choose
from a variety of interpolation techniques, but I'd recommend that you be

There are gridded analysis fields of precipitation available within the
continental United States referred to as StageII and StageIV.  These are a
combination of precipitation gauge data and radar data that are mapped to
a grid, but I believe there's a good amount of work that goes into those

So while interpolating point data onto a grid may not be that difficult,
I'd guess that it would be difficult to do a good job of it!  I'd worry
that you may be introducing errors into the gridded observation field in
the interpolation process.

That being said, you're welcome to use the MET tools however you like. 
And feel free to reuse any of the MET library code if you find it helpful.
 It's all open source.

I've copied Tressa Fowler, the statistician who leads the MET development,
on this message.  She may be able to offer more advice on the statistical
implications of interpolating from points to a grid.

Good luck,

> Thank you! I think I know the format now.
> And I want to know that if I need to write the program myself when I am
> processing the station data to grided data.
> As you know, I need to do the interpolation form station to grided. And in
> the Point-Stat Tool, there are some interpolation methods, but they are
> used to process grided data to station data. So is there any program can
> help me do the interpolation or I need to do it with my own program?
> ÔÚ2009-05-05£¬"John Halley Gotway" <johnhg at rap.ucar.edu> дµÀ£º
>>Just look in the output of the test scripts that you ran after building
>> MET.  The files in METv2.0/out/pcp_combine provide a good example.  Just
>> run "ncdump -h" on them to see how the dimensions and
>>variables are defined.  Your dimensions, variables, and variable
>> attributes should be named the same as in the example.  The variable name
>> "APCP_12" means 12 hour of accumulation.  If for example, you
>>only have 6 hours of accumulation, it should be named "APCP_6".  Also,
>> the attributes ending with "_ut" are for unixtime, the number of seconds
>> since Jan 1, 1970.
>>        lat = 129 ;
>>        lon = 185 ;
>>        float APCP_12(lat, lon) ;
>>                APCP_12:grib_code = 61 ;
>>                APCP_12:units = "kg/m^2" ;
>>                APCP_12:long_name = "Total precipitation" ;
>>                APCP_12:level = "SFC" ;
>>                APCP_12:_FillValue = -9999.f ;
>>                APCP_12:init_time = "20050807_000000" ;
>>                APCP_12:init_time_ut = 1123372800 ;
>>                APCP_12:valid_time = "20050807_120000" ;
>>                APCP_12:valid_time_ut = 1123416000 ;
>>                APCP_12:accum_time = "12 hours" ;
>>                APCP_12:accum_time_sec = 43200 ;
>>The one area you may need some help is writing the projection information
>> to the NetCDF file.  Your forecast grid is a "Lambert Conformal" one.
>> Listed below is how you should set the projection
>>information in the global attributes section for the forecast domain
>> defined in the sample GRIB file you sent:
>>                :Projection = "Lambert Conformal" ;
>>                :p1_deg = "60.0 degrees_north" ;
>>                :p2_deg = "30.0 degrees_north" ;
>>                :p0_deg = "24.796 degrees_north" ;
>>                :l0_deg = "101.408 degrees_east" ;
>>                :lcen_deg = "105.0 degrees_east" ;
>>                :d_km = "12.0 km" ;
>>                :r_km = "6367.470000 km" ;
>>                :nx = "195 grid_points" ;
>>                :ny = "159 grid_points" ;
>>Good luck.
>>zhxubinchaoshan wrote:
>>> Thank you for your reply!
>>> I think I will write the output in specialized format of NetCDF like
>>> PCP-Combine tool.But what is the format of output of PCP-Combine tool?
>>> Can you give me an example?
>>> Thank you!
>>> ÔÚ2009-05-05£¬"John Halley Gotway" <johnhg at rap.ucar.edu> дµÀ£º
>>>> Let me make sure I understand.  Your forecast files are the gridded
>>>> output of WRF Post-Processor in GRIB format.  And your observations
>>>> are GTS SYNOP point observations that you've processed using
>>>> ASCII2NC.  In MET, the only tool that enables you to compare a gridded
>>>> forecast to point observations is the Point-Stat tool.
>>>> Now if you'd like to and are able to process your point observations
>>>> in some way onto a grid, then you'd be able to use Grid-Stat, MODE,
>>>> and the Wavelet-Stat tools.  And yes, you're correct, that
>>>> they'd need to be on the same domain as the gridded forecast before
>>>> you could run Grid-Stat, MODE, or Wavelet-Stat.  If you do try to
>>>> process the point observations onto a grid, you could either write
>>>> the output in GRIB format or a specialized format of NetCDF.
>>>> Basically, you could make the NetCDF file look like the output of the
>>>> PCP-Combine tool, and the other MET tools should be able to read that.
>>>> Thanks,
>>>> John
>>>> zhxubinchaoshan wrote:
>>>>> Thank you for your hints!
>>>>> But now my server can not connect, there is some problem on my
>>>>> laboratory network. I will try it tomorrow!
>>>>> I still have a question.If I want to use the MODE Tool and
>>>>> Wavelet-Stat Tool, and the precipitation data I have is the GTS data,
>>>>> must I transform the GTS data to the grid data in GRIB format?
>>>>> ÔÚ2009-05-05£¬"John Halley Gotway" <johnhg at rap.ucar.edu> дµÀ£º
>>>>>> Using the actual filename or a link to a file should both work fine
>>>>>> - unless there's some problem with how the link is set up.
>>>>>> I did set up a link and tried running PCP-Combine on the link and
>>>>>> the actual file name.  And both of them worked fine.
>>>>>> Can you try running a simple PCP-Combine command to see if it works:
>>>>>> METv2.0/bin/pcp_combine -add WRFPRS_d01.006 6 WRFPRS_d01.006 6
>>>>>> tmp.nc
>>>>>> Are you still getting that same error message?
>>>>>> John
>>>>>> zhxubinchaoshan wrote:
>>>>>>> Thank you for your reply!
>>>>>>> But the file wrfprs_d01.006 is linking to WRFPRS_d01.006, so I
>>>>>>> think they are the same.
>>>>>>> wrfprs_d01.006  -> WRFPRS_d01.006
>>>>>>> Is that right ?
>>>>>>> ÔÚ2009-05-05£¬"John Halley Gotway" <johnhg at rap.ucar.edu> дµÀ£º
>>>>>>>> Xubin,
>>>>>>>> I received the file and was able to run it through PCP-Combine
>>>>>>>> without any problems.
>>>>>>>> It appears that the problem is just that you're passing the wrong
>>>>>>>> file name to the tool.  The file you sent me is named
>>>>>>>> "WRFPRS_d01.006" in all capital letters.  However, the error
>>>>>>>> message you're
>>>>>>>> seeing is about this file name "wrfprs_d01.006" in all lower-case
>>>>>>>> letters.  Since filenames in Linux are case sensitive, that won't
>>>>>>>> work.  You need to pass the correct filename on the command line
>>>>>>>> to
>>>>>>>> the MET tools.
>>>>>>>> Try correcting the filename by using "WRFPRS_d01.006" and
>>>>>>>> hopefully that'll do the trick for you.
>>>>>>>> Thanks,
>>>>>>>> John Halley-Gotway
>>>>>>>> johnhg at ucar.edu
>>>>>>>> zhxubinchaoshan wrote:
>>>>>>>>> I have check the hints you gave, but neither of those is the
>>>>>>>>> problem!
>>>>>>>>> And when I connect to the FTP you gave, I found there was not the
>>>>>>>>> irap/johnhg directory in the incoming directory. Also, I can not
>>>>>>>>> upload the grib file.
>>>>>>>>> So I want to sent the file to you by e-mail! Thank you very much!
>>>>>>>>> ------------------------------------------------------------------------
>>>>>>>>> _______________________________________________
>>>>>>>>> Met_help mailing list
>>>>>>>>> Met_help at mailman.ucar.edu
>>>>>>>>> http://mailman.ucar.edu/mailman/listinfo/met_help

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