title: Problems when i convert Grib2 to Netcdf4
body: <p>I got some problems when i try to convert some grib files form CFSR. 
  when i use <code>ncl_convert2nc</code>in the unix shell, the nc file information is like this :</p>
  
  <pre><code> netcdf flxf01.gdas.197901 {
  dimensions:
          ygrid_0 = UNLIMITED ; // (0 currently)
          lv_DBLL0 = 4 ;
  
  variables:
  
  // global attributes:
                  :creation_date = "Sun Nov 12 11:29:36 AEDT 2017" ;
                  :NCL_Version = "6.3.0" ;
                  :system = "Linux charcoalp-Lenovo-Erazer-Y50-70 4.10.0-38-generic #42-Ubuntu SMP Tue Oct 10 13:24:27 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux" ;
                  :Conventions = "None" ;
                  :grib_source = "flxf01.gdas.197901.grb2" ;
                  :title = "NCL: convert-GRIB-to-netCDF" ;
  }
  </code></pre>
  
  <p>and this only happen when i convert CFSR data grib2 files.</p>
  
  <p>here is part of information of the origin file:</p>
  
  <pre><code>Nio file:   flxf01.gdas.197901.grb2
     global attributes:
     dimensions:
        lat_0 = 576
        lon_0 = 1152
        lv_DBLL0 = 4
     variables:
        float TMP_P8_L1_GGA0_avg744h [ lat_0, lon_0 ]
           center :   US National Weather Service - NCEP (WMC)
           production_status :    Operational products
           long_name :    Temperature
           units :    K
           _FillValue :   1e+20
           grid_type :    Gaussian latitude/longitude
           parameter_discipline_and_category :    Meteorological products, Temperature
           parameter_template_discipline_category_number :    [8, 0, 0, 0]
           level_type :   Ground or water surface
           level :    0
           type_of_statistical_processing :   Average
           statistical_process_duration : 744 hours (ending at forecast time)
           forecast_time :    745
           forecast_time_units :  hours
           initial_time : 01/01/1979 (00:00)
        float TMP_P8_L103_GGA0_avg744h [ lat_0, lon_0 ]
           center :   US National Weather Service - NCEP (WMC)
           production_status :    Operational products
           long_name :    Temperature
           units :    K
           _FillValue :   1e+20
           grid_type :    Gaussian latitude/longitude
           parameter_discipline_and_category :    Meteorological products, Temperature
           parameter_template_discipline_category_number :    [8, 0, 0, 0]
           level_type :   Specified height level above ground (m)
           level :    2
           type_of_statistical_processing :   Average
           statistical_process_duration : 744 hours (ending at forecast time)
           forecast_time :    745
           forecast_time_units :  hours
           initial_time : 01/01/1979 (00:00)
  </code></pre>
  
  <p>as you can see it is able to find the variables from the grib file. So why  the nc file is empty? <code>ncl_convert2nc</code>  do work for Grib files of JRA-55 reanalysis dataset. Can anyone help me find where is the problem?</p>