Re: GRIB to netCDF

Since (presumably) each file has only one time step a time dimension is not created by default. Instead the time is attached to the variable as an attribute. You can get NCL to create an initial time dimension using the setfileoption "SingleElementDimensions". Try putting this before your addfiles call:

setfileoption("grb","SingleElementDimensions","Initial_time")

Be sure to look at the contents of the the aggregated file by uncommenting your "print(grib_in)" statement. The name of the time coordinate variable will be inital_time_x, where x is a number.
 -dave

On Apr 9, 2012, at 12:47 PM, Guo Zhang wrote:

> Hi,
>
> I have 3-hourly grib files and want to extract variables from these files.
> I use the example efficient GRIB to nc: http://www.ncl.ucar.edu/Applications/gribeff.shtml. But I cannot figure out how to assign time attribute. 'fatal:No valid instance of variable initial_time found in file list'.
>
> Please see my script and data information below. Also can access my some data and script via bluefire: /glade/home/zhangg/gld/nc.ncl GLDAS*.grb. Thanks.
>
> -Guo Zhang
>
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
> ;************************************************
> undef("rdVar4D")
> function rdVar4D (f:file, vName:string, time:integer)
> ; read GRIB 4D variable and assign new dimension names
> begin
> var = f->$vName$; read variable
> var!0 = "time" ; assign 'conventional' dimension names
> var!1 = "lat"
> var!2 = "lon"
> var&time = time ; assign time coord variable
> return(var)
> end
> begin
> ;***********************************************
> ; read in data from GRIB file
> ;***********************************************
> gribfil = systemfunc("ls *.grb")
> grib_in = addfiles(gribfil, "r")
> ; print(grib_in) ; file overview
> ; print(gribfil(0))
> TIME = grib_in[:]->initial_time ; time as "hours since"
> lat = grib_in[:]->g0_lat_0 ; latitude
> lon = grib_in[:]->g0_lon_1 ; longitude
> lat!0 = "lat" ; assign new dimension name
> lon!0 = "lon"
> TIME!0= "time"
>
> time = cd_calendar(TIME, -3) ; change to date syntax
> time!0= "time"
> time@units = "yyyymmddhh"
> snowf = rdVar4D (grib_in, "VAR_131_GDS0_SFC" ,time)
> rainf = rdVar4D (grib_in, "VAR_132_GDS0_SFC" ,time)
> ....
>
>
>
>
> file global attributes:
> dimensions:
> g0_lat_0 = 600
> g0_lon_1 = 1440
> lv_DBLY2 = 4
> variables:
> float PRES_GDS0_SFC ( g0_lat_0, g0_lon_1 )
> center : Zagreb
> long_name : Pressure
> units : Pa
> _FillValue : 1e+20
> level_indicator : 1
> gds_grid_type : 0
> parameter_table_version : 1
> parameter_number : 1
> forecast_time : 0
> forecast_time_units : hours
> initial_time : 01/01/2008 (00:00)
> float TMP_GDS0_SFC ( g0_lat_0, g0_lon_1 )
> center : Zagreb
> long_name : Temperature
> units : K
> _FillValue : 1e+20
> level_indicator : 1
> gds_grid_type : 0
> parameter_table_version : 1
> parameter_number : 11
> forecast_time : 0
> forecast_time_units : hours
> initial_time : 01/01/2008 (00:00)
> float WIND_GDS0_SFC ( g0_lat_0, g0_lon_1 )
> center : Zagreb
> long_name : Wind speed
> units : m/s
> _FillValue : 1e+20
> level_indicator : 1
> gds_grid_type : 0
> parameter_table_version : 1
> parameter_number : 32
> forecast_time : 0
> forecast_time_units : hours
> initial_time : 01/01/2008 (00:00)
> float SPF_H_GDS0_SFC ( g0_lat_0, g0_lon_1 )
> center : Zagreb
> long_name : Specific humidity
> units : kg/kg
> _FillValue : 1e+20
> level_indicator : 1
> gds_grid_type : 0
> parameter_table_version : 1
> parameter_number : 51
> forecast_time : 0
> forecast_time_units : hours
> initial_time : 01/01/2008 (00:00)
> float EVP_GDS0_SFC ( g0_lat_0, g0_lon_1 )
> center : Zagreb
> long_name : Evaporation
> units : kg/m^2
> _FillValue : 1e+20
> level_indicator : 1
> gds_grid_type : 0
> parameter_table_version : 1
> parameter_number : 57
> forecast_time : 0
> forecast_time_units : hours
> initial_time : 01/01/2008 (00:00)
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Received on Mon Apr 9 13:13:18 2012