## Components of a GrADS Data Descriptor File

DSET
CHSUB
DTYPE
INDEX
STNMAP
TITLE
UNDEF
UNPACK
FILEHEADER
XYHEADER
THEADER
HEADERBYTES
TRAILERBYTES
XVAR
YVAR
ZVAR
STID
TVAR
TOFFVAR
CACHESIZE
OPTIONS
PDEF
XDEF
YDEF
ZDEF
TDEF
EDEF
VECTORPAIRS
VARS
ENDVARS
ATTRIBUTE METADATA
COMMENTS

DSET data_filename | back to top
This entry specifies the filename of the data file being described. If the data and the descriptor file are not in the same directory, then data_filename must include a full path. If a ^ character is placed in front of data_filename, then data_filename is assumed to be relative to the path of the descriptor file. If you are using the ^ character in the DSET entry, then the descriptor file and the data file may be moved to a new directory without changing any entries in the data descriptor file, provided their relative paths remain the same. For example: If the data descriptor file is: /data/wx/grads/sa.ctl and the binary data file is: /data/wx/grads/sa.dat then the data file name in the data descriptor file can be: DSET ^sa.dat instead of: DSET /data/wx/grads/sa.dat If data_filename does not include a full path or a ^, then GrADS will only look for data files in the directory where you are running GrADS. GrADS allows you use a single DSET entry to aggregate multiple data files and handle them as if they were one individual file. The individual data files must be identical in all dimensions except time, and the time range of each individual file must be indicated it its filename. To accomplish this, the DSET entry has a substitution template instead of a filename. See the section on Using Templates for a description of all the possible components of the template. Second, the OPTIONS entry must contain the template keyword.
CHSUB t1 t2 string | back to top
(GrADS version 1.9b4) This entry is used with a new option for templating data files that allows for any user-specified string substitution, instead of only date string substitution. This is useful when none of the standard template options match the time ranges in the files you wish to aggregate, or if the files are located on different disks. When you put the `%ch` template in your DSET entry, then you also need to put additional CHSUB entries in the descriptor file. The string will be substituted for `%ch` in the data file name for the time steps beginning with t1 and ending with t2.See the section on Using Templates for examples.
DTYPE keyword | back to top
The DTYPE entry specifies the type of data being described. There are four options: grib, hdfsds, netcdf, or station. If the data type is none of these, then the DTYPE entry is omitted completely from the descriptor file and GrADS will assume the data type is gridded binary.
bufr | (GrADS version 1.9) Data file is a BUFR station data file. This data type must be accompanied by the following special entries: XVAR, YVAR, TVAR, STID. Optional special entries are: ZVAR, TOFFVAR.
grib | Data file is an indexed GRIB (version 1) file. This data type requires a secondary entry in the descriptor file: INDEX. The INDEX entry provides the filename (including the full path or a ^) for the GRIB index file. The index file is created by the gribmap utility. You must run gribmap and create the index file before you can display the GRIB data in GrADS.
grib2 | (GrADS version 2.0) Data file is an indexed GRIB2 file. This data type requires a secondary entry in the descriptor file: : INDEX. The INDEX entry provides the filename (including the full path or a ^) for the GRIB2 index file. The index file is created by the gribmap utility. You must run grib2map and create the index file before you can display the GRIB2 data in GrADS.
hdfsds | (GrADS version 1.9) Data file is an HDF Scientific Data Set (SDS). Although HDF-SDS files are self-describing and may be read automatically using the sdfopen/xdfopen commands, this DTYPE gives you the option of overriding the file's own metadata and creating a descriptor file for some or all of the variables in the file. This DTYPE may also be used if the metadata in the HDF-SDS file is insufficient or is not coards-compliant. This data type requires a special entry in the units field of the variable declaration. The undef and unpack entries contain special options for this dtype.
hdf5_grid | (GrADS version 2.0.a7+) Data file is HDF5 gridded format. The HDF5 format is extremely general and is designed to store a variety of data types. The GrADS interface is only for grids, and requires a complete descriptor file -- there is no sdfopen/xdfopen interface for HDF5.
netcdf | (GrADS version 1.9) Data file is NetCDF. Although NetCDF files are self-describing and may be read automatically using the sdfopen/xdfopen commands, this DTYPE gives you the option of overriding the file's own metadata and creating a descriptor file for some or all of the variables in the file. This DTYPE may also be used if the metadata in the NetCDF file is insufficient or is not coards-compliant. This data type requires a special entry in the units field of the variable declaration. The undef and unpack entries contain special options for this dtype.
station | Data file is in GrADS station data format. This data type requires a secondary entry in the descriptor file: STNMAP. The STNMAP entry provides the filename (including the full path or a ^) for the station data map file. The map file is created by the stnmap utility. You must run stnmap and create the map file before you can display the station data in GrADS.
INDEX filename | back to top
This entry specifies the name of the grib map file. It is required when using the DTYPE grib or grib2 entry to read GRIB formatted data. The file is generated by running the external utility gribmap. or grib2map. Filenaming conventions are the same as those described for the DSET entry.
STNMAP filename | back to top
This entry specifies the name of the station map file. It is required when using the DTYPE station entry to read GrADS-formatted station data. The file is generated by running the external utility stnmap. Filenaming conventions are the same as those described for the DSET entry.
TITLE string | back to top
This entry gives brief description of the contents of the data set. String will be included in the output from a query command and it will appear in the directory listing if you are serving this data file with the GrADS-DODS Server (GDS), so it is helpful to put meaningful information in the title field. For GDS use, do not use double quotation marks (") in the title.
UNDEF value <undef_attribute_name> | back to top
This entry specifies the undefined or missing data value. UNDEF is a required entry even if there are no undefined data. GrADS operations and graphics routines will ignore data with this value from this data set. (GrADS version 1.9b4) An optional second argument has been added for data sets of DTYPE netcdf or hdfsds -- it is the name of the attribute that contains the undefined value. This should be used when individual variables in the data file have different undefined values. After data I/O, the missing values in the grid are converted from the variable undef to the file-wide undef (the numerical value in the first argument of the UNDEF record). Then it appears to GrADS that all variables have the same undef value, even if they don't in the original data file. If the data require a transformation using the attributes named in the UNPACK entry, GrADS assumes the variable undef value corresponds to the data values as they appear in the file, i.e., before they are transformed using a scale factor and offset. Missing packed data values are thus assigned the file-wide undef value and are never unpacked. Attribute names are case sensitive, and it is assumed that the name is identical for all variables in the netcdf or hdfsds data file. If the name given does not match any attributes, or if no name is given, the file-wide undef value will be used. Example: UNDEF 1e+33 _FillValue
UNPACK scale_factor_attribute_name <add_offset_attribute_name> | back to top
(GrADS version 1.9) This entry is used with DTYPE netcdf, hdfsds, or hdf5_grid (GrADS version 2.0.a7+) for data variables that are 'packed' -- i.e. non-float data that need to be converted to float by applying the following formula: y = x * scale_factor + add_offset If your self-describing file does not have an offset attribute, the 2nd argument may be omitted, and the offset will be assigned the default value of 0.0. If your self-describing file has an offset attribute, but not a scale factor, use "NULL" for the scale_factor_attribute_name. (This "NULL" option is in GrADS version 2.0.0+). Attribute names are case sensitive, and it is assumed that the names are identical for all variables in the netcdf or hdfsds data file. If the names given do not match any attributes, the scale factor will be assigned a value of 1.0 and the offset will be assigned a value of 0.0. The transformation of packed data is done after the undef test has been applied. Examples: UNPACK scale_factor add_offset UNPACK NULL add_offset UNPACK Slope Intercept
FILEHEADER length | back to top
This optional entry tells GrADS that your data file has a header record of length bytes that precedes the data. GrADS will skip past this header, then treat the remaineder of the file as though it were a normal GrADS binary file after that point. This optional descriptor file entry is only valid for GrADS gridded data sets.
THEADER length HEADERBYTES length< | back to top
These two equivalent optional entries tell GrADS that the data file has a header record of length bytes preceding each time block of binary data. Use one or the other but not both. These entries are only valid for GrADS gridded data sets. See the section on structure of a gridded binary data file for more information.
TRAILERBYTES length | back to top
This optional entry tell GrADS that the data file has a trailer record of length bytes following each time block of binary data. This entry is only valid for GrADS gridded data sets. See the section on structure of a gridded binary data file for more information.
XYHEADER length | back to top
This optional entry tells GrADS that the data file has a header record of length bytes preceding each horizontal grid (XY block) of binary data. This entry is only valid for GrADS gridded data sets. See the section on structure of a gridded binary data file for more information.
XVAR x,y | back to top
(GrADS version 1.9) This entry provides the x,y pair for the station's longitude. This entry is required for DTYPE bufr.
YVAR x,y | back to top
(GrADS version 1.9) This entry provides the x,y pair for the station's latitude. This entry is required for DTYPE bufr.
ZVAR x,y | back to top
(GrADS version 1.9) This entry provides the x,y pair for the station data's vertical coordinate (e.g., pressure). This is an optional entry for DTYPE bufr.
STID x,y | back to top
(GrADS version 1.9) This entry provides the x,y pair for the station ID. This entry is required for DTYPE bufr.
TVAR yr x,y mo x,y dy x,y hr x,y mn x,y sc x,y | back to top
(GrADS version 1.9) This entry provides the x,y pairs for all the base time coordinate variables. Each time unit (year=yr, month=mo, day=dy, hour=hr, minute=mn, second=sc) is presented as a 2-letter abbreviation followed by the x,y pair that goes with that time unit. The time for any individual station report is the base time plus the offset time (see TOFFVAR). All six base time units are not required to appear in the TVAR record, only those that are in the data file. This entry is required for DTYPE bufr.
TOFFVAR yr x,y mo x,y dy x,y hr x,y mn x,y sc x,y | back to top
(GrADS version 1.9) This entry provides the x,y pairs for all the offset time coordinate variables. The syntax is the same as TVAR. The time for any individual station report is the base time plus the offset time. All six offset time units are not required to appear in the TOFFVAR record, only those that are in the data file. This is an optional entry for DTYPE bufr.
CACHESIZE bytes | back to top
(GrADS version 2.0.a8+) This entry overrides the default size of the cache for reading HDF5 or NetCDF4 files. It is not relevant for other data types. It should not be necessary to set the cache size explicitly unless the data file has especially large chunks. Please see the documentation on compression. |
OPTIONS keyword | back to top
This entry controls various aspects of the way GrADS interprets the raw data file. It replaces the old FORMAT record. The keyword argument may be one or more of the following:
pascals | (GrADS version 2.0) (For DTYPE grib2 only) Indicates that pressure values that appear in the descriptor file (in the ZDEF entry and in the GRIB2 codes in the variable declarations) are given in units of Pascals. The gribmap utility requires pressure to be given in Pascals. If this keyword is present, the pressure level values will be converted to millibars after the gribmap index is generated and the descriptor file is opened with GrADS. If this keyword is omitted, pressure levels will remain in Pascals, and many of the internal functions (which assume a vertical dimension in units of millibars) will not work properly.
yrev | Indicates that the Y dimension (latitude) in the data file has been written in the reverse order from what GrADS assumes. An important thing to remember is that GrADS still presents the view that the data goes from south to north. The YDEF statement does not change; it still describes the transformation from a grid space going from south to north. The reversal of the Y axis is done as the data is read from the data file.
zrev | Indicates that the Z dimension (pressure) in the data file has been written from top to bottom, rather than from bottom to top as GrADS assumes. The same considerations as noted above for yrev also apply.
template | Indicates that a template for multiple data files is in use. For more information, see the section on Using Templates.
sequential | Indicates that the file was written in sequential unformatted I/O. This keyword may be used with either station or gridded data. If your gridded data is written in sequential format, then each record must be an X-Y varying grid. If you have only one X and one Y dimension in your file, then each record in the file will be one element long (it may not be a good idea to write the file this way).
365_day_calendar | Indicates the data file was created with perpetual 365-day years, with no leap years. This is used for some types of model ouput.
byteswapped | Indicates the binary data file is in reverse byte order from the normal byte order of your machine. Putting this keyword in the OPTIONS record of the descriptor file tells GrADS to swap the byte order as the data is being read. May be used with gridded or station data.
The best way to ensure hardware independence for gridded data is to specify the data's source platform. This facilitates moving data files and their descriptor files between machines; the data may be used on any type of hardware without having to worry about byte ordering. The following three OPTIONS keywords are used to describe the byte ordering of a gridded or station data file:
big_endian | Indicates the data file contains 32-bit IEEE floats created on a big endian platform (e.g., sun, sgi)
little_endian | Indicates the data file contains 32-bit IEEE floats created on a little endian platform (e.g., iX86, and dec)
cray_32bit_ieee | Indicates the data file contains 32-bit IEEE floats created on a cray.
PDEF | back to top
PDEF is so powerful it has its own documentation page.
XDEF xnum mapping <additional arguments> | back to top
This entry defines the grid point values for the X dimension, or longitude. The first argument, xnum, specifies the number of grid points in the X direction. xnum must be an integer >= 1. mapping defines the method by which longitudes are assigned to X grid points. There are two options for mapping: LINEAR Linear mapping LEVELS Longitudes specified individually The LINEAR mapping method requires two additional arguments: start and increment. start is a floating point value that indicates the longitude at grid point X=1. Negative values indicate western longitudes. increment is the spacing between grid point values, given as a positive floating point value. The LEVELS mapping method requires one additional argument, value-list, which explicitly specifies the longitude value for each grid point. value-list should contain xnum floating point values. It may continue into the next record in the descriptor file, but note that records may not have more than 255 characters. There must be at least 2 levels in value-list; otherwise use the LINEAR method. Here are some examples:
XDEF | 144 | LINEAR | 0.0 2.5
XDEF | 72 | LINEAR | 0.0 5.0
XDEF | 12 | LEVELS | 0 30 60 90 120 150 180 210 240 270 300 330
XDEF | 12 | LEVELS | 15 45 75 105 135 165 195 225 255 285 315 345
YDEF ynum mapping <additional arguments> | back to top
This entry defines the grid point values for the Y dimension, or latitude. The first argument, ynum, specifies the number of grid points in the Y direction. ynum must be an integer >= 1. mapping defines the method by which latitudes are assigned to Y grid points. There are several options for mapping: LINEAR Linear mapping LEVELS Latitudes specified individually GAUST62 Gaussian T62 latitudes GAUSR15 Gaussian R15 latitudes GAUSR20 Gaussian R20 latitudes GAUSR30 Gaussian R30 latitudes GAUSR40 Gaussian R40 latitudes The LINEAR mapping method requires two additional arguments: start and increment. start is a floating point value that indicates the latitude at grid point Y=1. Negative values indicate southern latitides. increment is the spacing between grid point values in the Y direction. It is assumed that the Y dimension values go from south to north, so increment is always positive. The LEVELS mapping method requires one additional argument, value-list, which explicitly specifies the latitude for each grid point, from south to north. value-list should contain ynum floating point values. It may continue into the next record in the descriptor file, but note that records may not have more than 255 characters. There must be at least 2 levels in value-list; otherwise use the LINEAR method. The Gaussian mapping methods require one additional argument: start. This argument indicates the first gaussian grid number. If the data span all latitudes, start would be 1, indicating the southernmost gaussian grid latitude. Here are some examples:
YDEF | 73 | LINEAR | -90 2.5
YDEF | 180 | LINEAR | -90 1.0
YDEF | 18 | LEVELS | -85 -75 -65 -55 -45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85
YDEF | 94 | GAUST62 | 1
YDEF | 20 | GAUSR40 | 15
The NCEP/NCAR Reanalysis surface variables are on the GAUST62 grid. The final example shows that there are 20 Y dimension values which start at Gaussian Latitude 15 (64.10 south) on the Gaussian R40 grid
ZDEF znum mapping <additional arguments> | back to top
This entry defines the grid point values for the Z dimension. The first argument, znum, specifies the number of pressure levels. znum must be an integer >= 1. mapping defines the method by which level values are assigned to Z grid points. There are two options for mapping: LINEAR Linear mapping LEVELS Pressure levels specified individually The LINEAR mapping method requires two additional arguments: start and increment. start is a floating point value that indicates the level value at grid point Z=1. increment is the spacing between grid point values in the Z direction, or from lower to higher. increment must be non-zero and non0negative. The LEVELS mapping method requires one additional argument, value-list, which explicitly specifies the pressure level for each grid point in ascending order. value-list should contain znum floating point values. It may continue into the next record in the descriptor file, but note that records may not have more than 255 characters. Here are some examples:
ZDEF | 7 | LEVELS | 1000 850 700 500 300 200 100
ZDEF | 17 | LEVELS | 1000 925 850 700 600 500 400 300 250 200 150 100 70 50
(GrADS version 2.0) (For DTYPE grib2 only) If your Z axis is pressure, the gribmap utility requires the level values to be given in units of Pascals instead of millibars. Use the "options pascals" keyword to convert the unit of the level values to millibars after the gribmap index is generated and when the descriptor file is opened with GrADS. Pressure level values may remain in Pascals, but then many of the internal functions (which assume a vertical dimension in units of millibars) will not work properly.
TDEF tnum LINEAR start increment | back to top
This entry defines the grid point values for the T dimension. The first argument, tnum, specifies the number of time steps. tnum must be an integer >= 1. The method by which times are assigned to T grid points is always LINEAR. start indicates the initial time value at grid point T=1. start must be specified in the GrADS absolute date/time format: hh:mmZddmmmyyyy where:
hh | = | hour (two digit integer)
mm | = | minute (two digit integer)
dd | = | day (one or two digit integer)
mmm | = | 3-character month
yyyy | = | year (may be a two or four digit integer; 2 digits implies a year between 1950 and 2049)
If not specified, hh defaults to 00, mm defaults to 00, and dd defaults to 1. The month and year must be specified. No intervening blanks are allowed in the GrADS absolute date/time format. increment is the spacing between grid point values in the T direction. increment must be specified in the GrADS absolute time increment format: vvkk where:
vv | = | an integer number, 1 or 2 digits
kk | = | mn (minute) hr (hour) dy (day) mo (month) yr (year)
Here are some examples:
TDEF | 60 | LINEAR | 00Z31dec1999 1mn
TDEF | 73 | LINEAR | 3jan1989 5dy
TDEF | 730 | LINEAR | 00z1jan1990 12hr
TDEF | 12 | LINEAR | 1jan2000 1mo
TDEF | 365 | LINEAR | 12Z1jan1959 1dy
TDEF | 40 | LINEAR | 1jan1950 1yr
EDEF enum NAMES <list of names> | back to top
EDEF enum ensemble_record_1 ensemble_record_2 ... ensemble_record_enum ENDEDEF | back to top
(GrADS version 2.0) This entry defines the ensemble dimension. All ensemble members must have identical X, Y, and Z dimensions, the same list of variables, and the same time axis increment. There are two different syntaxes for the EDEF entry: the first is simpler and requires only the names for each ensemble member, the second expanded form contains a name, individual time axis information, and optional GRIB2 codes. Both EDEF syntaxes begin with the enum argument, an integer >=1 which specifies the number of ensemble members. If all of the ensemble members have an identical time axis (i.e. length, initial time, and increment are the same for each one), then it is only necessary to distinguish the ensembles by their names, and the simplified EDEF syntax with the NAMES keyword may be used. A simple space-delimited list of names is all that is required. Ensemble names must have between 1 and 15 alphanumeric characters, lower case only. (In version 2.0.0 and later, mixed case ensemble names are allowed). Some examples are:
EDEF | 10 | NAMES | 1 2 3 4 5 6 7 8 9 10
EDEF | 12 | NAMES | m01 m02 m03 m04 m05 m06 m07 m08 m09 m10 m11 ensm
EDEF | 7 | NAMES | e1 e2 e3 e4 e5 e6 e7
When the OPTIONS TEMPLATE entry is used with EDEF, the ensemble names are used in the %e substitution template to generate the file name. See Using Templates for more details. If the ensemble members do not have identical time axes (i.e., their lengths or initial times are not the same), or if you need to include the GRIB2 codes, then you must use the expanded EDEF syntax: a collection of records framed by EDEF and ENDEDEF. The format of the ensemble records is as follows: ensname length start <grib2 codes> The ensname is the 1-15 character "name" for the ensemble member. The length is the size of the time axis of the ensemble, which must be less than or equal to the tnum argument in the TDEF entry. (The time axis described by TDEF must span all the ensemble members.) The start argument is the initial time of the ensemble member and must be given in GrADS absolute date/time format. (See TDEF for details). The grib2 codes are required if (1) the DTYPE is grib2 and (2) there is more than one ensemble member (enum > 1). The expanded form of the EDEF entry must be used when grib2 codes are required, even if the length and start times are the same for all members. For GRIB2 ensembles, support currently exists for four different Product Definition Template (PDT) numbers: 1, 2, 11, and 12. These are grouped into two types: individual ensemble forecasts (PDT 1 and 11) or derived forecasts based on all ensemble members (PDT 2 and 12). For individual ensemble forecasts (PDT 1 and 11), two comma-delimited grib2 codes are required: the ensemble type and perturbation number. For derived forecasts based on all ensemble members (PDT 2 and 12), only one grib2 code is required: the derived forecast. Clarification of all the GRIB2 nomenclature may be found in the documentation at WMO and NCEP. Two examples are given below. The first example illustrates ensemble members with different lengths and start times:
TDEF 591 linear 12z09dec1980 12hr
EDEF 16
ensm | 591 | 12z09dec1980
m01 | 591 | 12z09dec1980
m02 | 589 | 12z10dec1980
m03 | 587 | 12z11dec1980
m04 | 585 | 12z12dec1980
m05 | 583 | 12z13dec1980
m06 | 571 | 12z19dec1980
m07 | 569 | 12z20dec1980
m08 | 567 | 12z21dec1980
m09 | 565 | 12z22dec1980
m10 | 563 | 12z23dec1980
m11 | 549 | 12z30dec1980
m12 | 547 | 12z31dec1980
m13 | 545 | 12z01jan1981
m14 | 543 | 12z02jan1981
m15 | 541 | 12z03jan1981
ENDEDEF
The second example illustrates the use of GRIB2 codes:
TDEF 31 linear 00z24apr2007 12hr
EDEF 23
p01 | 31 | 00z24apr2007 | 3,1
p02 | 31 | 00z24apr2007 | 3,2
p03 | 31 | 00z24apr2007 | 3,3
p04 | 31 | 00z24apr2007 | 3,4
p05 | 31 | 00z24apr2007 | 3,5
p06 | 31 | 00z24apr2007 | 3,6
p07 | 31 | 00z24apr2007 | 3,7
p08 | 31 | 00z24apr2007 | 3,8
p09 | 31 | 00z24apr2007 | 3,9
p10 | 31 | 00z24apr2007 | 3,10
p11 | 31 | 00z24apr2007 | 3,11
p12 | 31 | 00z24apr2007 | 3,12
p13 | 31 | 00z24apr2007 | 3,13
p14 | 31 | 00z24apr2007 | 3,14
p15 | 31 | 00z24apr2007 | 3,15
p16 | 31 | 00z24apr2007 | 3,16
p17 | 31 | 00z24apr2007 | 3,17
p18 | 31 | 00z24apr2007 | 3,18
p19 | 31 | 00z24apr2007 | 3,19
p20 | 31 | 00z24apr2007 | 3,20
c00 | 31 | 00z24apr2007 | 1,0
avg | 31 | 00z24apr2007 | 0
spr | 31 | 00z24apr2007 | 2
ENDEDEF
VECTORPAIRS U-component,V-component | back to top
(GrADS version 1.9b4) This entry is for explicity identifying vector component pairs. This is only necessary if the data are on a native projection other than lat/lon (i.e. you are using PDEF) and if the winds have to be rotated from a grid-relative sense to an Earth-relative sense. (GrADS has to retrieve both the u and v component in order to do the rotation calculation.) Using this entry replaces the old technique of putting 33 (for U) or 34 (for V) in the first element of the units field in the variable declaration. The U-component and V-component arguments should be variable names that appear in the VARS list. They are separated by a comma, with no spaces. More than one pair of components may be listed; in this case, the pairs should be separated by a space. For example: VECTORPAIRS u,v u10,v10 uflx,vflx
VARS varnum variable_record_1 variable_record_2 ... variable_record_varnum ENDVARS | back to top
This ensemble of entries describes all the variables contained in the data set. varnum indicates the number of variables in the data set and is therefore also equal to the number of variable records that are listed between the VARS and ENDVARS entries. ENDVARS must be the final line of the Grads data descriptor file. Any blank lines after the ENDVARS statement may cause open to fail! The format of the variable records is as follows: varname levs units description The syntax of varname and units is different depending on what kind of data format (DTYPE) you are describing. Details provided below:
varname | This is a 1-15 character "name" or abbreviation for the data variable. varname may contain alphabetic and numeric characters but it must start with an alphabetic character (a-z).
varname (DTYPE netcdf, hdfsds, or hdf5_grid) | (GrADS version 1.9+) For DTYPE netcdf or hdfsds, varname may have a different syntax. This syntax is required when the name of the data variable in the SDF does not conform to the GrADS naming conventions (see below for list of criteria), but it may also be used to shorten or change the variable name to make it easier to work with inside GrADS. The syntax is: SDF_varname=>grads_varname SDF_varname is the name the data variable was given when the SDF file was originally created. For NetCDF files, this name appears in the output from ncdump. It is important that SDF_varname exactly matches the variable name in the data file. SDF_varname may contain uppercase letters and non-alpha-numeric characters. The classic varname syntax (i.e., when "SDF_varname =>" is omitted) may be used if SDF_varname meets the criteria for GrADS variable names: it must be less than 16 characters, start with an alphabetic character, and cannot contain any upper case letters or non-alpha-numeric characters. (GrADS version 2.0.a3+) If the SDF_varname contains spaces, substitute "~" for each space -- the spaces in the variable name string will be swapped back in later after the descriptor file has been parsed. (GrADS version 2.0.a7+) For dtype hdf5_grid, the SDF_varname may be particularly long since it must contain the names of all the nested groups (separated by "/") to which the data set belongs. For example: /HDFEOS/GRIDS/EarthSurfaceReflectanceClimatology/Data~Fields/MonthlySurfaceReflectance=>msr
levs | This is an integer that specifies the number of vertical levels the variable contains. levs may not exceed znum as specified in the ZDEF statement. If levs is 0, the variable does not correspond to any vertical level. Surface variables (e.g. sea level pressure) have a levs value of 0. For DTYPE station or bufr, surface variables have a levs value of 0 and upper air variables have a levs value of 1. (Exception to this rule for bufr data: replicated surface variables are given a levs value of 2).
levs (DTYPE grib2) | (GrADS version 2.0) This is a comma-delimited list of integers that provide information about the vertical dimension of a variable. The first number in the list is the number of vertical levels the variable contains or zero if the variable doesn't vary in Z. The remaining numbers are the GRIB2 parameters that specify the veritcal level or layer. The levs field may contain up to four comma-delimited numbers: NLEVS,LTYPE,LVAL,LVAL2,LTYPE2 where
NLEVS | = | The number of vertical levels, or 0 if not Z-varying (Required)
LTYPE | = | The level type indicator (Required)
LVAL | = | The value of the 1st level (Not Required for all level types)
LVAL2 | = | The value of the 2nd level (Only Required for layers between 2 fixed levels)
LTYPE2 | = | The level type indicator for the 2nd level (Only required if different from LTYPE)
If NLEVS > 0 and is followed only by the LTYPE, the values for LVAL will be determined by the ZDEF entry. If a variable has an NLEVS entry that is > 0 but less than the number of levels declared in the ZDEF entry, then the values for LVAL will correspond to the first NLEVS values of the Z axis. If LTYPE is 100 (the GRIB2 code for an isobaric surface), the units of LVAL must be Pascals. If the values of LVAL are taken from the ZDEF entry, use OPTIONS pascals to convert the vertical coordinate to millibars once the descriptor file is opened with GrADS. Some level types such as "mean sea level" or "tropopause" do not require an LVAL. In this case, LVAL may be omitted (see the "slp" example below). If two LTYPE entries are required but LVAL and LVAL2 are not, then the LVAL entries may be omitted, with adjacent commas used to indicate missing values (see the "cloud" example below). Examples:
hgt | 26,100 | 0,3,5 | Geopotential Height [gpm]
hgt500 | 0,100,50000 | 0,3,5 | Geopotential Height at 500mb [gpm]
slp | 0,101 | 0,3,1 | Sea Level Pressure [Pa]
t2m | 0,103,2 | 0,0,0 | 2-meter Temperature [K]
soilt1 | 0,106,0,0.1 | 0,0,0 | Soil Temp, 0-0.10m below surface [K]
cloud | 0,1,,,8 | 0,6,1 | Total Cloud Cover, from surface to TOA [%]
The external utilities grib2scan and wgrib2 are quite useful in determining what the values for the levs field should be for a GRIB2 data file.
description | This is text description or long name for the variable, max 40 characters.
The units component of the variable record is used for data with DTYPE bufr, grib, netcdf, or hdfsds. It is also used for non-standard binary data files that require special "unpacking" instructions, and special cases of pre-projected wind components. If the data you are describing does not fall into any of these categories, put a value of 99 in the units field.
units | For flat binary files containing 4-byte floating-point data that are not pre-projected, this field is ignored but must be included. Put in a value of 99.
units (DTYPE bufr) | (GrADS version 1.9) For DTYPE bufr files, this field contains the x,y pair for the named variable.
units (DTYPE grib) | For DTYPE grib, the units field specifies the GRIB parameters of the variable. This information is used by the gribmap utility for mapping the variables listed in the descriptor file to the data records in the GRIB files. This parameter may contain up to four comma-delimited numbers: VV,LTYPE,LVAL,TRI or VV,LTYPE,LVAL,LVAL2 where,
VV | = | The GRIB parameter number (Required)
LTYPE | = | The level type indicator (Required)
LVAL | = | The value of the 1st level (Required if NLEVS=0)
LVAL2 | = | The value of the 2nd level (Optional)
TRI | = | The "time range indicator" (Optional)
The external utilities gribscan and wgrib are quite useful in determining what the values for the units field should be for a GRIB data file. Examples:
u | 39 | 33,100 | U Winds [m/s]
t | 39 | 11,100 | Temperature [K]
ts | 0 | 11,1 | Surface Temperature [K]
tb | 0 | 11,116,60,30 | Temperature, 30-60mb above surface [K]
dpt | 0 | 17,100,1000 | Dew Point Temperature at 1000 mb [K]
units (DTYPE grib2) | (GrADS version 2.0) This is a comma-delimited list of values that identify a GRIB2 parameter (variable): DISC,CAT,NUM,SP,SP2 where,
DISC | = | The parameter Discipline (Required)
CAT | = | The parameter Category (Required)
NUM | = | The parameter Number (Required)
SP | = | The Statistical Process used to derive the parameter (May be required if parameter is not an instantaneous value)
SP2 | = | The Spatial Process used to interpolate the parameter (Required only for Product Definition Template 4.15)
Some examples are:
u | 26,100 | 0,2,2 | U-Component of Wind [m/s]
v | 26,100 | 0,2,3 | V-Component of Wind [m/s]
t2max | 0,103,2 | 0,0,5 | 2-meter Temperature Maximum [K] (NCEP)
t2max | 0,103,2 | 0,0,0,2 | 2-meter Temperature Maximum [K] (TIGGE)
soilm1 | 0,106,0,0.1 | 2,0,192 | Soil Moisture, 0-0.10m below surface [K]
catave | 10,100 | 0,19,22,0,3 | Spatial Avg. of Clear Air Turbulence [%]
catmax | 10,100 | 0,19,22,2,3 | Spatial Max of Clear Air Turbulence [%]
units (DTYPE netcdf, hdfsds, or hdf5_grid) | (GrADS version 1.9) For DTYPE netcdf or hdfsds or hdf5_grid (GrADS version 2.0.a7+) , the units field is a comma-delimited list of the varying dimensions of the variable. Dimensions expressed as x, y, z, or t correspond to the four axes defined by XDEF, YDEF, ZDEF and TDEF. For example, a surface variable such as sea level pressure might look like this: presSFC=>psfc 0 y,x Surface Pressure A time-varying atmospheric variable such as geopotential height might look like this: Height=>hght 17 t,z,y,x Geopotential Height (m) The order of the dimensions listed in the units field does matter. They must describe the shape of the variable as it was written to the SDF data file. For NetCDf files, this information appears in the output from ncdump next to the variable name. If your data file contains a variable that also varies in a non-world-coordinate dimension (e.g. histogram interval, spectral band, ensemble number) then you can put a non-negative integer in the list of varying dimensions that will become the array index of the extra dimension. For example: VAR=>hist0 0 0,y,x First historgram interval for VAR VAR=>hist1 0 1,y,x Second historgram interval for VAR VAR=>hist2 0 2,y,x Third histogram interval for VAR Another option in this example would be to fill the unused Z axis with the histogram intervals: zdef 3 linear 1 1 ... VAR=>hist 3 z,y,x VAR Histogram In this case, it would appear to GrADS that variable 'hist' varies in Z, but the user would have to remember that the Z levels correspond to histogram intervals. The latter technique makes it easier to slice through the data, but is not the most accurate representation. And if you don't have an unsued world-coordinate axis available, then you still have a way to access your data.
units (non-standard binary) | For non-standard binary files, the units field is used to instruct GrADS how to read binary files that do not conform to the default structure or do not contain 4-byte float data. GrADS assumes the data were written in the following order (starting from the fastest varying dimension to the slowest): longitude (X), latitude (Y), vertical level (Z), variable (VAR), time (T). If your binary data set was created or "packed" according to a different dimension sequence, then you can use the units field to tell GrADS exactly how to unpack the data. For these non-standard binary files, the units field is a series of one or more comma-delimited numbers, the first of which is always -1. The syntax is as follows: -1, structure <,arg> There are four options for structure, outlined below. Some of these options have additional attributes which are specified with arg.
-1,10,arg | (GrADS 1.9 or earlier) This option indicates that "VAR" and "Z" have been transposed in the dimension sequence. The order is: longitude (X), latitude (Y), variable (VAR), vertical level (Z), time(T). Thus, all variables are written out one level at a time. This feature was designed to be used with NASA GCM data in the "phoenix" format. The upper air prognostic variables were transposed, but the diagnostic variables were not. Thus an arg of 1 means the variable has been var-z transposed, and an arg of 2 means the variable has not.
-1,20 | This option indicates that "VAR" and "T" have been transposed in the dimension sequence. The order is: longitude (X), latitude (Y), vertical level (Z), time(T), variable (VAR). Thus, all times for one variable are written out in order followed by all times for the next variable, etc. Data files for which "VAR" and "T" have been transposed may not be templated together.
-1,30 | (GrADS 1.9 or earlier) This option handles the cruel and unusual case where X and Y dimensions are transposed and the horizontal grids are (lat,lon) as opposed to (lon,lat) data. This option causes GrADS to work very inefficiently. However, it is useful for initial inspection and debugging.
-1,40,arg | This option handles non-float data. If there are multiple variables in the same file, they must all be the same type. The dimension sequence is assumed to be the default. The secondary arg tells GrADS what type of data values are in the binary file: units = -1,40,1 = 1-byte unsigned chars (0-255) units = -1,40,2 = 2-byte unsigned integers units = -1,40,2,-1 = 2-byte signed integers units = -1,40,4 = 4-byte integers
units (pre-projected wind components) | For pre-projected vector component data that require the use of PDEF and rotation, GrADS has to retrieve both the u and v component in order to do the rotation calculation. The new (and recommended) method for matching vector components is to use the VECTORPAIRS descriptor file entry. The old technique (for versions older than 1.9b4) is to use the units field of the variable record. The u-component variable must have a units value of 33, and the v-component variable must have a units value of 34. (This is the GRIB convention). If there are more than one u/v pairs, secondary units values are used.
@ varname attribute_type attribute_name attribute_value | back to top
(GrADS version 1.9b4) To supplement the metadata in your descriptor file, use attribute comments. The first two characters of the attribute comment must be "@" followed by a space -- this distinguishes it from an ordinary comment (see below). Attribute comments may appear anywhere in the descriptor file, and they will be ignored if used with older versions of GrADS. All file attributes may be retrieved with the 'query attr' command. varname may be set to "global" to describe general attributes that are valid for the entire data set. Set varname to "lon", "lat", "lev", or "time" to describe attributes of the four coordinate axes; otherwise, use one of the variable names listed in the variable declarations. If a variable name is aliased, use the grads_varname instead of the native SDF_varname. attribute_type should be one of the following case-sensitive types: String, Byte, Int16, UInt16, Int32, UInt32, Float32, Float64. attribute_name may be any single word or string with no spaces (e.g.: "units", "minimum_value") attribute_value can be any string as long as the length of the entire entry does not exceed 512 characters. For example: @ precip String units mm/day @ global String documentation http://put.your.documentation.url.here
* comment | back to top
You may put comments in your descriptor file by beginning the entry with * . Use @ for formatted attribute comments (see above).