TITLE string 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.

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CACHESIZE bytes

OPTIONS keyword

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:

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:

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:

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:

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 : mm Z ddmmmyyyy

where:

( 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:

( 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:

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 ( Version 2.0.1 or earlier )
varname levs <additional_codes> units description ( Version 2.0.2 or later )

The syntax of varname and units is different depending on what kind of data format (DTYPE) you are describing. The <additional_codes> are only necessary for certain types of GRIB2 data sets. Details provided below:

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

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

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).

( GrADS version 2.0 ) This is a comma-delimited list of numbers 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 five comma-delimited numbers:

For the Probability forecasts (PDT 5 or 9), the additional_codes field has 2 or 3 comma-delimted numbers, preceded by the letter "a" :

( Version 2.1.a3+ ) For the Percentile forecasts (PDT 6 or 10), the additional_codes field has only one number preceded by the letter "a" :

For the Aerosol Forecasts (PDT 48), the additional_codes field may have up to 7 comma-delimted numbers, preceded by the letter "a":

aATYPE,STYPE,S1,S2,WTYPE,W1,W2

The first size (in meters)

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 VV,LTYPE,LVAL,LVAL2 where,

( GrADS version 2.0 ) This is a comma-delimited list of values that identify a GRIB2 parameter (variable):

where,

( 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:

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 .

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 .

(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

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.

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

( 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