set sdfattr varname attribute_type attribute_name attribute_value
 Sets attribute metadata to be included in the output file created with the 
  sdfwrite command.
varname           May be set to    "global", "longitude", "latitude", "level",  "time", "ensemble", or 
                      a defined variable name
  attribute_type       May be one of five data type types (valid aliases in parentheses): 
                  "char" (String, Str, Url) 
                  "short" (Int16, UInt16)
                   "int" (long, Int32, UInt32)
                  "float" (Float32)
                  "double" (Float64). 
    attribute_name    May be any single word or string with no spaces (e.g.: "units", "long_name")
    attribute_value   May be any string as long as the length of the entire entry does not exceed 512 characters. 
                                        For numeric types, may be a list of comma-delimited numerals. 
This command is available in GrADS v2.0.a3 or higher.
The reset command will release all the attributes from memory. To do this without resetting all the other user-specified options, use  the clear sdfwrite command.
set sdfattr precip String units mm/day
set sdfattr precip String long_name Daily Precipiation 
set sdfattr precip double actual_range 0,57.5