METplus Practical Session Guide (Version 5.0) | MET Tool: Plot-Data-Plane > The Field String

Defining the field string

As you'll see throughout these exercises, the behavior of the MET and METplus tools is controlled using ASCII configuration files, and you will learn more about those options in the coming sessions. The field_string command line argument is actually processed as a miniature configuration file. In fact, that string is written to a temporary file which is then read by MET's configuration file library code.

In general, the name and level entries are required to extract a gridded field of data from a supported input file format. The conventions for specifying them vary based on the input file type:

  1. For GRIB1 or GRIB2 inputs, set name as the abbreviation for the desired variable or data type that appears in the GRIB tables and set level to a single letter (A, Z, P, L, or R) to define the level type followed by a number to define the level value. For example 'name = "TMP"; level = "P500";' extracts 500 millibar temperature from a GRIB file.
  2. For NetCDF inputs, set name as the NetCDF variable name and level to define how to extract a 2-dimensional slice of gridded data from that variable. For example, 'name = "temperature"; level = "(0,1,*,*)";' extracts a 2-dimensional field of data from the last two dimensions of a NetCDF temperature variable using the first two indicies as constant values.
  3. For Python embedding, set name as the python script to be run along with any arguments for that script and do not set level. For example, 'name = " input.txt";' runs a python script to read data from the specified input file.

Note that all field strings should be enclosed in single quotes, as shown above, so that they are processed on the command line as a single string which may contain embedded whitespace.

Examples for each of these input types are provided in the coming exercises. More details about setting the field string can be found in the Configuration File Overview section of the MET User's Guide. For example, if a field string matches multiple records in an input GRIB file, additional filtering criteria may be specified to further refine them. Additional options exist to explicitly specify the input file_type, define a function to convert the data, or define an operation to censor the data. Each configuration entry should be terminated with a semi-colon (;).

Since the field_string is processed using a temporary file, any syntax errors will produce a parsing error log message similar to the following:

ERROR  : yyerror() -> syntax error in file "/tmp/met_config_61354_1"

Error messages like this typically mean there is a problem in a configuration string or configuration file being read by MET.