MET Online Tutorial

MET Online Tutorial for METv3.0

Grid-Stat Tool: Output

The output of Grid-Stat is one or more ASCII files containing statistics summarizing the verification performed and a NetCDF file containing difference fields. In this example, the output is written to the tutorial/out/grid_stat directory as we requested on the command line. That output directory should now contain 15 files, 9 from the first Grid-Stat command and 6 from the second.

The first command generates CTC, CTS, CNT, SL1L2, NBRCTC, NBRCTS, and NBRCNT ASCII files, a STAT file, and a NetCDF difference fields file. The second command generates PCT, PSTD, PJC, and PRC ASCII files, a STAT file, and a NetCDF difference fields file.

The format of the CTC, CTS, CNT, and SL1L2 ASCII files are the same as was described for the Point-Stat tool. What's new for the Grid-Stat tool is the neighborhood method output (NBRCTC, NBRCTS, and NBRCNT) and the probability methods output (PCT, PSTD, PJC, and PRC). While Point-Stat is also able to use the probabilistic verification methods, it is NOT able to use the neighborhood verification methods since the observations are not gridded. Neighborhood verification is only available in Grid-Stat.

For the neighborhood methods, rather than comparing forecast/observation values at individual grid points, areas of forecast values are compared to areas of observation values. At each grid box, a fractional coverage value is computed for each field as the number of grid points within the neighborhood (centered on the current grid point) that exceed the specified raw threshold value. The forecast/observation fractional coverage values are then compared rather than the raw values themselves.

For the probability methods, the probabilistic forecast values are thresholded using multiple thresholds between 0 and 1 to define a multi-row contingency table. The observation field is also thresholded to define a binary yes/no field. The pairs of probabilistic forecast values and binary yes/no observation values are used to fill the multi-row contingency table. The output probability counts and statistics are derived from this multi-row contingency table.

arrow Since the lines of data in these ASCII files are so long, we strongly recommend configuring your text editor to NOT use dynamic word wrapping. The files will be much easier to read that way.

Open up the tutorial/out/grid_stat/grid_stat_240000L_20050808_000000V_nbrctc.txt NBRCTC file using the text editor of your choice and note the following:

  • The format of this file is almost identical to that of the CTC file.
  • The INTERP_MTHD column is set to NBRHD, indicating that the neighborhood method was applied.
  • The INTERP_PNTS column is set to 9 or 25, indicating that the neighborhood was defined over a 3-by-3 or 5-by-5 square.
  • The LINE_TYPE column is set to NBRCTC, indicating that the columns to follow contain neighborhood contingency table counts.
  • The COV_THRESH column is set to >=0.500 or >=0.7500, indicating the coverage thresholds that were applied to the coverage fields to define these contingency tables.

The same types of differences exist between the CTS and the NBRCTS files.

Close this file, open up the tutorial/out/grid_stat/grid_stat_240000L_20050808_000000V_nbrcnt.txt NBRCNT file, and note the following:

  • The format of this file is NOT very similar to that of the CNT files.
  • The two statistics included in this file are the Fractions Skill Score (FSS column) and the Fractions Brier Score (FBS column) and their corresponding confidence intervals. See the MET Users Guide for a description of the neighborhood methods.

Close this file and use the ncview utility (if avaliable on your machine) to view the NetCDF output of Grid-Stat:

    ncview tutorial/out/grid_stat/grid_stat_240000L_20050808_000000V_pairs.nc &

Click through the variable names in the ncview window to see plots of the forecast, observation, and difference fields for each masking region. Now dump the header using the ncdump utility (if available on your machine):

    ncdump -h tutorial/out/grid_stat/grid_stat_240000L_20050808_000000V_pairs.nc

View the NetCDF header to see how the variable names are defined.

Next, open up the tutorial/out/grid_stat/grid_stat_1080000L_20050808_000000V_pct.txt PCT probabilistic output file using the text editor of your choice and note the following:

  • The LINE_TYPE column is set to PCT, indicating that the columns to follow contain information about the probability contingency table counts.
  • Since the number of forecast thresholds the user may choose is variable, the number of columns in this line (and the other probability lines) is variable. This line contains columns named OY_i, ON_i, and THRESH_i for i = 1 to 5, the probability thresholds chosen.

Close this file and see the MET Users Guide for a description of the other output probability line types.