MET Tool: PCP-Combine

MET Tool: PCP-Combine
IMPORTANT NOTE: If you are returning to the tutorial, you must source the tutorial setup script before running the following instructions. If you are unsure if you have done this step, please navigate to the Verify Environment is Set Correctly page.

We now shift to a discussion of the MET PCP-Combine tool and will practice running it directly on the command line.

PCP-Combine Functionality

The PCP-Combine tool is used (if needed) to add, subtract, sum or derive accumulated field values, most commonly precipitation, from several gridded data files into a single NetCDF file containing the desired accumulation period. Its NetCDF output may be used as input to the MET statistics tools. PCP-Combine may be configured to combine any gridded data field you'd like. However, all gridded data files being combined must have already been placed on a common grid. The copygb utility is recommended for re-gridding GRIB files. In addition, the PCP-Combine tool will only sum model files with the same initialization time unless it is configured to ignore the initialization time.

PCP-Combine Usage

View the usage statement for PCP-Combine by simply typing the following:

Usage: pcp_combine  
  [[-sum] sum_args] | [-add input_files] | [-subtract input_files] | [-derive stat_list input_files]
(Note: "|" means "or")
  [-sum] sum_args Data from multiple files containing the same accumulation interval should be summed up using the arguments provided.
  -add input_files Data from one or more files should be added together where the accumulation interval is specified separately for each input file.
  -subtract input_files Data from exactly two files should be subtracted.
  -derive stat_list input_files The comma-separated list of statistics in "stat_list" (sum, min, max, range, mean, stdev, vld_count) should be derived using data from one or more files.
  out_file Output NetCDF file to be written.
  [-field string] Overrides the default use of accumulated precipitation (optional).
  [-name list] Overrides the default NetCDF variable name(s) to be written (optional).
  [-vld_thresh n] Overrides the default required ratio of valid data (1) (optional).
  [-log file] Outputs log messages to the specified file
  [-v level] Level of logging
  [-compress level] NetCDF file compression

Use the -sum, -add, -subtract, or -derive command line option to indicate the operation to be performed. Each operation has its own set of required arguments.

admin Mon, 06/24/2019 - 16:05

Run Sum Command

Run Sum Command

Since PCP-Combine performs a simple operation and reformatting step, no configuration file is needed.

  1. Start by making an output directory for PCP-Combine and changing directories:
mkdir -p ${METPLUS_TUTORIAL_DIR}/output/met_output/pcp_combine
cd ${METPLUS_TUTORIAL_DIR}/output/met_output/pcp_combine
  1. Now let's run PCP-Combine twice using some sample data that's included with the MET tarball:
pcp_combine \
-sum 20050807_000000 3 20050807_120000 12 \ \
-pcpdir ${METPLUS_DATA}/met_test/data/sample_fcst/2005080700
pcp_combine \
-sum 00000000_000000 1 20050807_120000 12 \ \
-pcpdir ${METPLUS_DATA}/met_test/data/sample_obs/ST2ml


The "\" symbols in the commands above are used for ease of reading. They are line continuation markers enabling us to spread a long command line across multiple lines. They should be followed immediately by "Enter". You may copy and paste the command line OR type in the entire line with or without the "\".

Both commands run the sum command which searches the contents of the -pcpdir directory for the data required to create the requested accmululation interval.

In the first command, PCP-Combine summed up 4 3-hourly accumulation forecast files into a single 12-hour accumulation forecast. In the second command, PCP-Combine summed up 12 1-hourly accumulation observation files into a single 12-hour accumulation observation. PCP-Combine performs these tasks very quickly.

We'll use these PCP-Combine output files as input for Grid-Stat. So make sure that these commands have run successfully!

admin Mon, 06/24/2019 - 16:13



When PCP-Combine is finished, you may view the output NetCDF files it wrote using the ncdump and ncview utilities. Run the following commands to view contents of the NetCDF files:

ncview &
ncview &
ncdump -h
ncdump -h

The ncview windows display plots of the precipitation data in these files. The output of ncdump indicates that the gridded fields are named APCP_12, the GRIB code abbreviation for accumulated precipitation. The accumulation interval is 12 hours for both the forecast (3-hourly * 4 files = 12 hours) and the observation (1-hourly * 12 files = 12 hours).

Note, if ncview is not found when you run it on your system, you may need to load it first.  For example, on hera, you can use this command:

 module load ncview

Plot-Data-Plane Tool

The Plot-Data-Plane tool can be run to visualize any gridded data that the MET tools can read. It is a very helpful utility for making sure that MET can read data from your file, orient it correctly, and plot it at the correct spot on the earth. When using new gridded data in MET, it's a great idea to run it through Plot-Data-Plane first:

plot_data_plane \ \ \
'name="APCP_12"; level="(*,*)";'
gv &

Ghostview (gv) can take a little while before it displays.  If you don't have gv on your computer, try using display, or any tool that can visualize PostScript files, e.g.:

display &

Another option is to create a PNG file from the PS file, also rotating it to appear the right way:

convert -rotate 90 \
display sample_fcst_12L_2005080712V_12A.png

Next try re-running the command list above, but add the convert(x)=x/25.4; function to the config string (Hint: after the level setting and ; but before the last closing tick) to change units from millimeters to inches. What happened to the values in the colorbar?

Now, try re-running again, but add the censor_thresh=lt1.0; censor_val=0.0; options to the config string to reset any data values less 1.0 to a value of 0.0. How has your plot changed?

The convert(x) and censor_thresh/censor_val options can be used in config strings and MET config files to transform your data in simple ways.

admin Mon, 06/24/2019 - 16:13

Add and Subtract Commands

Add and Subtract Commands

We have run examples of the PCP-Combine -sum command, but the tool also supports the -add, -subtract, and -derive commands. While the -sum command defines a directory to be searched, for -add, -subtract, and -derive we tell PCP-Combine exactly which files to read and what data to process. The following command adds together 3-hourly precipitation from 4 forecast files, just like we did in the previous step with the -sum command:

pcp_combine -add \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_03.tm00_G212 03 \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_06.tm00_G212 03 \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_09.tm00_G212 03 \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_12.tm00_G212 03 \

By default, PCP-Combine looks for accumulated precipitation, and the 03 tells it to look for 3-hourly accumulations. However, that 03 string can be replaced with a configuration string describing the data to be processed, which doesn't have to be accumulated precipation. The configuration string should be enclosed in single quotes. Below, we add together the U and V components of 10-meter wind from the same input file. You would not typically want to do this, but this demonstrates the functionality. We also use the -name command line option to define a descriptive output NetCDF variable name:

pcp_combine -add \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_03.tm00_G212 'name="UGRD"; level="Z10";' \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_03.tm00_G212 'name="VGRD"; level="Z10";' \ \

While the -add command can be run on one or more input files, the -subtract command requires exactly two. Let's rerun the wind example from above but do a subtraction instead:

pcp_combine -subtract \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_03.tm00_G212 'name="UGRD"; level="Z10";' \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_03.tm00_G212 'name="VGRD"; level="Z10";' \ \

Now run Plot-Data-Plane to visualize this output. Use the -plot_range option to specify a the desired plotting range, the -title option to add a title, and the -color_table option to switch from the default color table to one that's good for positive and negative values:

plot_data_plane \ \ \
'name="UGRD_MINUS_VGRD"; level="(*,*)";' \
-plot_range -15 15 \
-title "10-meter UGRD minus VGRD" \
-color_table ${MET_BUILD_BASE}/share/met/colortables/NCL_colortables/posneg_2.ctable

Now view the results:

gv &


admin Mon, 06/24/2019 - 16:14

Derive Command

Derive Command

While the PCP-Combine -add and -subtract commands compute exactly one output field of data, the -derive command can compute multiple output fields in a single run. This command reads data from one or more input files and derives the output fields requested on the command line (sum, min, max, range, mean, stdev, vld_count).

Run the following command to derive several summary metrics for both the 10-meter U and V wind components:

pcp_combine -derive min,max,mean,stdev \
${METPLUS_DATA}/met_test/data/sample_fcst/2005080700/wrfprs_ruc13_*.tm00_G212 \
-field 'name="UGRD"; level="Z10";' \
-field 'name="VGRD"; level="Z10";' \

In the above example, we used a wildcard to list multiple input file names.  And we used the -field command line option twice to specify two input fields.  For each input field, PCP-Combine loops over the input files, derives the requested metrics, and writes them to the output NetCDF file.  Run ncview to visualize this output:

ncview &

This output file contains 8 variables: 2 input fields * 4 metrics. Note the output variable names the tool chose.  You can still override those names using the -name command line argument, but you would have to specify a comma-separated list of 8 names, one for each output variable.

johnhg Tue, 07/23/2019 - 17:13