METplus Practical Session Guide (Version 5.0) | MET Tool: PCP-Combine > 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 &