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How to Use VIC as a Meteorological Forcing Disaggregator

In normal operation, VIC disaggregates daily forcings to a sub-daily time step, either the model time step (TIME_STEP) or the snow time step (SNOW_STEP) if this is different from the model time step. Normally, these disaggregated forcings are used internally but never written to the output files. However, there are many cases in which we want to store these disaggregated forcings; for example, preparing sub-daily forcings for other hydrologic models, or examining the inputs to VIC's physics functions.

For this reason, VIC has an option (OUTPUT_FORCE) to write its internal sub-daily forcings to output files. When OUTPUT_FORCE is TRUE, VIC will run as before, but after disaggregating each grid cell's forcings, it will move to the next grid cell without running a simulation.

Here is a detailed procedure for running VIC in OUTPUT_FORCE mode:

1. edit user_def.h

   set OUTPUT_FORCE to TRUE

2. recompile VIC

   make clean; make

3. create a global parameter file. Because VIC will not actually run a simulation, it will ignore some of the file's contents. The only settings that VIC will pay attention to in this mode are:
   • TIME_STEP

     Set this to the sub-daily interval that you want your forcings disaggregated to

   • SNOW_STEP

     Set this equal to TIME_STEP

   • STARTYEAR, STARTMONTH, STARTDAY, STARTHOUR

     Set these to the start date for your disaggregated forcings

   • ENDYEAR, ENDMONTH, ENDDAY

     Set these to the end date for your disaggregated forcings

   • All the variables in the forcing section should be the same as before, i.e. these describe the input (daily) forcings that you are reading: FORCING1, FORCING2 (if applicable), FORCE_FORMAT, FORCE_ENDIAN, N_TYPES, FORCE_TYPE (there must be one of these for each input variable, e.g. PREC, TMAX, TMIN, WIND), FORCE_DT, FORCEYEAR, FORCEMONTH, FORCEDAY, FORCEHOUR, GRID,_DECIMAL, WIND_H, MEASURE_H, and ALMA_INPUT
   • SOIL, ARC_SOIL (if applicable), SOIL_DIR (if applicable)

     These should be set to the appropriate soil parameter file - VIC needs this to get a list of cells whose forcings it will read

   • RESULT_DIR

     Set to the name of the directory where the disaggregated forcings should be written

   • OUT_STEP

     Set to 0

   • ALMA_OUTPUT

     For standard VIC forcings, set to FALSE; for ALMA-compliant forcings (often required by other models) set to TRUE

   • BINARY_OUTPUT

     Set this to FALSE to produce ASCII forcings, TRUE to produce BINARY forcings

   • SKIPYEAR

     We recommend setting this to 0

   • N_OUTFILES, OUTFILE, OUTVAR

     These can be omitted; by default, VIC will produce 1 output file per grid cell, names "full_data_lat_lon," where lat, lon = latitude and longitude of te grid cell's center. These default output files will contain the following variables:

     Name	Units (ALMA_OUTPUT FALSE)	Units (ALMA_OUTPUT TRUE)
     PREC	[mm/timestep]	[mm/sec]
     AIR_TEMP	[deg C]	[K]
     SHORTWAVE	[W/m2]	[W/m2]
     LONGWAVE	[W/m2]	[W/m2]
     DENSITY	[kg/m3]	[kg/m3]
     PRESSURE	[kPa]	[Pa]
     VP	[kPa]	[Pa]
     WIND	[m/s]	[m/s]

     If you wish to output different forcing variables from the ones wirtten by default, you will need to specify these in the global parameter file.

4. Run VIC

   vicNl -g global_param_file

5. Disaggregated forcings should now reside in the directory you specified in the global parameter file.
6. Turn OFF the OUTPUT_FORCE option:
   1. edit user_def.h; set OUTPUT_FORCE to FALSE
   2. recompile VIC ("make clean; make")