Merra2BC

Merra2BC interpolates MERRA-2 chemical species to the WRF-Chem grid and updates:

• initial conditions (wrfinput_d01)
• time-varying boundary conditions (wrfbdy_d01)

Requirements

• Python 3
• numpy
• scipy
• netCDF4
• WRF/WPS outputs from your case setup:
  • wrfinput_d01
  • wrfbdy_d01
  • met_em.d01.*.nc
• MERRA-2 files covering the full simulation period and spatial domain

Install Python dependencies (pip):

python3 -m pip install -r requirements.txt

Or create a Conda environment from the included file:

conda env create -f environment.yml
conda activate merra2bc

Configuration Model

Defaults are defined in src/config.py:

• species mapping (spc_map)
• file paths and filename masks
• whether to process IC/BC by default (do_IC, do_BC)

At runtime, command-line flags can override these defaults without editing the file.

Quick Start

1. Run real.exe with chem_in_opt = 0 to generate wrfinput_d01 and wrfbdy_d01.
2. Download required MERRA-2 collections:
   • M2I3NVAER_5.12.4
   • M2I3NVCHM_5.12.4
3. Update src/config.py defaults (especially spc_map, paths, and masks).
4. Zero relevant chemistry fields before interpolation:

   python3 zero_fields.py --do_IC=true --do_BC=true

5. Run interpolation:

   python3 main.py

6. Before running wrf.exe, in namelist.input under &chem set:
   • have_bcs_chem = .true. for boundary conditions
   • chem_in_opt = 1 for initial conditions
7. Run wrf.exe.

Command-line Interface

Print all options:

python3 main.py --help

main.py supported options:

• --wrf_input_file (full path to wrfinput_d01)
• --wrf_bdy_file (full path to wrfbdy_d01)
• --wrf_met_files (full-path glob mask for met_em files)
• --merra2_files (full-path glob mask for MERRA2 files)
• --do_IC=true|false
• --do_BC=true|false
• --init_co2_ch4=true|false (default false; sets co2=400 ppmv, ch4=1.7 ppmv; for BC also sets co2/ch4 tendencies to 0)

Important:

• Quote glob masks passed to --wrf_met_files and --merra2_files to prevent shell expansion.
• Use one MERRA collection per run (inst3_3d_aer_Nv or inst3_3d_chm_Nv) consistent with the active spc_map; do not mix both collections in one mask.

Example (explicit paths, process both IC and BC):

python3 main.py \
  --wrf_input_file /path/to/wrf/run/wrfinput_d01 \
  --wrf_bdy_file /path/to/wrf/run/wrfbdy_d01 \
  --wrf_met_files '/path/to/wps/run/met_em.d01.2010-*' \
  --merra2_files '/path/to/merra/MERRA2_*.nc4' \
  --do_IC=true --do_BC=true \
  --init_co2_ch4=false

Example (boundary conditions only):

python3 main.py --do_IC=false --do_BC=true

zero_fields.py uses a fixed zero value (1e-16) and supports shared config overrides, including --wrf_input_file, --wrf_bdy_file, --do_IC=true|false, --do_BC=true|false. The fields zeroed should be the same WRF fields that are updated later by main.py (based on the active spc_map).

python3 zero_fields.py \
   --wrf_input_file /path/to/wrf/run/wrfinput_d01 \
   --do_IC=true

OR

python3 zero_fields.py \
   --wrf_bdy_file /path/to/wrf/run/wrfbdy_d01 \
   --do_BC=true

Notes

• Interpolated values are added to existing WRF-Chem fields.
• Running zero_fields.py before main.py is recommended to avoid double counting.

Citation

If this utility is useful in your research, please cite:

Ukhov, A., Ahmadov, R., Grell, G., and Stenchikov, G.: Improving dust simulations in WRF-Chem v4.1.3 coupled with the GOCART aerosol module, Geosci. Model Dev., 14, 473–493, https://doi.org/10.5194/gmd-14-473-2021, 2021.