create_clm-forcing_icon.py

create_clm-forcing_icon.py#

{note} This page documents the script used to generate **CLM atmospheric forcing files from ICON model output**.

Overview#

create_clm-forcing_icon.py converts atmospheric output from the ICON model into forcing data suitable for the Community Land Model (CLM / eCLM).

The script performs the following steps:

  1. Reads ICON NetCDF files

  2. Extracts required meteorological variables

  3. Computes derived quantities required by CLM

  4. Reformats the data structure

  5. Writes CLM-compatible NetCDF forcing files

The resulting dataset contains atmospheric boundary variables required by CLM land surface simulations.

Script Location#

mkforcing_icon/create_clm-forcing_icon.py

Requirements#

Python Version#

Python ≥ 3.8 is recommended.

Python Dependencies#

Main libraries used:

Package Purpose

xarray NetCDF data processing numpy numerical calculations glob file discovery cftime time handling

Input Data#

The script processes ICON atmospheric output files stored in NetCDF format.

Input files are collected automatically using a filename pattern.

Example input path: /path/to/icon/output/ICON_out_EU-R13B5_inst_DOM01_ML_YYYYMM*.nc

where MM represents the month.

Required Variables#

The following variables must be present in the ICON dataset.

ICON variable Description

temp atmospheric temperature qv specific humidity u_10m zonal wind at 10 m v_10m meridional wind at 10 m sob_s shortwave radiation (down) sou_s shortwave radiation (up) thb_s longwave radiation (down) thu_s longwave radiation (up) tot_prec total precipitation pres_msl mean sea level pressure pres_sfc surface pressure

Output#

The script produces monthly CLM forcing NetCDF files.

Example output: YYYY-MM.nc

The output dataset contains the following variables.

CLM Variable Description

FSDS surface downward shortwave radiation FLDS surface downward longwave radiation WIND near-surface wind speed PRECTmms precipitation rate PSRF surface pressure TBOT near-surface air temperature QBOT near-surface relative humidity ZBOT reference height

Coordinates:

Coordinate Description

time simulation time xc longitude yc latitude ni grid cell index nj singleton dimension

Workflow#

The script follows several processing steps.

1. Collect ICON Files#

All monthly ICON files are collected automatically.

filelist = sorted(glob.glob(os.path.join(input_folder, pattern)))

2. Extract Required Variables#

Relevant variables are extracted from the dataset.

3. Compute Derived Variables#

Several CLM forcing variables must be computed from ICON output.

Shortwave Radiation#

FSDS = sob_s + sou_s

Longwave Radiation#

FLDS = thb_s + thu_s

Wind Speed#

WIND = sqrt(u10² + v10²)

Precipitation Rate#

ICON precipitation is converted from hourly accumulation to a rate:

PRECTmms = tot_prec / 3600

Relative Humidity#

Relative humidity is calculated from temperature, pressure, and specific humidity.

Example:

RH = 100 * e / es

where:

  • e is vapor pressure

  • es is saturation vapor pressure

4. Reshape Data#

CLM forcing files expect dimensions:

(time, nj, ni)

ICON data is therefore expanded:

var.expand_dims({“nj”:[1]}, axis=1)

5. Time Handling#

Time values are converted into hours since simulation start.

Example:

hours since YYYY-MM-01 00:00:00

This ensures CF-compliant time encoding.

6. Grid Handling#

Longitude and latitude are extracted from ICON grid coordinates.

clon → xc
clat → yc

Converted from radians to degrees.

7. Domain Truncation#

The dataset is truncated to the last 189976 grid cells. Needs to be adopted for others grids.

out = out.isel(ni=slice(-189976, None))

This matches the CLM target grid.

8. Metadata#

CF-conventions metadata are added to the output dataset.

Example:

Conventions = CF-1.6

Variable attributes include:

_FillValue = 1.e20
missing_value = 1.e20

9. Write Output#

Finally, the processed dataset is saved.

Example output filename: 2018-01.nc

Example Execution#

Run the script with:

python create_clm-forcing_icon.py

Ensure the input_folder variable inside the script points to your ICON output directory.

Example: /p/scratch/…/icon/

Possible Improvements#

Future improvements may include:

  • command line argument support

  • configurable grid truncation

  • support for multiple months in one run