solweig-gpu 1.2.22

GPU-accelerated SOLWEIG model for urban thermal comfort simulation

SOLWEIG-GPU: GPU-Accelerated Thermal Comfort Modeling Framework

SOLWEIG-GPU is a Python package and command-line interface for running the standalone SOLWEIG (Solar and LongWave Environmental Irradiance Geometry) model on CPU or GPU (if available). It enables high-resolution urban microclimate modeling by computing key variables such as Sky View Factor (SVF), Mean Radiant Temperature (Tmrt), and the Universal Thermal Climate Index (UTCI).

Cite this work as Kamath, H. G., Sudharsan, N., Singh, M., Wallenberg, N., Lindberg, F., & Niyogi, D. (2026). SOLWEIG-GPU: GPU-Accelerated Thermal Comfort Modeling Framework for Urban Digital Twins. Journal of Open Source Software, 11(118), 9535. https://doi.org/10.21105/joss.09535

SOLWEIG was originally developed by Dr. Fredrik Lindberg's group. Journal reference: Lindberg, F., Holmer, B. & Thorsson, S. SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and mean radiant temperature in complex urban settings. Int J Biometeorol 52, 697–713 (2008). https://doi.org/10.1007/s00484-008-0162-7

SOLWEIG GPU code is an extension of the original SOLWEIG Python model that is part of the Urban Multi-scale Environmental Predictor (UMEP). GitHub code: https://github.com/UMEP-dev/UMEP
UMEP journal reference: Lindberg, F., Grimmond, C.S.B., Gabey, A., Huang, B., Kent, C.W., Sun, T., Theeuwes, N.E., Järvi, L., Ward, H.C., Capel-Timms, I. and Chang, Y., 2018. Urban Multi-scale Environmental Predictor (UMEP): An integrated tool for city-based climate services. Environmental Modelling & Software, 99, pp.70-87. https://doi.org/10.1016/j.envsoft.2017.09.020

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For detailed documentation, see Solweig-GPU Documentation

Features

• CPU and GPU support (automatically uses GPU if available)
• Divides larger areas into tiles based on the selected tile size
• CPU-based computations of wall height and aspect are parallelized across multiple CPUs
• GPU-based computation of SVF, shortwave/longwave radiation, shadows, Tmrt, and UTCI
• Compatible with meteorological data from UMEP, ERA5, and WRF (wrfout)

Flowchart of the SOLWEIG-GPU modeling framework

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Required Input Data

• Building DSM: Includes both buildings and terrain elevation (e.g., Building_DSM.tif)
• DEM: Digital Elevation Model excluding buildings (e.g., DEM.tif)
• Tree DSM: Vegetation height data only (e.g., Trees.tif)

Currently tested only for hourly data

• Meteorological forcing:
  • Custom .txt file (from UMEP)
  • ERA5 (both instantaneous and accumulated)
  • WRF output NetCDF (wrfout)

ERA5 Variables Required

• 2-meter air temperature
• 2-meter dew point temperature
• Surface pressure
• 10-meter U and V wind components
• Downwelling shortwave radiation (accumulated)
• Downwelling longwave radiation (accumulated)

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Output Details

• Output directory: output_folder/ (under the directory you pass as base_path)
• Structure: One folder per tile (e.g., 0_0/, 1000_0/)
• SVF: Single-band raster
• Other outputs: Multi-band raster (e.g., 24 bands for hourly results)

If you need outputs in a different folder, set base_path to that directory and pass complete paths for the rasters: building_dsm_filename, dem_filename, trees_filename, and landcover_filename (optional).

UTCI for New Delhi, India, generated using SOLWEIG-GPU and visualized with ArcGIS Online.

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Installation

We recommend using conda environment (please see documentation)

conda create -n solweig python=3.10
conda activate solweig
conda install -c conda-forge gdal cudnn pytorch timezonefinder matplotlib sip #cudnn is required only if you are using nvidia GPU
pip install PyQt5
pip install solweig-gpu
#if you have older versions installed
pip install --upgrade solweig-gpu

Testing

Run the test suite with:

pytest -q

With coverage:

pytest --cov=solweig_gpu --cov-report=term-missing

CI runs tests on Linux and macOS across Python 3.10–3.12.

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Sample Data

Please refer to the sample dataset to familiarize yourself with the expected inputs. Sample data can be found at:

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Python Usage

Notes on sample data and forcing options

• The Input_raster folder in the sample contains the raster files required by SOLWEIG-GPU:

  1. Building_DSM.tif
  2. DEM.tif
  3. Trees.tif
  4. Landcover.tif (optional)

• SOLWEIG-GPU can be meteorologically forced in three ways:

  1. Using your own meteorological .txt file
  2. ERA5 reanalysis
  3. Weather Research and Forecasting (WRF) output files. Make sure filenames follow one of:
     • wrfout_d0x_yyyy-mm-dd_hh_mm_ss (preferred; works across operating systems)
     • wrfout_d0x_yyyy-mm-dd_hh:mm:ss
     • wrfout_d0x_yyyy-mm-dd_hh

• The Forcing_data folder in the sample data contains example data for all forcing methods.

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Examples with the provided sample data

Example 1: WRF

from solweig_gpu import thermal_comfort

thermal_comfort(
    base_path='/path/to/input',
    selected_date_str='2020-08-13',
    building_dsm_filename='Building_DSM.tif',
    dem_filename='DEM.tif',
    trees_filename='Trees.tif',
    landcover_filename=None,
    tile_size=1000,
    overlap=100,
    use_own_met=False,
    own_met_file='/path/to/met.txt',  # Placeholder as use_own_met=False
    start_time='2020-08-13 06:00:00',
    end_time='2020-08-14 05:00:00',
    data_source_type='wrfout',
    data_folder='/path/to/era5_or_wrfout',
    save_tmrt=False,  # True if you want to save TMRT, likewise below, default True
    save_svf=False,
    save_kup=False,
    save_kdown=False,
    save_lup=False,
    save_ldown=False,
    save_shadow=False
)

• The model simulation date is 2020-08-13
• The start and end dates provided to the model are 2020-08-13 06:00:00 UTC and 2020-08-14 05:00:00 UTC, respectively. These are the start and end times of wrfout in UTC. In local time, it is 2020-08-13 01:00:00 to 2020-08-13 23:00:00 (Austin, TX). UTC to local time conversion will be done internally.
• The tile_size depends on the RAM of the GPU, but can be set to 1000 in the example.
• overlap is set to 100 pixels, meaning the raster size will be 1100*1100 pixels. The additional 100 pixels are for shadow transfer between the tiles.

Example 2: ERA5

from solweig_gpu import thermal_comfort

thermal_comfort(
    base_path='/path/to/input',
    selected_date_str='2020-08-13',
    building_dsm_filename='Building_DSM.tif',
    dem_filename='DEM.tif',
    trees_filename='Trees.tif',
    landcover_filename = None,
    tile_size =1000,
    overlap = 100,
    use_own_met=False,
    own_met_file='/path/to/met.txt',  #Placeholder as use_own_met=False
    start_time='2020-08-13 06:00:00',
    end_time=  '2020-08-13 23:00:00',
    data_source_type='ERA5',
    data_folder='/path/to/era5_or_wrfout',
    save_tmrt=False, #True if you want to save TMRT, likewise below
    save_svf=False,
    save_kup=False,
    save_kdown=False,
    save_lup=False,
    save_ldown=False,
    save_shadow=False
)

• For the ERA-5, the sample data provided is from 2020-08-13 06:00:00 UTC to 2020-08-13 23:00:00 UTC. So the simulation will run from 2020-08-13 01:00:00 to 2020-08-13 18:00:00 local time (Austin, TX)
• Ony when ERA-5 data is used, the model can set the datetime automatically. For example, if the ERA-5 data are from 2020-08-13 00:00:00 UTC to 2020-08-14 23:00:00 UTC and the selected simulation date is 2020-08-13 along with start time of 2020-08-13 06:00:00 UTC and end time of 2020-08-14 05:00:00 UTC, the model will automatically process the data for the selected datetime provided there are ERA-5 data for those datetimes.

Example 3: Own File

from solweig_gpu import thermal_comfort
thermal_comfort(
    base_path='/path/to/input',
    selected_date_str='2020-08-13',
    building_dsm_filename='Building_DSM.tif',
    dem_filename='DEM.tif',
    trees_filename='Trees.tif',
    landcover_filename = None,
    tile_size =1000,
    overlap = 100,
    use_own_met= True,
    own_met_file='/path/to/met.txt',
    start_time='2020-08-13 06:00:00', # Placeholder
    end_time=  '2020-08-13 23:00:00', # Placeholder
    data_source_type='ERA5', # Placeholder
    data_folder='/path/to/era5_or_wrfout', # Placeholder
    save_tmrt=False, #True if you want to save TMRT, likewise below
    save_svf=False,
    save_kup=False,
    save_kdown=False,
    save_lup=False,
    save_ldown=False,
    save_shadow=False
)

Note for Windows Users

On Windows, Python uses the spawn start method for new processes: each worker re-imports your script. Without guarding the entry point, a top-level call to thermal_comfort() would run again in every child process, causing repeated execution and failures (e.g. BrokenProcessPool). Always call thermal_comfort() inside a main() function and use if __name__ == "__main__": (see example below).

from solweig_gpu import thermal_comfort
import multiprocessing as mp

def main():
    thermal_comfort(
        base_path='/path/to/input',
        selected_date_str="2020-08-13",
        building_dsm_filename="Building_DSM.tif",
        dem_filename="DEM.tif",
        trees_filename="Trees.tif",
        landcover_filename="Landcover.tif",
        tile_size=1000,
        overlap=100,
        use_own_met=False,
        own_met_file='/path/to/met.txt',  # placeholder; ignored when use_own_met=False
        start_time="2020-08-13 06:00:00",
        end_time="2020-08-14 05:00:00",
        data_source_type="era5",
        data_folder='/path/to/era5_or_wrfout',
        save_tmrt=False,
        save_svf=False,
        save_kup=False,
        save_kdown=False,
        save_lup=False,
        save_ldown=False,
        save_shadow=False,
    )

if __name__ == "__main__":
    mp.freeze_support()
    main()

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Command-Line Interface (CLI)

Example using sample ERA5 data on Windows

conda activate solweig
thermal_comfort --base_path '/path/to/input' ^
                --date '2020-08-13' ^
                --building_dsm 'Building_DSM.tif' ^
                --dem 'DEM.tif' ^
                --trees 'Trees.tif' ^
                --tile_size 1000 ^
                --landcover  'Landcover.tif' ^
                --overlap 100 ^
                --use_own_met False ^
                --data_source_type 'ERA5' ^
                --data_folder '/path/to/era5' ^
                --start '2020-08-13 06:00:00' ^
                --end '2020-08-13 23:00:00' ^
                --save_tmrt True ^
                --save_svf False ^
                --save_kup False ^
                --save_kdown False ^
                --save_lup False ^
                --save_ldown False ^
                --save_shadow False

Tip: Use --help to list all CLI options.

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GUI Usage

To launch the GUI:

conda activate solweig
solweig_gpu_gui

GUI Workflow

1. Select the base path containing input datasets.
2. Choose the Building DSM, DEM, Tree DSM, and Land cover (optional) raster files.
3. Set the tile size (e.g., 600 or 1200 pixels).
4. Select a meteorological source (metfile, ERA5, or wrfout):
   • If metfile: Provide a .txt file.
   • If ERA5: Provide a folder with both instantaneous and accumulated files.
   • If wrfout: Provide a folder with WRF output NetCDF files.
5. Set the start and end times in UTC (YYYY-MM-DD HH:MM:SS).
6. Choose which outputs to generate (e.g., Tmrt, UTCI, radiation fluxes).
7. Output will be saved in output_folder/, with subfolders for each tile.

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Contributing

Please refer to the documentation