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Community Radiative Transfer Model

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Community Radiative Transfer Model (CRTM)

The Community Radiative Transfer Model (CRTM) is a robust, efficient, and comprehensive tool designed for simulating the radiances observed by satellite-based remote sensing systems. It under development by the Joint Center for Satellite Data Assimilation (JCSDA) and is widely used for satellite data assimilation, numerical weather prediction, and climate modeling among other applications.

Overview

CRTM aims to bridge the gap between satellite observations and atmospheric state variables by providing a means to compute the transition of the radiance signal through Earth's atmosphere. It uses spectral bands to emulate the responses of specific satellite sensors to changes in atmospheric, surface, and subsurface conditions. The model has the ability to handle a variety of environmental conditions and is adaptable for a range of different satellite systems, allowing for broad utility in the fields of meteorology and climate science.

Architecture

The architecture of the CRTM model comprises of the clear-sky model, cloud/aerosol model, and surface model.

  • Clear-Sky Model: The clear-sky model in CRTM is a critical component that simulates the transmission of radiant energy through an atmosphere without the presence of clouds. This model relies on optical depth predictors, derived from atmospheric profiles of temperature, moisture, and trace gases, to execute its radiative transfer calculations.

To achieve its computations, the clear-sky model employs a number of specific parameters such as:

    • Temperature and Pressure: These parameters play a crucial role in determining the absorption and scattering of radiation in the atmosphere.
    • Atmospheric Composition: This includes the concentrations of various trace gases in the atmosphere, including carbon dioxide, ozone, methane, and others. Each of these trace gases has specific absorption characteristics that impact the transmission of radiation.
    • Aerosol Concentration: While the clear-sky model primarily concerns atmospheres without clouds, it still considers the role of aerosols in scattering and absorbing radiation.
    • Solar Zenith Angle: The angle of the sun above the horizon can significantly affect the pathlength of solar radiation through the atmosphere, thus influencing the total radiation absorbed and scattered.

The clear-sky model reconstructs the atmospheric transmittance using a regression against previously computed line-by-line calculations (using LBLRTM) for each layer of the atmosphere, integrating these to provide a comprehensive simulation of radiant transmission from the surface to the top of the atmosphere. The primary output of the clear-sky model is the top-of-atmosphere radiance for a given atmospheric and surface state. It's important to note that the clear-sky model is designed to be a high-speed model, aiming to provide a balance between computational efficiency and simulation accuracy.


  • Cloud/Aerosol Model: The cloud/aerosol model simulates the impact of clouds and aerosols on radiative transfer. Clouds and aerosols are treated as non-uniformly distributed layers within the atmospheric column.
  • Surface Model: The surface model predicts the emission and reflection of radiance from Earth's surface, including both land and sea, and also ice or snow-covered surfaces.

Applications

The CRTM finds its main application in satellite data assimilation. It plays a pivotal role in assimilating satellite radiance data into Numerical Weather Prediction (NWP) models by providing 'observation operators'. These operators map the atmospheric state variables in the NWP models to top-of-atmosphere radiance. The model is also used for validating satellite sensor data, generating synthetic satellite imagery, and climate modeling.

Licensing

The CRTM is public domain software. This means it is free and open for the public to use, modify, and distribute. The model is openly available and can be accessed, implemented, and modified without any restrictions. However, it is encouraged to acknowledge and reference the developers and contributors of the CRTM in any derived works or publications. The commitment to maintaining CRTM as public domain software ensures its continued broad usage and contribution to the advancement of meteorological and climate science.

Future Developments

CRTM is under constant development and updates by the JCSDA and the broader scientific community. Future updates are aimed at improving the accuracy and efficiency of radiative transfer calculations, including enhanced cloud and aerosol modeling, and the integration of more advanced surface models.

See also

References

Chen Y, F. Weng, Y. Han, and Q. Liu, 2008: Validation of the community radiative transfer model (CRTM) by using CloudSat Data. J. Geophys.Res., 113(D8), 2156–2202.

Ding, Shouguo, Ping Yang, Fuzhong Weng, Quanhua Liu, Yong Han, Paul Van Delst, Jun Li, and Bryan Baum, 2011: Validation of the community radiative transfer model. Journal of Quantitative Spectroscopy and Radiative Transfer 112 (6): 1050–1064.

Wei, S. W., Lu, C. H., Johnson, B. T., Dang, C., Stegmann, P., Grogan, D., ... & Hu, M. (2022). The influence of aerosols on satellite infrared radiance simulations and Jacobians: Numerical experiments of CRTM and GSI. Remote Sensing, 14(3), 683.

Johnson, B.T., Dang, C., Stegmann, P., Liu, Q., Moradi, I. and Auligne, T., 2023. The Community Radiative Transfer Model (CRTM): Community-Focused Collaborative Model Development Accelerating Research to Operations. Bulletin of the American Meteorological Society.

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