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.

Some of the primary parameters that influence this model's calculations are:

Cloud Properties: These include cloud phase (liquid, ice, or mixed), cloud particle size distributions, cloud water path, and cloud optical depth. The model treats clouds as non-uniform layers within the atmospheric column, which can have a profound impact on the absorption, reflection, and transmission of radiation.

Aerosol Properties: Key properties include aerosol type, concentration, size distribution, and the refractive index. Aerosols can both scatter and absorb radiation, depending on their physical and chemical properties.

Atmospheric Layers: The distribution of clouds and aerosols in different layers of the atmosphere also influences radiative transfer. The model treats each layer as a plane-parallel, homogeneous layer.

The cloud/aerosol model performs complex computations to predict the impact of clouds and aerosols on radiation passing through the atmosphere. This includes both shortwave (solar) and longwave (terrestrial) radiation. The model also considers multiple scattering events and the impact of cloud overlap.

While this model increases the complexity and computational demand of the CRTM, it significantly improves the accuracy of radiative transfer calculations under non-clear-sky conditions. This is essential for accurate satellite data assimilation, numerical weather prediction, and climate modeling.

Surface Model:

The surface model is a component of the CRTM that simulates the radiative properties and behaviors of the Earth's various surfaces. The model accounts for differences in surface types (such as oceans, land, ice, and snow) and their distinct reflection, absorption, and emission characteristics.

The calculations of the surface model hinge on several primary parameters:

Surface Type: Different surfaces exhibit unique radiative properties. For instance, water bodies, such as oceans, have high thermal emissivity and reflectivity properties. On the other hand, land surfaces, such as forests and deserts, have different albedo and emissivity values based on their composition and structure.

Surface Temperature: This is a significant variable that influences the emission of longwave (thermal) radiation from the surface.

Surface Roughness: The roughness of a surface affects its reflectivity and scattering of incoming radiation.

Snow and Ice Coverage: Snow and ice have high albedo values, meaning they reflect a significant portion of incoming solar radiation. They also affect thermal emission from the surface.

The surface model calculates both the upward emission of longwave radiation from the Earth's surface and the reflection of downwelling shortwave (solar) and longwave radiation. It takes into account both direct and diffuse radiation and also considers the zenith and azimuth angles of the incident radiation.

The output from the surface model feeds into the overall radiative transfer calculations, providing the lower boundary condition. It allows the CRTM to simulate satellite-observed radiances under a wide range of environmental conditions. It is worth noting that, like the other CRTM components, the surface model is designed to balance between computational efficiency and accuracy.

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.

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.

External links

JCSDA Projects - Community Radiative Transfer Model
MICROS, Monitoring of IR Clear-sky Radiances over Oceans for SST