Radiative Transfer

Absorption, emission, and scattering of radiation within the atmosphere are critical processes that impact our planet’s climate and allow the remote sensing of key atmospheric properties. Atmospheric and Environmental Research (AER) is internationally recognized as the premier global authority on these processes and their effects.

Radiative Transfer in the Earth’s Energy Budget

A better understanding of Earth’s present and future requires computer codes that accurately simulate the movement of solar and thermal radiation through the atmosphere.

Atmospheric and Environmental Research (AER) employs some of the world’s foremost experts on atmospheric radiation, and the accuracy and efficiency of our radiative transfer codes are relied upon by government programs and scientists worldwide. An example is our RRTMG code, used by many of the world’s most important climate and weather prediction models to account for the impact of atmospheric radiation, including the greenhouse effect, the key driver of global climate change.

State-of-the-science radiation codes

The diversity of geophysical applications that depend on atmospheric radiation creates a need for radiative transfer codes that have an appropriate combination of accuracy, speed, and spectral coverage for each application. AER has developed state-of-the-science codes that cover all spectral regions from the microwave to the ultraviolet to meet these diverse needs:

Line-By-Line Radiative Transfer Model (LBLRTM)

This is the foundational code for all our radiation code development. LBLRTM computes the absorption and emission of radiation by gaseous molecules (e.g., carbon dioxide) in the atmosphere at its most fundamental level, accounting for the effects of many thousands of individual absorption lines. A central focus of AER’s radiation scientists is the validation of LBLRTM with respect to high-quality spectral radiation measurements from satellite-, aircraft-, and ground-based instruments. The results of these studies are the basis for regular quality upgrades to this publically available code.

Optimal Spectral Sampling (OSS)

AER's patented OSS model efficiently and accurately computes the hyperspectral radiances measured by Earth orbiting satellites. These radiance measurements are used to determine the properties of the Earth’s atmosphere, oceans, and surface for environmental and defense applications, weather prediction, and global change studies.

The OSS method is well suited for both remote sensing applications and assimilation of satellite observations in numerical weather prediction models. Both applications require extremely fast and accurate models.

Benefits of the OSS approach in analyzing data sets from high-spectral-resolution environmental sensors include accuracy and speed. The absorption properties of gaseous species are obtained from LBLRTM. Originally formulated for satellite sensor simulation, the OSS approach can be applied to any spectral domain and instrument viewing geometry, and to the general problem of flux or radiance computation in emitting and scattering atmospheres. Because OSS is a monochromatic approach, the gradient of the OSS-based radiative transfer model can be produced at low computational cost and sensor simulations in scattering situations are made possible.

RRTM and RRTMG

Rapid calculations of radiative flux and heating rates are needed by climate and weather prediction models to perform accurate simulations. AER’s RRTM and RRTMG codes provide these calculations with an effective accuracy equivalent to that provided by LBLRTM but with an enormous increase in speed. Among the prestigious modeling centers worldwide that use AER’s rapid radiation codes operationally are the National Center for Atmospheric Research (NCAR), the European Centre for Medium-Range Weather Forecasts (ECMWF), the U.S. National Centers for Environmental Prediction (NCEP), and the China Meteorological Administration (CMA).

For recent developments and more information on our radiation codes and related databases, please visit the AER radiative transfer website.

Radiation research studies

AER scientists are involved in numerous radiation-related research studies and have an extensive record of peer-reviewed publications. Recent research topics include:

Validation and improvement of spectroscopic parameters used by our detailed radiative transfer codes

To ensure that our radiation codes provide accurate calculations for the full range of atmospheric conditions observed on our planet, AER radiation experts are constantly looking for new sources of well-calibrated high-spectral-resolution measurements for validation studies.

Recent comparisons have utilized satellite-based measurements from the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric InfraRed Sounder (AIRS).

AER scientists are also prepared to go to greater lengths to obtain desired measurement data. Recently, an AER scientist co-led two field campaigns that focused on improving our knowledge of specific radiative processes that govern atmospheric dynamics in the middle-to-upper troposphere. The Radiative Heating in Underexplored Bands Campaigns (RHUBC) were held in Barrow, Alaska, in 2007, and at a site at an altitude of 5400 m in the Atacama Desert of Chile in 2009, locations chosen for their low abundances of water vapor. Results from the first campaign have led to key improvements in AER’s radiation codes [Delamere et al., 2010]. Analysis of the data from RHUBC-II is ongoing.

Retrieval of temperature and constituent profiles from remotely sensed spectral measurements

An accurate radiative transfer (forward) model is a crucial component for accurate retrievals of temperature, water vapor and trace gases from remote measurements, such as those from satellites.

AER’s radiative transfer models have provided foundations for the operational forward models used in the analysis of measurements from satellite instruments such as the Tropospheric Emission Spectrometer (TES) on the NASA Aura satellite and the Infrared Atmospheric Sounding Instrument (IASI) on the European MetOp platform. For TES, AER has played a leading role in the interpretation of information in the operational retrievals [Payne et al., 2009] and in the development of new retrievals of trace gases with signals that are at or below the noise level of the instrument, such as ammonia (NH3) and methanol (CH3OH).

The radiation-related research at AER is principally supported by the National Aeronautics and Space Agency (NASA), the Departments of Defense and Energy, the NASA Jet Propulsion Laboratory (JPL), and the Joint Center for Satellite Data Assimilation (JCSDA).

To learn more about AER's Radiative Transfer expertise, please contact us.