Summary of the paper
Introduction
Radiative forcing is "a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism.’’ [1] As the concentration of greenhouse gases increases in the atmosphere, the radiative forcing of those gases becomes increasingly more important. Due to the increase in those gases since the 1950s, there is a net increase of radiative forcing of about +2.4 W/m^2 with an uncertainty of around 10%. The positive sign represents an overall absorption of radiation by the atmosphere.
Basic Understanding of Radiation Models (mainly RRTMG)
Line-by-Line Radiative Transfer Model (LBLRTM)
This radiative model is very accurate and efficient. The model stays relevant by being constantly updated and validated to measured real-world data. LBLRTM considers the influence of important atmospheric gases that absorb radiation from the HITRAN 2004 data to ensure accurate predictions. There is also a continuum model that accurately represents water vapor's broadening effects. The algorithm behind LBLRTM is very accurate, having a 0.5% error margin, and coupled with another independent program RADSUM, the integrated fluxes and heating rates are evaluated. It helps understand how much heat is entering and leaving each layer of the atmosphere. LBLRTM/RADSUM uses three angles and six streams to integrate flux, that is, radiation coming from three different directions and divided into six bands of radiation. [2]
CHARTS (code for high-resolution accelerated radiative transfer with scattering)
CHARTS "is a monochromatic plane-parallel radiative transfer model for the line-by-line calculation of radiances and fluxes at a single level for thermal and solar regimes in general scattering atmospheres" [3]. In summary, CHARTS is a sophisticated model used to study how radiation and energy move through the atmosphere, especially in cases where scattering plays a significant role. It's applied to various scenarios, from analyzing solar radiation to understanding heat transfer, and its results are checked and compared to high-resolution spectral radiance measurements.
Rapid Radiative Transfer Model (RRTM & RRTMG)
The goal of the RRTM model is to get results that are close to those obtained by line-by-line models for longwave and shortwave radiation. A few details regarding the RRTM model:
An extensive list (water vapor, carbon dioxide, ozone, methane, nitrous oxide, oxygen, nitrogen and the halocarbons in the longwave and water vapor, carbon dioxide, ozone, methane and oxygen) of molecular absorbers are included in the RRTM scheme and their absorption coefficients are from the LBLRTM.
The water vapor continuum is based on CKD_v2.4 in the versions of RRTM applied to this work, and molecular line parameters are based on HITRAN 2000 for water vapor and HITRAN 1996 for all other molecules.
Extinction from aerosols, clouds and Rayleigh scattering are also included, and the discrete ordinates algorithm DISORT [5] is used for multiple scattering calculations.
Cloud liquid and ice parameterizations are available that allow the specification of cloud fraction and cloud physical or optical properties. However, the shortwave is limited to fully clear or overcast calculations.
RRTM_LW utilizes four angles (eight streams) and RRTM_SW/DISORT uses eight angles (16 streams) for flux integration.[4]
The new proposed scheme is RRTMG, basically, the RRTM model that can be used with GCMs (general circulation models). It is more efficient than the RRTM model with a minimum loss of accuracy. Below is an outline of the major changes:
RRTMG_LW
Accuracy is improved in some spectral bands in the LW code by replacing the standard diffusivity angle (secant 1.66) used for angular integration with a diffusivity angle that varies with total column water vapor. [6]
The total number of g-points used has been reduced from 256 to 140. Fluxes are accurate to within 0.5 W m-2 and cooling rate within 0.1 K day-1 relative to the standard RRTM_LW, which is itself accurate to within 1 W m-2 of the data-validated line-by-line radiative transfer model, LBLRTM. [7]
RRTMG_SW
The version of RRTMG_SW provided here utilizes a reduced complement of 112 g-points, which is half of the 224 g-points used in the standard RRTM_SW, and a two-stream method for radiative transfer. Additional minor changes have been made to enhance computational performance.
Total fluxes are accurate to within 1-2 W m-2 relative to the standard RRTM_SW (using DISORT) in clear sky and in the presence of aerosols and within 6 W m-2 in overcast sky. RRTM_SW with DISORT is itself accurate to within 2 W m-2 of the data-validated multiple scattering model, CHARTS.
This model can also utilize McICA, the Monte-Carlo Independent Column Approximation, to represent sub-grid scale cloud variability such as cloud fraction and cloud overlap. [8]
References
Solomon, S., et al. (2007), Technical summary, in Climate Change 2007: The Physical Science Basis—Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon et al., pp. 19 – 91, Cambridge Univ. Press, Cambridge, U. K.
Turner, D. D., et al. (2004), The QME AERI LBLRTM: A closure experiment for downwelling high spectral resolution infrared radiance, J. Atmos. Sci., 61, 2657 – 2675, doi:10.1175/JAS3300.1.
Moncet, J.-L., and S. A. Clough (1997), Accelerated monochromatic radiative transfer for scattering atmospheres: Application of a new model to spectral radiance observations, J. Geophys. Res., 102, 21,853 – 21,866, doi:10.1029/97JD01551.
Iacono, M. J., E. J. Mlawer, S. A. Clough, and J.-J. Morcrette (2000), Impact of an improved longwave radiation model, RRTM, on the energy budget and thermodynamic properties of the NCAR community climate model, CCM3, J. Geophys. Res., 105, 14,873 – 14,890, doi:10.1029/ 2000JD900091
Stamnes, K., S. C. Tsay, W. Wiscombe, and K. Jayaweera (1988), A numerically stable algorithm for discrete-ordinate-method radiative transfer in scattering and emitting layered media, Appl. Opt., 27, 2502 – 2509.
Peter Norris et al., Transition to the RRTMG Shortwave Radiation Code in GEOS Models, National Aeronautics and Space Administration, June 2020.
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