Intercomparison Of Radiation Codes Research Articles

Spectral model for clear sky atmospheric longwave radiation

An efficient spectrally resolved radiative model is used to calculate surface downwelling longwave (DLW) radiation (0 ∼ 2500 cm−1) under clear sky (cloud free) conditions at the ground level. The wavenumber spectral resolution of the model is 0.01 cm−1 and the atmosphere is represented by 18 non-uniform plane-parallel layers with pressure in each layer determined on a pressure-based coordinate system. The model utilizes the most up-to-date (2016) HITRAN molecular spectral data for 7 atmospheric gases: H2O, CO2, O3, CH4, N2O, O2 and N2. The MT_CKD model is used to calculate water vapor and CO2 continuum absorption coefficients. Longwave absorption and scattering coefficients for aerosols are modeled using Mie theory. For the non-scattering atmosphere (aerosol free), the surface DLW agrees within 2.91% with mean values from the InterComparison of Radiation Codes in Climate Models (ICRCCM) program, with spectral deviations below 0.035 W cm m−2. For a scattering atmosphere with typical aerosol loading, the DLW calculated by the proposed model agrees within 3.08% relative error when compared to measured values at 7 climatologically diverse SURFRAD stations. This relative error is smaller than a calibrated parametric model regressed from data for those same 7 stations, and within the uncertainty (+/− 5 W m−2) of pyrgeometers commonly used for meteorological and climatological applications. The DLW increases by 1.86 ∼ 6.57 W m−2 when compared with aerosol-free conditions, and this increment decreases with increased water vapor content due to overlap with water vapor bands. As expected, the water vapor content at the layers closest to the surface contributes the most to the surface DLW, especially in the spectral region 0 ∼ 700 cm−1. Additional water vapor content (mostly from the lowest 1 km of the atmosphere) contributes to the spectral range of 400 ∼ 650 cm−1. Low altitude aerosols ( ∼ 3.46 km or less) contribute to the surface value of DLW mostly in the spectral range 750 ∼ 1400 cm−1.

A k‐distribution technique for radiative transfer simulation in inhomogeneous atmosphere: 2. FKDM, fast k‐distribution model for the shortwave

A new technique for developing k‐distributions applied to longwave radiation parameterization has been presented in a preceding paper. Now we discuss an extension of this technique to the shortwave spectral range. A fast k‐distribution model (FKDM) for gaseous absorption calculations suitable for use in weather and climate prediction is described. FKDM has been created using 15 k‐distribution terms only, less than in other comparable codes. The molecular species represented in the model are H2O, CO2, O3, and O2. In k‐distribution terms, characterized by strong absorption, representative absorption cross section is treated as a function of absorber amount along the direct solar radiation path, thus allowing improved fitting of solar fluxes and heating rates in upper troposphere and stratosphere. This technique has been applied to derive effective single‐scattering properties of clouds in each term for a more accurate treatment of cloud optical properties by taking into account correlation between water vapor and liquid water or ice absorption. It is shown that disregarding the above correlation in radiation models can essentially distort simulated fluxes and heating rates. FKDM has been developed and validated using a fast line‐by‐line model (FLBLM). Both FKDM and FLBLM used a Monte‐Carlo code. Validations have covered the tropical, midlatitude summer, midlatitude winter, subarctic summer, subarctic winter, and U.S. standard atmospheres, four atmospheres from the Spectral Radiance Experiment campaign, and a case of an observed tropical atmosphere. It is found that the FKDM heating rate accuracy for clear‐sky conditions is as follows: ∼0.1 and ∼0.2 K d−1 in the troposphere for standard and real atmospheres, respectively, and ∼0.5 K d−1 in all the cases at altitudes below 70 km. Downward flux errors are below 1%, upward flux errors are below 2% (usually ∼1.5 W m−2), and total atmospheric absorption errors are below 3% (usually 1.5–3 W m−2) in every case. The Intercomparison of Radiation Codes in Climate Models (ICRCCM) cloud models have also been used for the validations. It has been demonstrated that the usage of the technique to derive effective cloud optical properties halves maximal errors in calculated radiation fluxes absorbed by cloud.

A Fast, Accurate Method of Computing Near-Surface Longwave Fluxes and Cooling Rates in the Atmosphere

As radiation plays a key role in the determination of the near-surface thermal environment, great accuracy is required in the computation of radiative fluxes, especially because a small error in the fluxes can lead to large errors in estimated cooling rates. A new code that employs a novel numerical scheme for making precise estimates of longwave fluxes and cooling rates near the surface of the earth, for arbitrary surface emissivities, is presented here. The code is a development of the infrared band model of Chou, Ridgway, and Yan. Unacceptable oscillations found near the surface in the cooling rates provided by this code have been overcome through the new numerical scheme. The new code gives results in excellent agreement with available results from Intercomparison of Radiation Codes used in Climate Models (ICRCCM) test cases and with the line-by-line calculations of Clough, Iacono, and Moncet. The code has no restriction on the number of grid points, yields fluxes accurate to a prescribed tolerance, and permits a discontinuity in temperature at the surface (although this is not used in the results presented). The computing times are comparable (for given accuracy) to those demanded by current codes in use elsewhere. It is found that, as surface emissivity $\varepsilon_ {g}$ departs from unity, the cooling rate rises dramatically near the surface, reaching values as high as nearly 40 K $day^{-1}$ at $\varepsilon_ {g}$ = 0.8 in the midlatitude summer atmosphere, and the effect of the surface is noticeable at heights of up to about 1 km. An analysis of spectral distribution shows that, when the surface is not radiatively black, the major contributions to near-surface cooling rates (due to water vapor) come from the two wavenumber bands, 340–540 $cm^{-1}$ and 1215–1380 $cm^{-1}$ (located on either side of the atmospheric window), in which both absorption and radiative flux are significantly high.

The Spectral Radiance Experiment (SPECTRE): Project Description and Sample Results

Abstract The fundamental climatic role of radiative processes has spurred the development of increasingly sophisticated models of radiative transfer in the earth-atmosphere system. Since the basic physics of radiative transfer is rather well known, this was thought to be an exercise in refinement. Therefore, it came as a great surprise when large differences (30–70 W m−2) were found among longwave infrared fluxes predicted by over 30 radiation models for identical atmospheres during the intercomparison of radiation codes used in climate models (ICRCCM) exercise in the mid-1980s. No amount of further calculation could explain these and other intermodel differences; thus, it became clear that what was needed was a set of accurate atmospheric spectral radiation data measured simultaneously with the important radiative properties of the atmosphere like temperature and humidity. To obtain this dataset, the ICRCCM participants charged the authors to develop an experimental field program. So, the authors develop...

Preface [to special section on The Intercomparison of Radiation Codes in Climate Models

Journal of Geophysical Research: AtmospheresVolume 96, Issue D5 p. 8921-8923 Free Access Preface [to special section on The Intercomparison of Radiation Codes in Climate Models] V. Ramaswamy, V. RamaswamySearch for more papers by this authorM. D. Schwarzkopf, M. D. SchwarzkopfSearch for more papers by this authorShaw C. Liu, Shaw C. LiuSearch for more papers by this author V. Ramaswamy, V. RamaswamySearch for more papers by this authorM. D. Schwarzkopf, M. D. SchwarzkopfSearch for more papers by this authorShaw C. Liu, Shaw C. LiuSearch for more papers by this author First published: 20 May 1991 https://doi.org/10.1029/91JD00875AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume96, IssueD520 May 1991Pages 8921-8923 This article also appears in:The Intercomparison of Radiation Codes in Climate Models RelatedInformation

Calculation of longwave radiation fluxes in atmospheres

A technique for the computation of longwave radiative quantities using the line‐by‐line approach has been developed in the Soviet Union. The method has been applied to obtain fluxes and cooling rates for standard atmospheric profiles used in the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM) sponsored by the World Meteorological Organization. The sensitivity of the result to changes in the vertical quadrature scheme, the angular integration, and the spectral line shape is evaluated. Fluxes and cooling rates in the troposphere are in general agreement with those obtained with different line‐by‐line models. Results from parameterized models, including a wideband statistical model and one employing the integral transmission function, have been compared to the line‐by‐line results. Flux errors in the simplified schemes are of the order of 10 W/m2. The sensitivity of these models to changes in atmospheric profiles, or to an increase in CO2 amount, is similar to that of the line‐by‐line calculations.

Intercomparison of Radiation Codes in Climate Models (ICRCCM): Longwave Clear-Sky Results—A Workshop Summary

An international program of intercomparison of radiation models has been initiated because of the central role of radiative processes in many proposed climate change mechanisms. Models ranging from the most detailed (line-by-line) to the most-highly parameterized have been compared with each other and with selected aircraft observations. Although line-by-line-model fluxes tend to agree with each other to within one percent (if the water-vapor–continuum absorption is ignored), the less-detailed models show a spread of 10–20 percent. The spread is even larger (30–40 percent) for the sensitivities of the models to changes in important radiation variables, such as carbon dioxide amounts and water-vapor amounts. These spreads are disturbingly large. Lacking highly accurate flux observations from within the atmosphere, it has been customary to regard line-by-line–model results as “the truth.” However, uncertainties in the physics of line wings and in the proper treatment of the water-vapor continuum make it impossible for the line-by-line models to provide an absolute reference for evaluating less-detailed models. Therefore, a dedicated surface-based field measurement program is recommended in order to properly evaluate model performance; the goal would be to use sophisticated spectrometers to measure accurately spectral radiances rather than integrated fluxes.