Intercomparing shortwave radiation codes for climate studies

Abstract

As a second step of the international program of Intercomparison of Radiation Codes Used inClimate Models (ICRCCM), an intercomparison of shortwave radiation models was initiated. Among the 26 codes that participated in the comparison were very detailed (line-by-line), narrow-band (high-spectral resolution), as well as highly parameterized (low-spectral resolution) models. A considerable spread was detected in the response of these models to a set of well-defined atmospheric profiles. Substantial discrepancies exist among models even for the simplest case of pure water vapor absorption with standard deviation ranging from 1% to 3% for the downward fluxes at the surface and from 6% to 11% for the total atmospheric absorption. The divergences in downward surface flux increase to nearly 4% when all absorbers and the molecular scattering are considered. In cloudy conditions the divergences range from 4% to 10%, depending on the cloud optical thickness. Another major uncertainty that has been identified is the spectral averaging of the scattering properties which can result in very significant errors for low spectral resolution codes. Since these errors appear to be systematic, they may induce unrealistic feedback mechanisms in numerical climate models. The amplitude of the differences between models is in many cases larger than the accuracy required for the achievements of several objectives of the World Climate Research Program. While reference solutions for the absorption and scattering in atmospheres can be obtained based on the state-of-the-art spectroscopic knowledge and rigorous computational techniques, the absolute tests of the validity of the radiation algorithms would be comprehensive field experiments in which the radiative and all relevant atmospheric parameters are measured to a high degree of accuracy. INTRODUCTIONRadiation is a primary process governing the general circulation of the atmosphere and is the basic means through which significant changes in the climate may occur. As such, its treatment in climate models requires careful attention. Instead, the idea that radiation is a science with a very well-established physical basis is widespread in the climate community. As a consequence, relatively little attention has been paid until recently to a careful calibration of the various hypotheses, simplifications, or parameterizations on which the radiation codes in climate models are based. It is widely Table 1) were selected and distributed among the community. A majority of the cases are for clear-sky conditions (i.e., no clouds). The test cases considered included clear-sky and overcast atmospheres. It was decided at the outset that intercomparisons between the various models for clear skies had to be analyzed and placed 8955

A new multiple‐band solar radiative parameterization for general circulation models

“…The surface and TOA fluxes are likewise improved from the >10% errors that were seen to occur in RF92. Results are presented for three cloud locations: 180-900, 300-900, and 500-900 mbar using the CS drop model from the ICRCCM specifications [Fouquart et al, 1991] …”

Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment

“…Fouquart et al (1991) examined 26 radiation schemes ranging from high to low spectral resolution and found substantial discrepancies in computed fluxes for even the simplest prescription of only pure water vapor absorption. When including highly scattering aerosols and a fixed surface albedo, the relative standard deviation for the eleven models considered ranged from 23 to 114 % as the solar zenith angle (SZA) decreased from 75 • to 30 • (Fouquart et al, 1991;Boucher et al, 1998).…”

“…Fouquart et al (1991) examined 26 radiation schemes ranging from high to low spectral resolution and found substantial discrepancies in computed fluxes for even the simplest prescription of only pure water vapor absorption. When including highly scattering aerosols and a fixed surface albedo, the relative standard deviation for the eleven models considered ranged from 23 to 114 % as the solar zenith angle (SZA) decreased from 75 • to 30 • (Fouquart et al, 1991;Boucher et al, 1998). Boucher et al (1998) found that the relatively high (8 %) standard deviation in zenith angle-average broadband forcing due to prescribed non-absorbing sulfate aerosols was due to differences in the treatment of Mie scattering, multiple scattering, phase functions, and spectral and angular model resolution.…”

Ten year radiation budget of the Earth: 1984–93

“…These models have been derived from a detailed radiative-convective model (Vardavas and Carver, 1984) and have been successfully tested (see Vardavas and Koutoulaki, 1995;Hatzianastassiou et al, 1999) according to the Intercomparison of Radiation Codes in Climate Models (ICRCCM) program (Fouquart et al, 1991). The same models have already been used to predict the long-term mean monthly 10°zonal radiation budgets for both SW and LW radiation (Vardavas and Koutoulaki, 1995;Vardavas, 1999a,b, 2001a,b;Hatzianastassiou et al, 1999Hatzianastassiou et al, , 2001, and also to derive the downwelling LW flux at the surface on a 2.5°resolution for the entire globe (Pavlakis et al, 2004).…”