Evaluation of the ECHAM family radiation codes performance in the representation of the solar signal

摘要

Solar radiation is the main source of energy for the Earth's atmosphere andin many respects defines its composition, photochemistry, temperature profileand dynamics. The magnitude of the solar irradiance variability stronglydepends on the wavelength, making difficult its representation in climatemodels. Due to some deficiencies in the applied radiation codes, severalmodels fail to show a clear response in middle stratospheric heating rates tosolar spectral irradiance variability; therefore, it is important to evaluatemodel performance in this respect before doing multiple runs. In this work weevaluate the performance of three generations of ECHAM (4, 5 and 6) solar radiation schemes by a comparison with the reference high-resolution libRadtran code. We found that all original ECHAM radiation codes miss almost all solar signals in the heating rates inthe mesosphere. In the stratosphere the two-band ECHAM4 code (E4) has analmost negligible radiative response to solar irradiance changes and thesix-band ECHAM5 code (E5c) reproduces only about half of the referencesignal, while representation in the ECHAM6 code (E6) is better – it misses amaximum of about 15% in the upper stratosphere. On the basis of thecomparison results we suggest necessary improvements to the ECHAM familycodes by the inclusion of available parameterizations of the heating rate dueto absorption by oxygen (O₂) and ozone (O₃). Improvement ispresented for E5c and E6, and both codes, with the introducedparameterizations, represent the heating rate response to the spectral solarirradiance variability simulated with libRadtran much better without asubstantial increase in computer time. The suggested parameterizations arerecommended to be applied in the middle-atmosphere version of the ECHAM-5 and6 models for the study of the solar irradiance influence on climate.