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Piers M. De F. Forster, Keith P. Shine, and Ann R. Webb

Abstract

High-resolution measurements in the spectral region of 280–400 nm using a double monochromator are compared with detailed radiative transfer calculations at Reading, United Kingdom (52°N, 0°), for clear and totally overcast days, using aerosol and cloud information deduced from empirical methods. For clear skies, instrument and model agree well in the UVA (320–400 nm), but agreement is worse in the UVB (280–320 nm). A number of possible reasons for the discrepancies are explored. Volcanic aerosols in the stratosphere of the model are found to improve agreement between the model and the instrument for high solar zenith angles by increasing the model UVB irradiances by as much as 6%. Convolving the model surface irradiances with the bandpass of the instrument leads to smaller differences between instrument and model at short wavelengths and also reduces the noisiness of the difference. When the model included stratospheric aerosol and the instrument's bandpass function, UVB irradiances within 10% of the measured irradiances could be produced by the model for clear skies. For cloudy conditions, differences between instrument and model are larger, reaching 20%, integrated over the UVB.

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V. Eyring, N. R. P. Harris, M. Rex, T. G. Shepherd, D. W. Fahey, G. T. Amanatidis, J. Austin, M. P. Chipperfield, M. Dameris, P. M. De F. Forster, A. Gettelman, H. F. Graf, T. Nagashima, P. A. Newman, S. Pawson, M. J. Prather, J. A. Pyle, R. J. Salawitch, B. D. Santer, and D. W. Waugh

Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.

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