Comparison of radiation schemes for calculating UV radiation

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Abstract
Increases in ultraviolet radiation (UV) caused by ozone depletion in the stratosphere are expected to have physiological effects on plants and animals. Biologists require high wavelength resolution UV data to adequately assess these effects. Numerical simulations of scattering and absorption in the atmosphere provide a useful way of predicting the global UV irradiance reaching the ground. Results from radiative transfer calculations are presented here. Two different methods are used to calculate diffuse irradiances, either the Discrete-Ordinate (D-O) method or the Delta-Eddington (D-E) approximation. Surface UV irradiances obtained with the D-O scheme are compared to those using the D-E approximation for a wide variety of atmospheric conditions, to assess how changes in tropospheric aerosol and cloud affect the accuracy of the D-E approximation. Stratospheric aerosol has been shown to be capable of increasing the UV irradiances at the ground. The magnitude of these increases is shown to depend strongly on the absorption of multiple scattered photons by ozone. The prediction of this increase by numerical models provides a good test of the accuracy of any radiative transfer approximations used. It is shown that the Delta-Eddington approximation appears to be deficient for high wavelength resolution surface irradiance prediction, in certain circumstances.

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