A COMPARISON OF 2 RADIATION SCHEMES FOR CALCULATING ULTRAVIOLET-RADIATION

Increases in ultraviolet radiation (UV) caused by ozone depletion in the stratosphere are expected to have physiological effects on plants and animals. Biologists require that the UV data have high wavelength resolution to assess these effects adequately. Numerical simulations of scattering and absorption in the atmosphere provide a useful way of predicting the UV irradiance reaching the ground and the results from such radiative transfer calculations are presented. Two different methods were used to calculate the diffuse irradiances: the Discrete Ordinate (D-O) method and the Delta-Eddington (D-E) approximation. Surface UV irradiances for a wide variety of atmospheric conditions, obtained with the D-O method were compared with those obtained using the D-E approximation 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 depending heavily on the absorption by ozone of multiply-scattered photons. The prediction of these increases by means of numerical models provides a good test of the accuracy of the radiative transfer approximations used. It is shown that the D-E approximation appears to be deficient for the prediction of surface irradiance at high spectral resolution, under certain circumstances.