An Algorithm for Inferring Surface UV Irradiance Including Cloud Effects

Abstract

Recent extratropical ozone depletion and the concomitant increase in surface ultraviolet (UV) radiation may be expected to adversely influence the biosphere. Since few long-term, high quality datasets of surface UV are available for assessing these effects, there is a need to develop techniques for estimating past levels of biologically harmful UV at a particular location and thus derive long-term trends. This paper presents a semiempirical algorithm, making use of readily available meteorological variables and total column ozone, for inferring historical UV levels at a particular location, including cloud cover effects. Where input data are available for a network of locations, the technique can be used to generate geographical distributions of surface UV. Measurements made at Lauder (45.04°S, 169.68°E), from November 1993 to October 1994, were used to establish the relationship between cloud-induced reductions of erythemal UV and broadband irradiance, as a function of solar zenith angle?termed cloud cover modifier functions. To demonstrate the performance of the algorithm, these functions were used to derive 10-min surface erythemal UV irradiances for each day in November 1994. The algorithm makes use of measured broadband irradiances and clear-sky erythemal and broad-band irradiances calculated using the following: statistical derivation from measured data and output from a surface spectral irradiance model. Error analysis of the November 1994 surface erythemal UV irradiances indicates an accuracy of 0.12 ± 0.69 µW cm?2. The percentage error increases with solar zenith angle, with an accuracy of 2 ± 10% for solar zenith angles less than 55°. Although the accuracy deteriorates for larger solar zenith angles, these errors contribute little to the error in calculated erythemal daily doses except in winter where the daily doses are small.