| description 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. | |
