MEASUREMENTS OF SPECTRAL SOLAR UV IRRADIANCE IN TROPICAL AUSTRALIA

Measurements of global spectral irradiance in the UV and visible range were carried out during December 1995 and January 1996 in Townsville, Australia (19.33 degrees S, 146.76 degrees E, 30 m above sea level (a sl)) using the mobile spectroradiometer of the Fraunhofer Institute fo r Atmospheric Environmental Research, Germany. These are, to our knowl edge, the first reported spectral UV measurements in the tropics of Au stralia. For cloudless days, the spectral measurements are consistent with results of a radiative transfer model. In the UVA, measurement an d model agree within +/-10% for solar elevations above 10 degrees. In the UVB, the differences are larger (approximate to 15% at 300 nm), wh ich can be explained by uncertainties in the model input parameters of total ozone column and ozone absorption cross section. The variation of the daily erythemal irradiation was found to be +/-24% (+/- 1 sigma ) for the period of the campaign. The variation of the total ozone col umn contributes less than +/-3% to this variability; the main part is introduced by clouds. These attenuate UV irradiation less than total i rradiation (300 nm - 3000 nm); the daily total irradiation averaged ov er the period of the campaign was reduced by a factor of 0.71 owing to cloudiness compared to the clear-sky case while the erythemal irradia tion was only diminished by a factor of 0.78. Using long-term records of total irradiation and ozone column, the parameters influencing UV r adiation on Earth were found to be typical for the period and site of the campaign. The maximum erythemally weighted irradiance measured dur ing the campaign was 429 mW/m(2), and the average daily erythemal irra diation for this period was 6.06 kJ/m(2). These high radiation levels were found to exceed the corresponding values for Garmisch-Partenkirch en (47.5 degrees N, 11.0 degrees E, 730 m asl). Germany, by between 55 and 70%. This pronounced difference in the radiation environment betw een Australia and Germany is explained by the higher solar elevation a nd the lower ozone column in the tropics.