UNCERTAINTIES IN MODELED UV IRRADIANCES DUE TO LIMITED ACCURACY AND AVAILABILITY OF INPUT DATA

Uncertainties in modeled spectral UV irradiances under cloud-free cond itions are analyzed with respect to limited measurement accuracy of ac tual atmospheric input parameters or their nonavailability under the a ssumption that no uncertainty results from the used model or from the spectral values of the extraterrestrial solar irradiance and the gaseo us absorption coefficients. The resulting mean uncertainty of spectral UV irradiance is calculated using a root-mean-square (rms) procedure for various scenarios, defined by differing qualities of the used sets of input values. The results are discussed with respect to the possib ility of reducing the uncertainty in modeled UV irradiances by additio nal measurements of input parameters and, on the other hand, assessing which of such measurements may be redundant since greater measurement expense leads to no significant improvement in accuracy of modeled ir radiances. The uncertainties in modeled UV irradiances are mainly prod uced by the uncertainties of the measured ozone amount, by the aerosol optical depth if it is not directly measured, and by the soot concent ration of the aerosol in the haze layer. Additional uncertainties can arise where snow cover is present. If O-3 and SO2 contents, spectral a erosol optical depth, and aerosol soot concentration near the ground a re measured under actual conditions, the uncertainties in input parame ters result in a mean uncertainty of about 5% for spectral integrals o f UV irradiance. These results cannot be improved significantly, even when measured values of vertical profiles of all atmospheric constitue nts are used. Using only the observed visibility without the measureme nt of aerosol optical properties, the mean uncertainty for modeled UV integrals is about 10-15%.