Method to determine snow albedo values in the ultraviolet for radiative transfer modeling

For many cases modeled and measured UV global irradiances agree to within /-5% for cloudless conditions, provided that all relevant parameters for de scribing the atmosphere and the surface are well known. However, for condit ions with snow-covered surfaces this agreement is usually not achievable, b ecause on the one hand the regional albedo, which has to be used in a model , is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method i s given to determine the regional albedo values for conditions with snow co ver by use of a parameterization on the basis of snow depth and snow age, r outinely monitored by the weather services. An algorithm is evolved by mult iple linear regression between the snow data and snow-albedo values in the UV, which are determined from a best tit of modeled and measured UV irradia nces for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18-0.5 range. Since the constants of the regr ession depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with diff erent snow conditions improves the accuracy of modeled UV irradiances consi derably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average dev iations between measured and modeled UV global irradiances. (C) 1999 Optica l Society of America.