An ultraviolet (290 to 325 nm) irradiation model for southern Canadian conditions

A numerical model to estimate spectral and broadband ultraviolet irradiance (290 to 325 nm) for Canadian conditions is described and validated with Br ewer spectroradiometer measurements at four stations. The model applies the delta-Eddington algorithm to a 50-level, 100-km, plane-parallel atmosphere with cloud inserted between 2 and 3 km. It requires measured total atmosph eric ozone depth and hourly observations of cloud amount. In the absence of ozone soundings, model ozone profiles are scaled by the ratio of measured (from the Brewer instrument) to model total atmospheric ozone depths. In th e model calculations, SUSIM ATLAS 3 extraterrestrial irradiance measurement s are averaged for each nanometer of wavelength to mimic the triangular fil ter used by the Brewer instrument. Ozone absorption is calculated from the temperature-dependent coefficients of Pauer and Bass(1985), Rayleigh optica l depths after Elterman (1968), and aerosol optical properties from MODTRAN . Surface albedo is a function of snow depth and 0.05 for snow-free ground. Model and measured spectral irradiances for cloudless skies agree well, bu t model values are smaller than measurements for wavelengths below about 30 5 nm because of enhancement of the Brewer signal by stray light. Model valu es of daily cloudless sky irradiance using lidar aerosol optical depth meas urements from York University after the Mt. Pinatubo eruption in 1991 agree well with measurements. Cloud optical depths were calculated iteratively f or overcast conditions. A fixed optical depth of 45 was used to calculate c loudy sky irradiances at the four stations. These agree well with measureme nts. Mean bias error (MBE) is less than 5% of the mean measured daily irrad iance and Foot mean square error (RMSE) less than 25%, decreasing to below 12% for 10-day averages. Agreement between mean daily measured and calculat ed spectral irradiances over a month is also good.