UV-B cloud optical properties for Canada

摘要

Cloud optical properties play a highly significant role in the amount of UV-B irradiance reaching the ground. Broadband values of UV-B cloud optical properties are calculated for nine Canadian stations from 26 years of data. Cloud single scattering albedo ω_c and asymmetry factor g_c are computed from Mie theory for two values of equivalent droplet radius; 7 μm for arctic stations and 10 μm for midlatitude and subarctic stations. Overcast cloud optical depths τ_c are estimated iteratively for a model cloud layer located between 2 and 3 km above the surface from hourly integrated spectral Brewer spectrophotometer measurements for snow-free cases using either the discrete ordinate radiative transfer (DISORT) or the delta-Eddington algorithms. Median τ_c values calculated by both algorithms compare to within 3%. Median values are smaller for arctic stations (9-18) and between 26 and 38 for the rest. Both mean and median values are negatively correlated with latitude. Aerosol effect on τ_c varies between 9 and 18% on average.