Influence of inhomogeneous surface albedo on UV irradiance: Effect of a stratus cloud

The influence of inhomogeneous surface albedo on UV irradiance is investiga ted assuming the sky to be overcast by a horizontally homogeneous stratus c loud. From calculations with a three-dimensional (3-D) radiative transfer m odel the snow-induced relative UV irradiance enhancement as well as the 1-D albedo is derived for two different snow bidirectional reflectance distrib ution functions (Lambertian and anisotropic) and snow coverages varying bet ween 0 and 100%. The 1-D albedo is a quantity defined to consider effects o f inhomogeneous surface albedo even in 1-D radiative transfer codes. It tur ned out that anisotropy of snow reflectance is of minor importance for quan tification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud-free atmosphere. Furthermore, th e comparison of the new results with corresponding data obtained for cloudl ess sky indicates that a stratus cloud, on the one hand, enhances the surfa ce albedo effect by about a factor 2-3, depending on wavelength. On the oth er hand, the area significantly influencing UV irradiance via its surface a lbedo is distinctly smaller. Looking, e.g., at wavelength 330 nm and clear sky, the maximum albedo effect of the surface surrounding a quadratic area of 6400 km(2) on UV irradiance at area center is 3%. In the case of a strat us cloud the same effect is found for an area of only 1500 km(2).