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

The influence of inhomogeneous surface albedo on UV irradiance is investigated assuming the sky to be overcast by a horizontally homogeneous stratus cloud. From calculations with a three-dimensional (3-D) radiative transfer model the snow-induced relative UV irradiance enhancement as well as the 1-D albedo is derived for two different snow bidirectional reflectance distribution functions (Lambertian and anisotropic) and snow coverages varying between 0 and 100%. The 1-D albedo is a quantity defined to consider effects of inhomogeneous surface albedo even in 1-D radiative transfer codes. It turned out that anisotropy of snow reflectance is of minor importance for quantification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud-free atmosphere. Furthermore, the comparison of the new results with corresponding data obtained for cloudless sky indicates that a stratus cloud, on the one hand, enhances the surface albedo effect by about a factor 2-3, depending on wavelength. On the other hand, the area significantly influencing UV irradiance via its surface albedo 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 stratus cloud the same effect is found for an area of only 1500 km(2).