Multiple reflections across a linear discontinuity in surface albedo

Abstract Solar radiation received and absorbed at the surface beneath an overcast sky is calculated as a function of distance d from a linear discontinuity in surface albedo, cloud base altitude h , cloud optical thickness τ, surface albedo α, and precipitable water. Horizontal variation of multiple reflections between surface and cloud is accounted for. Cases of snow or ice surfaces adjacent to open water or snow‐free land surfaces are considered because of large albedo differences. Irradiance on all surfaces and absorptance on the darker surfaces are shown to depend on d, h , τ, α and water vapour beneath the cloud base. Absorption by snow and ice surfaces is less sensitive to changes in d, h and τ because of weak absorptance in the spectral region where multiple reflections are most prevalent; λ<0.7 μm. Ground‐based, radiometrically measured (point) surface albedo depends weakly on d and h but more strongly on τ. Estimates of cloud base albedo from surface measurements may be in error if surface irradiances are measured too close to large albedo discontinuities.