On estimating the effect of clouds on atmospheric absorption based on flux observations above and below cloud level

Recent attempts to estimate the contribution of clouds to absorption of solar radiation by the atmosphere have been based on the difference between measurements of solar flux at the top of-the-atmosphere, or in the atmosphere above cloud level, and measurements of solar flux at the surface, or in the atmosphere below cloud level. One problem with this approach is that fluxes below cloud level depend on the position of the clouds relative to the sun, which is not the case for measurements above cloud level. For example, at any point in the atmosphere below cloud level, the net downward flux varies considerably, depending on whether or not the clouds block the direct rays of the sun. As a result, the net downward flux above cloud level is generally a maximum for clear-sky conditions, but not so for the net downward flux below cloud level. This effect, if not taken into account, would bias the methods used to infer the contribution of clouds to atmospheric absorption, and lead to an overestimate of the contribution.