ON ESTIMATING THE EFFECT OF CLOUDS ON ATMOSPHERIC ABSORPTION BASED ONFLUX OBSERVATIONS ABOVE AND BELOW CLOUD LEVEL

Recent attempts to estimate the contribution of clouds to absorption o f 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 t he surface, or in the atmosphere below cloud level. One problem with t his approach is that fluxes below cloud level depend on the position o f the clouds relative to the sun, which is not the case for measuremen ts above cloud level. For example, at any point in the atmosphere belo w cloud level, the net downward flux varies considerably, depending on whether or not the clouds block the direct rays of the sun. As a resu lt, the net downward flux above cloud level is generally a maximum for clear-sky conditions, but not so for the net downward flux below clou d level. This effect, if not taken into account, would bias the method s used to infer the contribution of clouds to atmospheric absorption, and lead to an overestimate of the contribution.