On the climate forcing consequences of the albedo continuum between cloudy and clear air

Abstract

It has been long understood that the partly cloudy atmosphere manifests a continuum of states between the end members ‘clear’ and ‘cloud.’ Nevertheless, many research methods are premised on a dichotomy of states—for example, those that use ‘cloud cover’ or ‘cloud-clearing.’ Here we consider the consequences of this practice for studies of aerosolclimate effects. Aerosols affect the Earth’s energy budget primarily by affecting albedo; therefore, we explore the nature of albedo variability in the partly cloudy marine boundary layer on scales down to a few tens of metres. We employ two diagnostic tools: a cloud resolving model and an albedo proxy derived from high altitude lidars. We show that a continuum of albedo values results from indeterminate and variable combinations of hydrated aerosol and wispy (including subvisible) clouds. Two consequences arise. First, cloud-clearing schemes employed by different observational methods are mutually inconsistent and are sensitive to concentrations of unactivated aerosol particles. Second, aerosol radiative forcing (the sensitivity of overall albedo to changes in aerosol concentration) is inaccurately calculated as the average of clear and overcast conditions. Together, these results imply that dividing the aerosol forcing problem into ‘direct’ and ‘indirect’ components may lead to substantial errors.