Implications of the albedo continuum between hydrated intra-cloud aerosol and activated cloud droplets on aerosol radiative forcing in scattered-to-broken sky cover

Major uncertainties arise in climate projections from difficulties in measuring and modeling albedo of the earth and its atmosphere, especially in quantifying the influences of aerosols and clouds. Conventional measurement and modeling approaches to these effects on albedo are based on separately calculating the properties of clear and cloudy air and convolving them to arrive at the combined effect. We consider results from airborne and satellite borne lidar and large-eddy simulations, and find they all indicate that the probability distribution of atmospheric albedo does not neatly separate into clear and cloudy conditions but rather that a broad continuum exists encompassing clear, cloudy, and partly cloudy air. This continuum is caused primarily by fragmented clouds and hydrated intra-cloud aerosol particles. We find that separating the albedo continuum into clear and cloudy conditions (equivalent to separating the direct and indirect aerosol forcings of climate) may significantly bias the calculated magnitude of climate forcing by anthropogenic aerosol as estimated from large-scale models and deduced from satellite radiometry.