Unrealistic desiccation of marine stratocumulus clouds by enhanced solar absorption

THE absorption of solar radiation by clouds affects the distribution of heat that drives atmospheric and ocean circulations. Many investigators have measured cloud solar absorption values exceeding theoretical estimates, although the discrepancies have been. regarded as inconclusive(1). But more recent measurements indicate that clouds absorb up to three times as much solar energy than conventional theory predicts(2,3). Marine stratocumulus clouds are particularly sensitive to solar absorption because marine boundary-layer mixing is typically driven by cloud radiative processes. Here we present model simulations of a stratocumulus-topped marine boundary layer, incorporating different levels of solar absorption. With conventional absorption, the simulations reproduce observed cloud behaviour. But those with artificially enhanced absorption result in an unrealistic daytime depletion of cloud water, because a reduction in cloud radiative cooling results in decreased boundary-layer mixing. Moreover, we show that it is unlikely that liquid water or any plausible dissolved material can absorb the energy required by the recent measurements of solar absorption. Our results therefore indicate that either enhanced solar absorption occurs only in clouds other than marine stratocumulus, or the enhancement of cloud solar absorption indicated hy recent measurements(2,3) is overestimated.