Wednesday, 5 June 2002
Evaluating the semi-direct aerosol effect: Thinning of stratocumulus due to absorption of solar radiation by aerosols
The term 'semi-direct effect' was coined by Hansen et al (JGR,1997) to describe the impact of absorbing aerosols on cloud cover. Absorption of solar radiation by aerosols may reduce cloud cover and Liquid Water Path (LWP), via changes in relative humidity and atmospheric stability. This is distinct from both the direct effect of aerosol on the radiation budget and the indirect effects of aerosol, which impact on the cloud properties via changes in effective radius. Hansen et al (JGR,1997) showed that the semi-direct effect has a significant impact on the climate sensitivity for absorbing aerosols due to reductions in cloud amount. However, they use a relatively crude general circulation model and it is important to assess whether this mechanism operates in models which more explicitly account for cloud processes. Here we use the UK Met Office Large Eddy Model to investigate the semi-direct effect in marine stratocumulus. Large-Eddy-Simulations show significant reductions of cloud thickness and LWP in marine stratocumulus due to absorbing aerosols. This leads to a reduction in planetary albedo and upwelling radiation at the top of the atmosphere (consistent in sign with Hansen et al's result). The significance of the semi-direct aerosol effect will be considered and possible mechanisms will be discussed.