The semi-direct aerosol effect: Impact of absorbing aerosols on marine stratocumulus

Aerosols that absorb solar radiation may lead to a decrease of low-cloud cover and liquid-water path (LWP), leading to a positive radiative forcing. A large-eddy model was used to investigate this 'semi-direct effect' for marine stratocumulus and examine the dependency on the vertical distribution of the aerosol. In this study, the aerosols influenced clouds by directly altering the short-wave heating rate (the semi-direct effect), but did not interact with the cloud microphysics (i.e. indirect aerosol effects are excluded). Absorbing aerosols within the boundary layer (BL) decreased LWP by 10 g m(-2), leading to a positive semi-direct forcing. Even for mildly absorbing aerosols (mid-visible single-scattering albedo of 0.96), the semi-direct forcing was three times stronger, and opposite in sign, to the aerosol direct forcing. The semi-direct forcing was found to be proportional to aerosol single-scattering co-albedo (tested to a value of 0.12). Conversely, with the absorbing aerosol layer above the cloud, the LWP increased by 5 to 10 g m(-2), leading to a negative semi-direct forcing. Absorbing aerosols located in the BL heat the cloud layer, enhancing the daytime decoupling and thinning of the stratocumulus layer. Absorbing aerosols immediately above the BL increased the contrast in potential temperature across the inversion, leading to a lower cloud-top entrainment rate. With aerosol both within and above the BL, the semi-direct forcing was positive but half the magnitude of that experienced when aerosol was only in the BL. As marine stratocumulus covers about 20% of the globe, the semi-direct effect could significantly influence the radiative forcing by absorbing aerosols.