Evaluation of sulfate aerosols indirect effect in marine stratocumulus clouds using observation‐derived cloud climatology

Abstract

The indirect shortwave effect of anthropogenic sulfate aerosol augmentation in marine stratocumulus clouds has been evaluated using global cloud climatology, sulfate aerosol data from a chemical model, and cloud albedo augmentation obtained from a large‐eddy simulation (LES) cloud model with explicit microphysics. It is found that the annually and globally (over the oceans) averaged indirect shortwave forcing is −1.1 Wm−2 with a hemispheric difference of 0.4 Wm−2. Given the many uncertainties in the aerosol‐CCN‐cloud microstructure links, this estimate, most likely, should be considered as an upper limit of the aerosol indirect forcing. The hemispheric forcing has a strong seasonal cycle with the Northern Hemisphere (NH) forcing exceeding the Southern Hemisphere (SH) forcing during the NH spring and summer and the SH forcing prevailing during the SH spring and summer. We also estimate that the single layer St/Sc contributes most to the aerosol indirect forcing (60%) with an additional 25% from the St/Sc overlapped with Ci/Cs/Cc, and 15% from St/Sc overlapped with As/Ac.

A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks

“…This results in a buffer of less hygroscopic particles to stabilize the CDNC as per the paradigm noted by Stevens and Feingold (2009). While it is likely that the validity of the derived regression relationship is limited to the regions from which it derives, those regions are the most significant for the indirect forcing of climate by aerosols (e.g., Kogan et al, 1996). Hence, the relationship could be of considerable value in both modeling exercises and remote sensing pertaining to the climate change issue.…”

“…Indeed, they are influenced by a variety of aerosol sources including industrial pollution, biomass burning, and marine emissions (cf., Hegg et al, 2010;Chand et al, 2010;Haywood et al, 2003).These decks are a major factor in the radiative balance of the atmosphere (Klein and Hartmann, 1993) and, due to a combination of cloud extent, frequency, and the cloud-type dependent sensitivity of cloud albedo to aerosol modulation, the climatic impact of aerosols on cloud microphysics (the indirect effect) is largely determined by these decks (Warren et al, 1988;Platnick and Twomey, 1994;Allen et al, 2011). For example, Kogan et al (1996) estimate the contribution of low stratiform clouds to the indirect forcing by aerosol as ∼60 %. Hence, quantifying the CDNCaerosol relationship in simple terms for these stratocumulus decks alone would have great benefit for understanding indirect aerosol radiative forcing globally.…”

Improving aerosol distributions below clouds by assimilating satellite-retrieved cloud droplet number

“…These conditions represent low stratiform clouds, which persistently cover large regions around the world (e.g., stratocumulus decks off the west coasts of Africa and South and North America) and are pointed out as the main players in aerosol indirect forcing (33). While this first approach to N d assimilation does not resolve the vertical N d gradients and ice and graupel phases that arise from convection, convective clouds are often accompanied by or form from low clouds where this technique can be applied.…”

CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

“…Low stratus clouds that predominate the east sides of oceans provide a majority of the indirect aerosol effect (IAE; Warren et al 1988;Platnick and Twomey 1994;Kogan et al 1996), which remains the largest climate uncertainty (Alley et al 2007). The high radiative temperatures and large albedo contrasts with the underlying ocean provide substantial global cooling that can be increased by the advection and injection of anthropogenic cloud condensation nuclei (CCN), which increase cloud droplet concentrations N c (albedo; first IAE; Twomey 1977a) and decrease droplet sizes (lifetime; second IAE; Albrecht 1989).…”