Abstract
Abstract. The effects of different aerosol types on cloud albedo are analysed using the linear relation between total albedo and cloud fraction found on a monthly mean scale in regions of subtropical marine stratocumulus clouds and the influence of simulated aerosol variations on this relation. Model experiments from the Coupled Model Intercomparison Project phase 5 (CMIP5) are used to separately study the responses to increases in sulfate, non-sulfate and all anthropogenic aerosols. A cloud brightening on the month-to-month scale due to variability in the background aerosol is found to dominate even in the cases where anthropogenic aerosols are added. The aerosol composition is of importance for this cloud brightening, that is thereby region dependent. There is indication that absorbing aerosols to some extent counteract the cloud brightening but scene darkening with increasing aerosol burden is generally not supported, even in regions where absorbing aerosols dominate. Month-to-month cloud albedo variability also confirms the importance of liquid water content for cloud albedo. Regional, monthly mean cloud albedo is found to increase with the addition of anthropogenic aerosols and more so with sulfate than non-sulfate. Changes in cloud albedo between experiments are related to changes in cloud water content as well as droplet size distribution changes, so that models with large increases in liquid water path and/or cloud droplet number show large cloud albedo increases with increasing aerosol. However, no clear relation between model sensitivities to aerosol variations on the month-to-month scale and changes in cloud albedo due to changed aerosol burden is found.
Highlights
Aerosol particles have an impact on the radiation budget of the Earth, directly through interaction with radiation and indirectly via interaction with clouds
Four of the models utilized (CSIRO, HadGEM, IPSL, MIROC) are included in a previous modelintercomparison study performed by Shindell et al (2013) within the ACCMIP (Atmospheric Chemistry and Climate Model Intercomparison Project; Lamarque et al, 2013), who found a general agreement with observations in terms of total aerosol optical depth (AOD) but pointed at model underestimates over east Asia and Europe, and large intermodel differences
The same anthropogenic aerosol emissions by mass are used for all models, the AOD varies among models
Summary
Aerosol particles have an impact on the radiation budget of the Earth, directly through interaction with radiation and indirectly via interaction with clouds. The magnitude of the cooling remains uncertain (Boucher et al, 2013) and smaller forcings and narrower uncertainty ranges have been suggested (Stevens, 2015), the most recent report from the Intergovernmental Panel on Climate Change (IPCC) estimates the effective radiative forcing due to aerosols, including cloud adjustments, to −0.9 Wm−2, with a 90 % uncertainty range of −1.9 to −0.1 Wm−2. Assuming a constant liquid water path (LWP), a cloud with a larger number of available CCN will have more numerous and smaller cloud droplets, and a higher cloud albedo. This cloud brightening effect due to aerosols is known as the cloud albedo or Twomey effect (Twomey, 1977).
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