Abstract

Abstract. Aerosols affect the climate system by changing cloud characteristics. Using the global climate model ECHAM5-HAM, we investigate different aerosol effects on mixed-phase clouds: The glaciation effect, which refers to a more frequent glaciation due to anthropogenic aerosols, versus the de-activation effect, which suggests that ice nuclei become less effective because of an anthropogenic sulfate coating. The glaciation effect can partly offset the indirect aerosol effect on warm clouds and thus causes the total anthropogenic aerosol effect to be smaller. It is investigated by varying the parameterization for the Bergeron-Findeisen process and the threshold coating thickness of sulfate (SO4-crit), which is required to convert an externally mixed aerosol particle into an internally mixed particle. Differences in the net radiation at the top-of-the-atmosphere due to anthropogenic aerosols between the different sensitivity studies amount up to 0.5 W m−2. This suggests that the investigated mixed-phase processes have a major effect on the total anthropogenic aerosol effect.

Highlights

  • The interactions of aerosols with clouds is the largest source of uncertainty for estimating the total anthropogenic forcing since pre-industrial times (Forster et al, 2007)

  • We investigate the contribution of thermophoresis to contact freezing, which could be of importance in subsaturated cloudy regions where the temperature gradient would favor collisions between a contact ice nuclei (IN) and an evaporating cloud droplet as evaluated by Phillips et al (2007)

  • The importance of these effects is investigated by varying the parameterization for the onset of the Bergeron-Findeisen process and the threshold coating thickness of sulfate (SO4-crit), which is required to convert an externally mixed aerosol particle into an internally mixed particle

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Summary

Introduction

Climate models estimate the sum of all anthropogenic aerosol effects (total indirect plus direct) to be −1.2 W m−2 with a range from −0.2 to −2.3 W m−2 in the change in the top-of-the-atmosphere net radiation since pre-industrial times (Denman et al, 2007), whereas inverse estimates constrain the indirect aerosol effect to be between −0.1 and −1.7 W m−2 (Hegerl et al, 2007). If in present times more dust aerosols are internally mixed immersion nuclei they are worse IN than in preindustrial times, where more of them acted as contact nuclei Similar findings were obtained in the LACIS cloud chamber by Niedermeier et al (2009) for Arizona test dust for coatings with succinic acid, sulfuric acid and ammonium sulfate at temperatures between 233 and 241 K. These two studies suggest a de-activation effect of sulfate coatings on mineral dust nuclei acting as deposition nuclei. We investigate the contribution of thermophoresis to contact freezing, which could be of importance in subsaturated cloudy regions where the temperature gradient would favor collisions between a contact IN and an evaporating cloud droplet as evaluated by Phillips et al (2007)

Model description
Set-up of the simulations
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