Abstract
Abstract. Recent simulations of deep convection with a spectral microphysics cloud model show that an increase in aerosol concentration can have a significant effect on the nature of convection with more ice precipitation and less warm rain in polluted air. The cloud lifetime and the area covered by cloud anvils of deep convection are also larger for polluted air. Therefore, it is possible that the increase of anthropogenic aerosols in most of the 20th century has increased humidity and perhaps also cloudiness in the mid- to upper troposphere. Satellite data of upper tropospheric relative humidity in 1979–1997 and observed changes in cloudiness support this hypothesis. As changes in upper tropospheric humidity strongly affect longwave radiation, it is possible that anthropogenic aerosols have had a significant warming effect in addition to their other known effects on radiation.
Highlights
Climate sensitivity to the doubling of atmospheric CO2 strongly depends on feedbacks associated with water vapor and clouds
Ramanathan et al (2001) suggested that the suppression of precipitation associated with increasing amounts of aerosols could be associated with an increase of upper tropospheric humidity and that this could counter some of the aerosol cooling effect
There is some evidence for such an effect of aerosols from biomass burning and other sources on the observed positive moisture trend in the stratosphere (e.g. Sherwood, 2002; Notholt et al, 2005)
Summary
Climate sensitivity to the doubling of atmospheric CO2 strongly depends on feedbacks associated with water vapor and clouds. Aerosols’ effect on precipitation can be even qualitatively different in pristine and polluted environments (Rosenfeld et al, 2008; Wild, 2009) Complicated as their role may be, aerosols are of first order importance to the climate variations associated with anthropogenic changes in the composition of the atmosphere. It is an intriguing question whether aerosols have yet another, so far overlooked effect on the Earth’s atmosphere. We draw attention to an effect that was pointed out by Ramanathan et al (2001) As they stated, “Suppression of precipitation in the updrafts of very deep clouds, especially in the tropics, can transport more water and aerosols into the upper troposphere and the lower stratosphere. We hope that this paper encourages research on the effect of changes in microphysics on tropospheric humidity, and especially in the upper troposphere
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