Abstract
Anthropogenic aerosol particles exert an—quantitatively very uncertain—effective radiative forcing due to aerosol-cloud interactions via an immediate altering of cloud albedo on the one hand and via rapid adjustments by alteration of cloud processes and by changes in thermodynamic profiles on the other hand. Large variability in cloud cover and properties and the therefore low signal-to-noise ratio for aerosol-induced perturbations hamper the identification of effects in observations. Six approaches are discussed as a means to isolate the impact of anthropogenic aerosol on clouds from natural cloud variability to estimate or constrain the effective forcing. These are (i) intentional cloud modification, (ii) ship tracks, (iii) differences between the hemispheres, (iv) trace gases, (v) weekly cycles and (vi) trends. Ship track analysis is recommendable for detailed process understanding, and the analysis of weekly cycles and long-term trends is most promising to derive estimates or constraints on the effective radiative forcing.
Highlights
The radiative forcing anthropogenic aerosol particles exert when they serve as cloud condensation nuclei (CCNs) and
Since fall velocity is a non-linear function of particle size, a change in size spectra alters this process and, the precipitation formation
It has been postulated that the delay in precipitation formation may lead to larger cloud cover [5], but this is disputed [6,7,8,9]
Summary
The radiative forcing anthropogenic aerosol particles exert when they serve as cloud condensation nuclei (CCNs) andThis article is part of the Topical Collection on Climate FeedbacksThe shift in particle size spectra impacts cloud microphysical and dynamical processes. Since fall velocity is a non-linear function of particle size, a change in size spectra alters this process and, the precipitation formation. It has been postulated that the delay in precipitation formation may lead to larger cloud cover [5], but this is disputed [6,7,8,9]. Smaller particles more readily evaporate due to the larger surface-tovolume ratio, so an increase in CCN may lead to less, thinner clouds [6]. The cloud-top entrainment rate is, besides its dependency on the thermodynamic profiles, a function of precipitation rate for shallow, stratiform clouds [12]. Depending on the humidity above the cloud layer, a reduction in precipitation may lead to more entrainment of dry air
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