Extent and Nature of Anthropogenic Perturbations of Clouds

Starting with the characteristic parameters of cloud particle precursors and the sensitivities of warm and cold cloud formation to these characteristics, Kreidenweis et al. (Chapter 13) discuss changes to these parameters and propose recommendations for future closure exercises between modeled and experimentally characterized cloud formation processes.

Feingold and Siebert (Chapter 14) extend the microphysical interaction of aerosols and cloud processes to the cloud scale, which involves vertical motions and turbulent mixing processes inside clouds as well as at their borders. Many uncertainties remain on this scale. In particular, there is scant observational evidence of aerosol effects (positive or negative) on surface precipitation. In addition, clouds and precipitation modify the amount of aerosol through both physical and chemical processes so that a three-way interactive feedback between aerosol, cloud microphysics, and cloud dynamics must be considered. Feingold and Siebert demonstrate the dubious utility of simple constructs to separate aerosol effects from the rest of the cloud system. Both observations and modeling suggest that the magnitude (as well as perhaps the sign) of these effects depend on the larger-scale meteorological context in which aerosol-cloud interactions are embedded. They also consider alternate approaches and the possibility of self-regulation processes, which may act to limit the range over which aerosol significantly affects clouds.

The most obvious, yet controversial, perturbation of clouds through willful human intervention—cloud seeding—is addressed in Chapter 15 by Cotton. Here, he reviews research that confirms or refutes the existing concepts for increasing rainfall, decreasing hail damage, and reducing hurricane intensity, and provides a critical overview of the existing atmospheric concepts for climate engineering to counter greenhouse warming.

The physical hypothesis that air pollution in the form of small particles should lead to less efficient formation of precipitation has been established for several decades and is considered by some to be scientifically sound. Ayers and Levin (Chapter 16) provide strong arguments that there is as yet no convincing proof that such a microphysical control of precipitation efficiency has been the prime cause of rainfall reduction in any area of the globe. They emphasize the need for new experimental designs to test this hypothesis in a holistic way, taking into account all possible confounding influences on rainfall trends in a climate that is clearly nonstationary in the face of global warming and natural decadal variability.

Nakajima and Schulz (Chapter 17) broaden the scope of anthropogenic perturbations of clouds to the global scale, using satellite data and global models. Recent observations have detected what appear to be signatures of large-scale changes in the atmospheric aerosol amount and associated changes in cloud fraction and microphysical structures on a global scale. Models can simulate these signatures fairly well, but problems still exist, thus necessitating further improvements. Fields of anthropogenic aerosol optical depth from several atmospheric models have been found to be consistent with the spatial pattern obtained from satellite-derived products. Further studies are needed (a) to improve our ability to differentiate between natural and anthropogenic aerosols, (b) to interpret observed temporal and regional trends in aerosol parameters, and (c) to interpret the extent to which the covariation of satellite-derived aerosol and cloud characteristics can be utilized to advance understanding of aerosol-cloud interactions.

Chuang et al. (Chapter 18) emphasize the daunting task of identifying the myriad effects which must be considered, and the consequences of these for relevant cloud-related processes. These effects include those on microphys-ics, radiation (both reflected short wave and emitted longwave), precipitation (both rain and snow), dynamics (attributable to the redistribution of energy by clouds), and on chemical processes in clouds and on the composition of precipitation. Three sorts of perturbations are noted: those attributable to aerosols, perturbations of greenhouse gases (which involve changes in dynamics), and changes in the land surface. Three categories of gaps in understanding are identified: conceptual gaps, knowledge or data gaps (which are deficits that could be filled using present-day instruments and data, but for some reason, e.g., lack of resources, have not), and tool gaps or deficits in our ability to make relevant measurements. However, some points seem clear. For example, Chuang et al. emphasize the need to consider multiple scales. The constraints imposed by limitations of available observations were exemplified by the present impossibility to measure small supersaturations in the field (cf. Grabowski and Petch, Chapter 9, who state that it is possible to generate accurately known supersaturations in the laboratory). In addition, Chuang et al. discuss the apparent constancy of global albedo over the past ca. 10 millennia in the context that this stability implies constancy of cloud properties. The possibility exists that as yet unidentifi ed feedbacks might be responsible for such stasis. The difficulty of understanding and quantifying cloud fraction (i.e., the fractional area of a region or the globe covered by clouds) was highlighted as a key problem. Once again, Chuang et al. identified the need for longevity of satellite observations of 30+ years, as well as the need for new sorts of instruments in new orbits (e.g., L1 satellite).

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