To address the multitude of issues that contribute to a thorough understanding of clouds, science needs to consider and incorporate a wide variety of biological, physical, and chemical processes that are embedded in flows. A host of processes emerge from the categories of atmospheric dynamics—kinetics, diffusion, and turbulence—and span scales from the millimeter, or smaller, scale to the planetary dimension (see Figure 12.1). The development of a simulation system, or theory, that would incorporate the full range of all relevant cloud-controlling factors does not appear to us to be foreseeable, and it is unlikely that we will ever have a reference model that incorporates all relevant processes and their interactions.

Nonetheless, a variety of models or theories continue to be developed in an attempt to describe specific sets of processes over a subset of relevant scales. These models and theories, as well as the connections between them, form an informal network or hierarchy. We refer to this network as being informal because the connections between models cannot be rigorously justified; we term it hierarchical in the sense that the flow of information through the hierarchy tends to have an aggregative character, wherein one attempts to represent the statistical behavior of clouds on small scales in larger-scale models.

The informal nature of the network raises two important issues: How can formalism (i.e., the connections) be increased among processes and models at different scales? How should the network be justifi ed in the context of each problem one wishes to solve, in the present case perturbed clouds.

By "perturbed clouds," we mean clouds that are perturbed either by anthropogenic aerosols, which initiate such perturbations at the smallest spatial

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