Current Understanding and Quantification of the Effects of Clouds in the Changing Climate System and Strategies to Reduce the Critical Uncertainties

Anthropogenic aerosols are thought to exert a significant indirect radiative forcing because they act as CCN in warm cloud formation and as ice nuclei in cold cloud-forming processes. Haywood et al. (Chapter 19) address this issue by comparing the radiative forcing from the indirect effect of aerosols with those from other radiative forcing components, such as that from changes in well-mixed greenhouse gases. They highlight problems in assessing the effect of anthropogenic aerosols upon clouds under the strict definitions of radiative forcing provided by the IPCC (2007). Straightforward scaling between forcing and the temperature change it induces is significantly compromised in the case of aerosols, where feedbacks from indirect aerosol effects are responses to both radiative and cloud microphysical perturbations. Haywood et al. argue that additional characterization, such as climate efficacy, is required when comparing indirect aerosol effects with other radiative forcings. They suggest using the radiative flux perturbation associated with a change from preindus-trial to present-day composition, calculated in a global climate model with fixed sea-surface temperature and sea ice, as a supplement to IPCC's definition of forcing.

Collins and Satoh (Chapter 20) discuss the differences of cloud responses to increasing greenhouse gas concentrations using global cloud-resolving models (GCRMs) in comparison to conventional global climate models with cloud parameterization. They demonstrate that high clouds behave differently within these models, suggesting the questions: How is high cloud amount sensitive to cloud processes such as cloud generation, precipitation efficiency, or sedimentation of cloud ice? How are model results of high clouds comparable to current satellite observations such as CloudSat and CALIPSO? How can we understand the change in dynamic fields such as narrowing the precipitation regions, increase in transport of water, and relative humidity?

Strategies to reduce critical uncertainties in our understanding of inadvertent anthropogenic perturbations of clouds are discussed on micro- to mesoscales by Brenguier and Wood (Chapter 21). They emphasize that the challenge is to establish the links between two contrasting forcings, i.e., to understand how clouds respond to changes in the general circulation in order to quantify how this response might be modulated by changes in their microphysical properties. The two generic classes of micro- to mesoscale observational strategies, the Eulerian column closure and the Lagrangian cloud system evolution approaches, are described using examples of low-level cloud studies, and recommendations are made on how they should be combined with large-scale information to address this issue.

Illingworth and Bony (Chapter 22) extend this strategic discussion from the mesoscale to larger scales, where the response of clouds to climate change remains very uncertain because of an incomplete knowledge of the cloud physics and the difficulties in simulating the different properties of clouds. They propose an observational strategy to improve the representation of clouds in large-scale models and reduce uncertainties in the future change of cloud properties. This consists first in determining what key aspects of the simulation of clouds are the most critical with respect to future climate changes, and then in using specifi c methodologies and new datasets to improve the simulation of these aspects in large-scale models.

A critical review of the representation of clouds in large-scale models by Lohmann and Schwartz (Chapter 23) reveals a major unresolved problem. This is attributable to the high sensitivity of radiative transfer and water cycle to cloud properties and processes, an incomplete understanding of these processes, and the wide range of scales over which these processes occur. Small changes in the amount, altitude, physical thickness, and/or microphysical properties of clouds which result from human influences can exert changes in the Earth's radiation budget that are comparable to the radiative forcing by anthropogenic greenhouse gases, thus either partly offsetting or enhancing greenhouse warming. Because clouds form on aerosol particles, changes in the amount and/or composition of aerosols affect clouds in a variety of ways. Because of the forcing of the radiation balance that results from aerosol-cloud interactions, major uncertainties exist and must be addressed before accurate results can be obtained.

Quaas et al. (Chapter 23) focus on the necessity of models at all scales, especially global, and note the apparent lack of progress in quantifying the cloud-albedo-climate feedback, even though this problem has been identified for more than two decades. Substantial discussion centers on our need for present-day observational proxies to extrapolate future cloud perturbations. In addition, Quaas et al. emphasize the role of small-scale models to describe processes in large-scale models. Substantial effort seems to be required if we are to be able to identify and isolate key cloud-related processes. Quass et al. discuss the problem of applying the concept of climate forcing (Wm-2) to systems in which the fast response of the system via feedbacks changes the initial forcing itself. They recommend the use of a different terminology (i.e., the term radiative flux perturbation) to avoid misapplication of the concept of forcing. This new forcing concept, however, could be defined in an even more rigorous way, with explicit statements about the maximum response time of system adjustments in the models that are allowed. Quaas et al. note the necessity of developing process-based evaluation of large-scale models: What aspects of clouds do we need to represent to achieve an accurate assessment of aerosol-cloud interactions? Is it possible to design an observational program to detect and quantify aerosol indirect effects?

0 0

Post a comment