Cloudcontrolling Factors

Since the atmosphere allows only the observation of clouds and does not permit us to control the initial and/or boundary conditions, laboratory studies are important tools for the examination and understanding of microphysical cloud processes under well-defined and repeatable conditions. Stratmann et al. (Chapter 7) provide an overview of the capabilities and limitations of laboratory facilities (ranging in scales from bench-top instruments to vertical mine shafts), wherein clouds are generated artificially and studied under controlled conditions. In this context, hygroscopic growth and activation of aerosol particles, droplet dynamic growth, ice nucleation, and droplet-turbulence interactions can be investigated. Stratmann et al. offer suggestions for future research topics, including investigations into particle hygroscopic growth and activation, the accommodation coefficients of water vapor on liquid water and ice, aerosol effects on primary ice formation in clouds, aerosol-based parameterizations of cloud ice formation, secondary ice formation/multiplication, the production and characterization of particles suitable for cloud simulation experiments, and experiments which combine turbulence and microphysics. The latter is emphasized because of the potential importance of interactions between the micro-physical (activation, growth, freezing) and turbulent transport processes within clouds, and the difficulty of studying these through any other approach.

Stevens and Brenguier (Chapter 8) review how meteorological and aerosol factors determine the statistics and climatology of layers of shallow (boundary layer) clouds, with an emphasis on factors that may be expected to change in a fluctuating climate. They identify the paramount role of theory, both to advance our understanding and to improve our modeling and attribution of specific cause and effects. In particular, they argue that limits to current understanding of meteorological controls on cloudiness make it difficult, and in many situations perhaps impossible, to attribute changes in cloudiness to perturbations in the aerosol. Suggestions for advancing our understanding of low cloud-controlling processes include renewing our focus on theory, model craftsmanship, and increasing the scope and breadth of observational efforts.

In Chapter 9, Grabowski and Petch address deep convection, which plays a key role in the Earth's atmospheric general circulation and is often associated with severe weather. They argue that an understanding of the role of deep convection in the climate system, as well as in predictions of climate change, necessitates modeling efforts across all scales, from the micro- to global scale, using a variety of models. Traditional atmospheric general circulation models, in which representation of deep convection and how it may change in the perturbed climate is highly uncertain, are not sufficient. Grabowski and Petch review the relevant aspects of the problem, highlight limitations of current modeling and observational approaches, and suggest areas for future research.

Bretherton and Hartmann (Chapter 10) emphasize current limits in modeling accurately the interaction of clouds and dynamics in the present-day climate.

To guide thinking about the real atmosphere, they demonstrate that horizontal gradients in top-of-the-atmosphere cloud radiative forcing act as atmospheric circulation feedbacks, and that cloud shading helps regulate sea surface temperatures. This relates closely to our lack of fundamental understanding of the empirical controls on tropical deep and low cloud forcing. Bretherton and Hartmann advocate the use of new high-resolution modeling tools, discussed elsewhere in the volume (Chapters 8, 9, and 18) and suggest that new observations may lead to progress if cleverly applied. They caution that scientists should tread carefully and test comprehensively when adding components to general circulation models, such as aerosols and soluble trace gases which interact closely with clouds.

Clouds in the upper troposphere and tropopause region that lack a liquid water phase are called cirrus. They represent a special cloud type, not only because of their formation mechanisms and characteristics but also because of their evident anthropogenic perturbations in terms of contrails. Factors controlling cirrus clouds comprise small- and large-scale atmospheric dynamics, ice nucleation behavior of natural and anthropogenic particles, and interaction with terrestrial and solar radiation. Current understanding of these factors is summarized by Karcher and Spichtinger in Chapter 11. Key uncertainties in this active area of research are outlined, along with viable approaches to minimize them. These areas of concern include relative humidities in the cirrus regions, vertical velocities, the understanding of ice initiation and growth processes, and accurate data on small ice particles.

Siebesma et al. (Chapter 12) address the shortcomings that arise in atmospheric models attributable to the interactions between resolved and unresolved (i.e., parameterized) cloud-related processes. These problems occur because it is necessary to consider simultaneously a wide range of scales of cloud-related processes, from molecular to global (cf. Figures 12.1 and 12.2). One way to do this is to use smaller-scale process models to improve the representation of clouds in climate models. However, problems and questions arise in deciding just what level of complexity is needed in global models. Siebesma et al. suggest three different pathways to improve the representation of cloud-related processes in future climate models. They recognize that there are many open issues concerning the description of cloud particle formation, right down to questions about the behavior of the water molecule during phase transitions. They echo statements made by Anderson et al. (Chapter 6), in terms of the difficulties in describing the ice phase; however, they add the case of mixed-phase clouds as another cloud process with a serious knowledge deficit. In terms of observations, Siebesma et al. note that polar-orbiting satellites, which are necessary for global, high-resolution coverage with some classes of instruments, do not provide adequate information about the diurnal variations of clouds (e.g., the mid-day convection maximum).

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