Cloud Feedback

INCREASES IN SuRFACE and tropospheric temperatures produce changes in cloud properties that, in turn, produce changes in temperature. This effect is known as cloud feedback. Cloud feedback has been identified by the United Nations Intergovernmental Panel on Climate Change (IPCC) as one of the most uncertain processes in climate models. Cloud particles (hydrometers) affect both thermal and solar radiation. Cloud particle size and concentration are strongly-affected by updraft velocity, humidity, temperature, and cloud nuclei concentration. In addition, the lifetime of cloud particles depends on the humidity of the ambient air. Thus, cloud properties and coverage are sensitive to climate change.

Clouds produce extremely complex climate feedbacks that lead to large uncertainty in climate simulations because cloud processes are not well understood. Clouds cool the Earth by scattering solar radiation and increasing the amount of solar radiation returned to space. That is, clouds increase the planet's albedo. They also absorb solar radiation, increasing the atmosphere's temperature, and cooling the surface by reducing the amount of solar radiation reaching it. In addition, clouds produce a greenhouse effect by absorbing thermal radiation and re-emitting it at their own temperature. Clouds tend to keep nighttime surface temperatures warmer than they would be in their absence, and keep daytime temperatures cooler.

Cloud feedback is the combined result of changes in cloud cover, cloud height, cloud emissivity, and cloud albedo. Because various cloud processes contribute to cloud feedback, and because different types of clouds such as cumulus, stratus, and cirrus have different effects on the budget of solar and thermal radiation, cloud feedback is complicated. Climate models cannot even determine if the overall cloud feedback is positive or negative. High clouds, such as cirrus, are optically thin and cold. Because they are reasonably opaque to thermal radiation, they scatter a relatively small amount of solar radiation when compared to the amount of thermal radiation they emit. Thus, high clouds usually have a net warming effect on the planet.

Low clouds, such as stratus, are warm and highly opaque to solar and thermal radiation. However, because low clouds are not much colder than the surface, their main effect is to increase the amount of solar radiation scattered to space. Therefore, low clouds usually produce a net cooling of the planet. On the other hand, low clouds over bright and cold surfaces such as snow or ice can have a positive climate feedback. The net result depends on details of cloud properties, such a particle size distribution and concentration.

The effects of clouds on climate are uncertain, because they depend on the cloud particle number, size distribution, phase, cloud height, temperature, and many other physical parameters. Cloud feedback is the source of the largest uncertainty in the Global Climate Models (GCMs) used to calculate human-induced global climate changes. The International Satellite Cloud Climatology Project (ISCCP), started in 1983, was the first internationally-coordinated satellite cloud climatology project. This pioneering program served as a prototype to many other global satellite cloud climatology projects. The U.S. Department of Energy (DOE) Atmospheric Radiation Measurement Program (ARM) has focused on surface and cloud measure ments. These measurements are used to study cloud processes to produce solid cloud climatology.

SEE ALSO: Climate Models; Clouds, Cirrus; Clouds, Cumulus; Clouds, Stratus.

BIBLIOGRAphY. J.P. Huang, et al., "Advanced Retrievals of Multilayered Cloud Properties Using Multispectral Measurements," Journal of Geophysical Research (v. 110, 2005); G.G. Mace, et al., "Cloud Radiative Forcing at the Atmospheric Radiation Measurement Program Climate Research Facility: 1. Technique, Validation, and Comparison to Satellite-Derived Diagnostic Quantities," Journal of Geophysical Research (v.111, 2006); G.G. Mace, S. Benson, and S. Kato, "Cloud Radiative Forcing at the Atmospheric Radiation Measurement Program Climate Research Facility: 2. Vertical Redistribution of Radiant Energy by Clouds," Journal of Geophysical Research (v.111, 2006); N.O. Renno, P.H. Stone, and K.A. Emanuel, "A Radiative-Convective Model with an Explicit Hydrological Cycle: 2. Sensitivity to Large Changes in Solar Forcing," Journal of Geophysical Research (v.99, 1994); W.B. Rossow, A.W. Walker, and L.C. Garder, "Comparison of ISCCP and Other Cloud Amounts," Journal of Climate (v.6, 1993); G.L. Stephens, "Dirty Clouds and Global Cooling," Nature (v.370, 1994); R.T. Wetherald and S. Manabe, "Cloud Feedback Processes in a General Circulation Model," Journal of Atmospheric Science (v.45/8, 1988).

Nilton O. Renno University of Michigan

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