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For more information on these science news and feature story tips, please contact the public information officer at the end of each item at (703) 292-8070. Contents of this News Tip:
WILD CARD IN PREDICTING GLOBAL CLIMATE CHANGE: CLOUDSThe role of clouds is one of the wild cards in predicting global climate change, say atmospheric scientists. Clouds play a critical role in absorbing and reflecting solar radiation, and in producing precipitation. But whether they enhance or moderate global climate change is not known. Researchers at the National Science Foundation (NSF) supported National Center for Atmospheric Research (NCAR) in Boulder, Colorado are investigating various aspects of cloud physics that range in scale from the global effect of clouds on climate, to the physics of microscopic interactions in individual clouds. CLOUDS ABSORB LARGE AMOUNTS OF SOLAR ENERGYNCAR researcher Jeffrey Kiehl reports that clouds absorb about four times as much solar energy as had been thought previously. Kiehl, a climate modeler, says that before researchers can say for certain what effect this new knowledge will have on global climate change scenarios, scientists will first have to learn how clouds absorb much more energy than accounted for by current theory. By using the new values in an NCAR climate model, Kiehl arrived at preliminary results that predict a warmer, drier global climate than that shown in earlier models. CLOUDS AS ATMOSPHERIC HEAT ENGINESRefining the mathematical models that describe how clouds affect the atmosphere on a global scale is the focus of recent work by Mitchell Moncrieff, another NCAR scientist. Moncrieff is researching how clouds transport momentum and energy, and how they operate as atmospheric heat engines. In order to better describe how clouds affect climate, Moncrieff has been developing a model that simulates how different processes (radiation, microphysics, convection and turbulence) are coupled and interact in clouds. This will allow climate modelers to use relatively simple formulas to describe the effects of clouds on climate, rather than trying to derive those effects from descriptions of the behavior of individual clouds or the processes in them. HOW MICROSCOPIC CLOUD PARTICLES PRODUCE RAINWorking at the minute of the scale, several NCAR researchers are investigating the behavior of microscopic particles. In a project that, like Kiehl's, suggests the possibility of a drier global climate in the future, Al Cooper is studying the tiny particles that form the nuclei of cloud droplets and raindrops. Cooper says that the burning of tropical forests and grasslands may be the largest source of these particles. Other cloud condensation nuclei are dust, sea salt and other materials not created by human activities. The increase in these nuclei could result in a decrease in rainfall. When water vapor condenses into various sized drops, the larger ones fall faster than the smaller ones. As fasterfalling drops overtake slower ones, they merge with them, creating even larger drops in a cascading process that results in rain. But too many nuclei competing for a fixed amount of water vapor may create many small drops that fall at about the same speed, and move around each other instead of colliding. Fewer collisions between droplets could mean that fewer large drops are formed and that rainfall is inhibited. Cooper and others will be studying this and other questions related to cloud physics in the Small Cumulus Microphysics Study as part of a field program near Cape Canaveral, Florida from July 3 through August 17 this year. CLOUDS AND POLAR OZONE DEPLETIONNCAR researchers are also studying very high stratospheric clouds that are crucial to the processes creating polar ozone depletion. Jim Dye, Darrel Baumgardner, Bruce Gandrud and others have developed an aerosol spectrometer that is measuring the characteristics of stratospheric aerosols and polar stratospheric clouds. Chlorine compounds react on the surfaces of these particles to release ozone-destroying chlorine molecules. The spectrometer uses a laser to determine the size, concentration, and optical properties of stratospheric particles. From a particle's optical characteristics, researchers can deduce information about its chemical composition. The spectrometer may also shed light on whether clouds absorb more energy than theory predicts. The instrument will measure haze droplets as small as 0.3 micrometers in diameter; such small droplets have been typically ignored in previous cloud measurements.
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