1995-14 For Release: June 1995 Contact: Joan Vandiver Frisch Manager, UCAR Media Relations Boulder, CO 80307-3000 Telephone: 303-497-8607; fax (303) 497-8610 E-mail: jfrisch@ucar.edu Tip Sheet on Effects of Clouds on Global Warming BOULDERŅThe role of clouds is one of the wild cards in predicting global climate change. Clouds play a critical role in absorbing and reflecting solar radiation and in producing precipitation. But whether they will enhance or moderate global warming is still not well understood. Researchers at the National Center for Atmospheric Research (NCAR) 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. Recent media reports of articles coauthored by NCAR's Jeffrey Kiehl illustrate the growing interest in cloud studies. In two papers published in the January 27 edition of Science, Kiehl and other researchers from various institutions reported that clouds absorb about four times as much solar energy as had been thought previously. The accuracy of the new absorption value is supported by the fact that each study reached the new value by a different method. Kiehl, a climate modeler, says that before researchers can say for certain what effect this new knowledge will have on global warming scenarios, scientists will first have to learn how clouds absorb much more energy than current theory can account for. By using the new values in NCAR's community climate model, Kiehl arrived at preliminary results that predict a warmer, drier climate than that shown in earlier models. Refining 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. He has been 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 trying to create a model that simulates how different kinds of processes (radiation, microphysics, convection, and turbulence) are coupled and interact in clouds considered as general systems. This idea of cloud systems 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. Working at the other end 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 climate in the future, Al Cooper is currently 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 nuclei could result in a decrease in rainfall. When water vapor condenses into different sized drops, the larger ones fall faster than the smaller ones. As faster-falling 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 of cloud physics in the Small Cumulus Microphysics Study as part of a field program near Cape Canaveral from July 3 until August 17 this year. NCAR 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 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 tropospheric clouds absorb more energy than theory predicts. The instrument will measure haze droplets as small as 0.3 micrometer in diameter. Such small droplets have been typically ignored in previous cloud measurements. The spectrometer was deployed four times from New Zealand between March and October 1994 to study the Antarctic ozone hole. It will be flown over the Arctic on NASA's DC-8 and on the National Science Foundation's C-130, operated by NCAR, from Hobarth, Tasmania next year. NCAR is managed by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation. Below is contact information for the scientists mentioned in this report and other researchers at UCAR member universities and at NASA who are actively involved in researching cloud physics and climate change. Thomas Ackerman, radiative transfer, global climate, Pennsylvania State University, College Station, 814-865-2915 Darrel Baumgardner, cloud and aerosol physics, NCAR, 303-497-1054 William Cooper, cloud physics, NCAR, 303-497-8938 Robert Charlson, University of Washington, aerosol and cloud chemistry, 206-453-2537 James Dye, cloud physics, NCAR, 303-497-8944 Bruce Gandrud, atmospheric chemistry, NCAR, 303-497-1038 Peter Hobbs, cloud and aerosol physics, University of Washington, 206-543-6027 Jeffrey Kiehl, climate modeler, NCAR, 303-497-1350 Mitchell Moncrieff, cloud systems modeler, NCAR, 303-497-8960 Rudy Pueschel, aerosol cloud interactions, NASA Ames Research Center, Mountain View, CA, 415-604-5254 Larry Radke, cloud and aerosol physics, NCAR, 303-497-1032 Writer Intern: John Dawson ˙