Explaining Away Tropical Clouds


J. W. Stewart

When it comes to climate modeling, predictions should often be taken with a grain of salt--or, perhaps, aerosol soot. Consider, for example, the ongoing dialogue about global warming. Only recently did one of the strongest arguments against that possibility--based on what were presumed to be fairly constant ocean temperatures--come unglued. Now another steady assumption, this time about the dynamics of cloud cover, has cracked.

Indeed, conventional wisdom held that higher levels of aerosol pollution in the atmosphere should cool the earth's climate because aerosols can increase cloudiness; they not only reduce precipitation, which raises the water content in clouds, but they also increase the size of the individual water droplets, which in turn causes more warming sunlight to be reflected back into space.

But using a new simulation that relies on data from the Indian Ocean Experiment (INDOEX), researchers have demonstrated an exactly opposite mechanism--one in which aerosol pollution actually intensifies solar absorption and reduces cumulus cloud cover, which significantly offsets whatever cooling it might cause within other layers of the atmosphere.

"Solar absorption by aerosols during the northeast monsoon over the Indian Ocean can reduce daytime cloud coverage by nearly half in a specific case of trade cumulus," the scientists wrote in a paper appearing in the May 12 issue of Science. The six-person team included Andrew S. Ackerman of the NASA Ames Research Center, Owen Brian Toon of the University of Colorado, D. E. Stevens of the Lawrence Livermore National Laboratory, Andrew J. Heymsfield of the National Center for Atmospheric Research, V. Ramanathan of the Scripps Institution of Oceanography and E.J. Welton of Science Systems and Applications.

Essential to the group's work was the INDOEX observation of scarce cumulus clouds and deep layers of dark, sunlight-absorbing haze, present during 99 percent of the data-gathering phase. Earlier work, shown in the animation at top right, revealed that "intense aborption of solar energy can desiccate an optically thick stratocumulus cloud layer," turning it into cumulus clouds. The trade cumulus themselves, though, are governed by very different dynamics.

To figure out what was going on with these lower, lighter clouds, the researchers adopted a basic simulation of trade cumulus found over warm tropical waters and subjected it to varying degrees of aerosol-induced solar heating. In the baseline simulations, cumulus convection arose an hour into the exercise and generated clouds that very much resembled in appearance at least those photographed during INDOEX. Solar heating lowered relative humidities in the cloud layer through late afternoon, and stronger convection arose after sunset. Early and mature growth stages of convective episodes appeared clearly in snapshots of the simulation, which are illustrated at right.

When they incorporated aerosol soot into the simulations, the resulting haze even more dramatically enhaced solar heating, thereby further increasing temperatures and lowering relative humidities. At haze levels comparable to those seen during INDOEX in 1998 and 1999, cloud coverage was reduced by 25 percent and 40 percent, respectively. It is important to note that in additional simulations, they demonstrated that the effect of aerosol pollution on cumulus cloud coverage is not absolute; it can cause an increase or decrease, depending on other meteorological conditions. But this itself is an important finding--hopefully one that will help other scientists to refine their own models in the future.

Animation: ANDREW ACKERMAN; Images: K. LEUTWYLER, after Ackerman et al in Science
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