Convective clouds include cumulonimbus clouds, which can be many miles thick with a base near Earth's surface and a top frequently at an altitude of 33,000 feet or more.
The research team, which included atmospheric scientists from Caltech, the Massachusetts Institute of Technology, NASA's Jet Propulsion Laboratory and UC San Diego, linked the changing cloud patterns to the increasing pollution through a series of computer studies.
The scientists also examined satellite data from the Atlantic region during the same periods, since pollution from North America follows the prevailing winds to Europe. But they did not find any similar pattern of cloud changes or increase in storm intensity.
The Pacific pollution also may affect other pervasive patterns of air circulation that shape world climate.
"If the trend to intensified storms in this region persists, it will likely have profound implications on climate change," said Robert McGraw, a senior atmospheric chemist at Brookhaven National Laboratory on Long Island, who was not involved in the study.
Among other consequences, the more energetic Pacific storm track could be carrying warmer air and more black soot farther north into the Canadian Arctic, where it may accelerate the melting of polar ice packs, the researchers said.
The researchers emphasized that it would take much more sustained study to understand the international climate ramifications.
Until recently, most scientists believed that, with its adverse effects on health and plant life, such aerosol pollution was mostly a local problem. If anything, it helped rather than hindered the climate -- at least in terms of global warming -- by offsetting the heat-trapping effects of greenhouse gases such as carbon dioxide and methane.
At low altitudes, the haze of aerosol particles reflects the sun's energy back into space, cooling Earth's surface slightly. At the same time, the particles help form brighter low-altitude clouds that also shield the surface from solar heat.
But once these tiny particles reach the upper atmosphere, they generate fierce downpours from super-cooled droplets and ice particles instead of gentle warm showers.
At monitoring sites along the U.S. West Coast, scientists have been detecting pollutants that originated from smokestacks and tailpipes thousands of miles to the west.
Recently, researchers at the University of Washington have captured traces of ozone, carbon monoxide, mercury and particulate matter from Asia at monitoring sites on Mt. Bachelor in Oregon and Cheeka Peak in Washington state.
Cliff and his colleagues have been picking up the telltale chemical signatures of Asian particulates and other pollutants at several monitoring sites north of San Francisco and, during the last year, around Southern California.
The pollutants, however, are suspended at high altitude. It is unclear how much of them reach ground level or what their direct effect on local weather might be.
"The air above Los Angeles is primarily from Asia," Cliff said. "Presumably that air has Asian pollution incorporated into it."
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lee.hotz@latimes.com
