Western US's stronger storms traced to Asia pollution

Winter cyclones may be gaining intensity as they pick up soot and other particles, says a US research team.

from the March 12, 2007 edition

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The US and Europe have imposed tough air-pollution laws, and, indeed, no such cloud trends were seen over the north Atlantic during the study period. Asian countries, with their rapidly growing economies, have been slow to follow suit.

For this study, the scientists used satellite photos of winter storm clouds from two sets of satellites. They used images taken throughout each January between 1984 and 2005.

Then they compared cloud information averaged over the first 10 years with cloud data from the second 10 years. The number of so-called deep convective clouds – driven by strong updrafts and typical of intense thunderstorms – rose by 20 to 50 percent over much of the region. Then the team used one of the latest computer models capable of simulating cloud activity to test the notion that aerosols might be driving the change.

The team found a marked increase in deep convective clouds when the aerosols were included. Cloud formation reverted to more normal types when aerosols were left out of the calculations.

Some researchers say the team's explanation for the increase in convective clouds is plausible. But they also note that the physical processes involved challenge the ability of computer models to simulate them.

Wind patterns are changing

Still, the results imply a one-two punch, adds Henry Diaz, an atmospheric scientist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.

Long-term changes in large-scale wind patterns in the north Pacific, driven by rising ocean temperatures in the tropics, appear to be "forcing more intense cyclones, and those are better able to use the particulates to enhance themselves," he says.

Aerosols' effects, however, may be on the decline. Last year, China adopted a five-year plan than calls for a 20 percent reduction in sulfur-dioxide emissions by 2010.

Last December, researchers at the Argonne National Laboratory in Argonne, Ill., published a study indicating that soot and sulfate aerosols over China have been declining since the mid-1990s. The change, they suggest, came from increased use of cleaner ways to burn coal in power plants and factories and a reduction in household use of solid fuels for heat and cooking.

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