The effect of work-week pollution on weekly weather patterns has been a topic of research for some time. Providing new evidence that bolsters a pollution-precipitation connection, recent rainfall data recorded from space indicate that summertime storms in the southeastern United States shed more rainfall midweek than on weekends.
The findings, from NASA's Tropical Rainfall Measuring Mission satellite (TRMM), are from a study led by Thomas Bell, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. He and his colleagues reported their results in a January issue of the Journal of Geophysical Research-Atmospheres.
What they found was that midweek storms tend to be stronger with more rain and span a larger area across the Southeast than on weekends, which were dryer and calmer. To identify any kind of significant weekly rainfall trend, Bell looked at the big picture from Earth's orbit. The data from TRMM were used to estimate daily summertime rainfall averages from 1998 to 2005 across the entire Southeast.
The researchers found that, on average, it rains more between Tuesday and Thursday than from Saturday through Monday. Newly analyzed satellite data show that summer 2007 echoes the midweek trend with peak rainfall occurring late on Thursdays. However, midweek increases in rainfall are more significant in the afternoon, when the conditions for summertime storms are in place. Bell noted that afternoon rainfall peaks on Tuesday, with 1.8 times more rainfall than on Saturday, which experiences the least amount of afternoon rain.
The team used ground-based data from rain gauges, along with vertical wind speed and cloud height measurements, to help confirm the weekly trend in rainfall observed from space.
"If two things happen at the same time, it doesn't mean one caused the other," Bell comments, on the correlation of pollution and amount of rainfall. "But it's well known that particulate matter has the potential to affect how clouds behave, and this kind of evidence makes the argument stronger for a link between pollution and heavier rainfall."
Researchers know clouds are "seeded" by particulate matter. [But] some researchers think increased pollution thwarts rainfall by dispersing the same amount of water over more of these cloud condensation nuclei, preventing the smaller droplets from growing large enough to fall as rain. When conditions are ripe for big storms, however, updrafts carry the smaller, pollution-seeded raindrops high into the atmosphere where they condense and freeze. "It's the freezing process that gives the storm an extra kick, causing it to grow larger and climb higher into the atmosphere," Bell says. That extra kick results in heavier rain.
The trend doesn't mean that it will always rain on weekday afternoons during summertime in the Southeast. Rather, "it's a tendency," says Bell. With the help of satellites, new insights into pollution's effect on weather may one day help improve the accuracy of rainfall forecasts, which Bell says, "probably underpredict rain during the week and overpredict rain on weekends." (SOURCE: AGU)
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ECHOES
"Metrorail condemns the behavior of commuters following delays caused by the storm, which it had no control over."
-THOKOZANI ZITHASHE, South African Press Association spokeswoman on angry commuters setting fire to two trains in Pretoria, South Africa, after severe storms delayed services. The storm brought trains to a standstill after the system broke down and the backup system was struck by lightning. Commuters became impatient and torched four train coaches. (SOURCE: Reuters)
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