14 December 2009

Brown Haze May Alter Rainy Days in Asia

Posted by Michael McFadden

Brown haze over northern India, from Jacques Descloitres, MODIS Land Rapid Response Team at NASA GSFCAfter visiting India on family trips, I’m no stranger to the thick brown haze that hangs in the air.  Soot from dirty cooking fires, diesel exhaust, and other particles form a dingy miasma that always took a toll on my lungs and ruined the (formerly) killer views from the Himalayan foothills.

But despite the haze’s omnipresence, it never occurred to me that sooty air might be altering the large-scale weather patterns in India. Yet new field studies, presented in an afternoon session on Monday (A13K, Aerosols and Climate in Asia II: Field Experiments), are suggesting exactly that: aerosols may be changing rain patterns across the subcontinent.

A group, led by J. R. Kulkarni of the Indian Institute of Tropical Meteorology in Pune and Daniel Rosenfeld of the Hebrew University in Jerusalem, found that lots of aerosols suppressed rain in small clouds, but caused harder rains from large, juicy clouds full of moisture.

To accomplish this task, the team flew from Kashmir in the north to the hi-tech hub of Bangalore in the south to measure everything from the concentration of aerosols to the size and concentration of cloud condensation nuclei, which are points where particulate pollution can seed raindrops. Then, they investigated how pollution altered the type of rain drops seeded in these clouds.

The team noticed more polluted clouds seeded tiny raindrop nuclei that evaporated too quickly to form rain. And in these clouds, the rain that did form came from higher up, at sub-freezing temperatures. That meant smaller clouds that don’t reach as high didn’t form rain at all, while large, towering clouds produced even more violent rains and thunderstorms.

Sadly, the team couldn’t find an area in India with “pristine” clouds to compare to the polluted ones.  The hazy layer could extend up to 6 kilometers into the atmosphere, and even after a heavy monsoon rain the haze was 4 kilometers high the next afternoon, Rosenfeld said.

The idea that aerosol pollution can alter the rains isn’t new; Rosenfeld and his colleagues suggested this hypothesis in a 2008 Science paper.  In another presentation, Zhanqing Li of College Park, Maryland, found hints of a similar trend across mainland China.

–Tia Ghose, UC Santa Cruz Science Communication Graduate Student