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Summer 1998

Fire and rain: Indonesian fires ignite cloud study

Fire and smoke over Kalimantan. (Photo by Roelof Bruintjes.)

Last year's out-of-control forest fires on the Indonesian islands of Sumatra and Java and in Kalimantan, the Indonesian part of Borneo, were a disaster for all of Southeast Asia. In Indonesia they took lives and left hundreds of thousands of acres deforested, and throughout Southeast Asia they created massive atmospheric pollution and serious respiratory problems for an estimated 70 million people.

But out of the ashes came one bit of good: Indonesia and two neighbor nations collaborated in a scientific study of the smoke plumes. Called the Brief Assessment of the Effects of Smoke on Indonesian Clouds (BASIC), the two-week airborne experiment was the first collaborative effort of its kind in Southeast Asia. The project had two goals: to study any effects of the smoke aerosols on precipitation efficiency, and to decide which, if any, kind of cloud seeding could be used to mitigate drought.

Roelof Bruintjes (NCAR Research Aviation Program, or RAP) was the only westerner participating in the program. He was asked because of his several years' interactions with cloud seeding personnel in Indonesia and also in Thailand, where last year he and Roy Rasmussen (also in RAP) helped train scientists who had just acquired two King Air research aircraft. The other BASIC participants were from Indonesia's national weather service, BPP Teknologi (the research arm of the government), and LAPAN (space agency) and Thailand's Bureau of Royal Rainmaking and Agricultural Aviation (BRRAA) and space agency, as well as a representative from Malaysia.

"This was the first time so many Southeast Asian nations had collaborated on a scientific experiment," said Bruintjes, who has just completed a similar ten-day study of Mexican wildfires. "The program was organized on a very short time scale due to the significant smoke event in Indonesia. Although the measurement program only lasted for ten days, most of the objectives were achieved, and an excellent data set was collected."

Farmers, plantation owners, and logging companies set fires every year in Indonesia to clear forest land. They time the fires to occur before the monsoon and count on its rains to end the burning. But last year, with a strong El Niño changing the weather regime, the monsoon rains never arrived. By August and September "the fires erupted," Bruintjes said. Satellite images from over Kalimantan at this time show many areas completely obscured by a dense haze. During the fall, Indonesia and her neighbors quickly planned BASIC. The field phase took place in mid-December.

By that time, most of the fires had been put out with the help of international aid, but "we could still find fires burning and areas of smoke," said Bruintjes. The Thai King Airs flew for more than 30 hours during the program, making 12 flights over Kalimantan, Sumatra, and Java. BASIC was the first time the Indonesian government had allowed unrestricted flights over these islands. The researchers sampled smoke plumes, clouds without smoke, and clouds in an area where cloud seeding was taking place.

Cloud seeding probably doesn't spring to mind when considering lush, tropical Southeast Asia. Bruintjes explains, "With rice farming, the agriculture is so water-intensive that even a 5% reduction in rainfall means a drought." Additionally, Thailand has lost rainfall because of extensive deforestation, and Java suffers because all of its rivers are short and heavily polluted by human use. With problems like these on their hands, most countries in the region have been seeding clouds for decades, although the methods they use have little scientific basis.

Since cloud droplets coalesce around nuclei such as soot particles, one might think that adding more aerosol particles to a cloud would automatically mean more rain. However, nothing about the atmosphere is quite that predictable. The BASIC scientists had hints of what to expect from an earlier study of smoke from biomass burning in the Amazon Basin. Yoram Kaufman (NASA) and Teruyuki Nakajima (University of Tokyo) found that the additional aerosols over the Amazon did indeed lead to more droplets in clouds. However, these droplets were about two-thirds the size of those in clouds not affected by the burning--9 versus 14 microns in radius. The smaller-sized droplets weren't heavy enough to rain out, so the smoke-affected clouds produced less rain than the smoke-free ones.

Although data analysis for BASIC is not complete, the data show that, as in the Amazon case, the cloud droplets near the Indonesian cloudtops were considerably smaller than in smoke-free clouds. The scientists don't know whether the Asian clouds will behave in the same way as the Amazon ones, but if so, it could mean that the smoky Indonesian clouds are not able to develop rain as well as unaffected clouds.

Besides the data collected on the flights, BASIC scientists are using measurements from NOAA's advanced very high resolution radiometer (AVHRR) satellite instrument. Data analysis from BASIC is being coordinated by NCAR. Flight data are being analyzed in Indonesia, Thailand, and at NCAR; AVHRR data are being analyzed at the Desert Research Institute in Nevada and Hebrew University (Jerusalem, Israel), under the leadership of Melanie Wetzel and Danny Rosenfeld, respectively.

Bruintjes hopes BASIC will be only the precursor of a longer-term effort in Southeast Asia to study both natural aerosols and those produced by pollution, biomass burning, and cloud seeding, and to determine their effects on rain development in clouds. The current economic problems in the entire region have dimmed hopes for a locally funded program, but discussion with donor nations and the Association of Southeast Asian Nations (ASEAN) are continuing.

For further information about BASIC, contact Bruintjes (303-497-8909 or roelof@ucar.edu). Kaufman and Nakajima's 1993 paper, "Effect of Amazon smoke on cloud microphysics and albedo analysis from satellite imagery," was published in the Journal of Applied Meteorology 32(4), 729-744. Also, see "Haze," a Web site with many links on the Southeast Asian fires.

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:55:01 MDT 2000