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Re: Dust Aloft Late Breaking-OT

I just found this report today addressing rain inhibitions from smoke in the
atmosphere.  It is a wee bit off topic  but addresses a recent thread about
atmospheric dust. Source: Cambridge Conference.

>From NASANews@hq.nasa.gov
David E. Steitz
Headquarters, Washington, DC                  Oct. 5, 1999
(Phone:  202/358-1730)

Allen Kenitzer
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-2806)

Harvey Leifert
American Geophysical Union, Washington, DC
(Phone:  202/777-7507)

RELEASE:  99-110


     For the first time, researchers have proven that smoke from 
forest fires inhibits rainfall. The findings, to be published in 
the Oct. 15 issue of Geophysical Research Letters, are based on an 
extensive analysis of data taken from NASA's Tropical Rainfall 
Measuring Mission (TRMM) spacecraft.

     The study shows that the "warm rain" processes that often 
create rain in tropical clouds are practically shut off when the 
clouds are polluted with heavy smoke from forest fires.  In these 
clouds, scientists found, the cloud tops must grow considerably 
above the freezing level (16,000 feet) in order for them to start 
producing rain by an alternative mechanism.

     "We've seen evidence of decreased precipitation in clouds 
contaminated by smoke, but it wasn't until now that we had direct 
evidence showing that smoke actually suppresses precipitation 
completely from certain clouds," said Dr. Daniel Rosenfeld, the 
paper's author and a TRMM scientist at the Institute of Earth 
Sciences, Hebrew University of Jerusalem.

     Scientists have a keen interest in how changes in global 
precipitation affect human activities, such as crop production, 
and the global rainfall weather pattern.  More precise information 
about rainfall and its variability is crucial to understanding the 
global climate and predicting climate change.

     In his paper, Rosenfeld highlights one specific area: 
Kalimantan, Indonesia.  During the satellite's overpass on March 
1, 1998, the southeastern portion of the island was engulfed in 
smoke, while the northwestern portion was relatively smoke free.  
The spacecraft's radar detected precipitation in smoke-free 
clouds, but almost none in the smoke-plagued clouds, showing the 
impact of smoke from fires on precipitation over the rainforest. 

     "It's important to note that this is not a unique case," said 
Rosenfeld.  "We observed and documented several other cases that 
showed similar behavior.  In some instances even less severe smoke 
concentration was found to have comparable impacts on clouds."

     This research further validates earlier studies on urban air 
pollution showing that pollution in Manila, the Philippines, has 
an effect similar to forest fires, according to Rosenfeld.

     "Findings such as these are making the first inroads into the 
difficult problem of understanding humanity's impacts on global 
precipitation," said Dr. Christian Kummerow, TRMM project 
scientist at NASA's Goddard Space Flight Center, Greenbelt, MD.

     Raindrops in the atmosphere grow by two means.  In the first, 
called the "warm rain" process, a few cloud drops get large enough 
to start falling.  As they fall, they pick up other cloud drops 
until they become big enough to fall to Earth as raindrops.

     The second process requires ice particles and water colder 
than 32 degrees Fahrenheit.  Ice particles surrounded by this 
"supercooled" water may grow extremely rapidly as water freezes 
onto the ice core.  As these large ice particles fall, they 
eventually melt and become raindrops.

     Scientists have known for some time that smoke from burning 
vegetation suppresses rainfall, but did not know to what extent 
until now.  Thanks to TRMM observations, scientists are able to 
see both precipitation and cloud droplets over large areas, 
including clouds in and out of smoke plumes.

     TRMM has produced continuous data since December 1997.  
Tropical rainfall, which falls between 35 degrees north latitude 
and 35 degrees south latitude, comprises more than two-thirds of 
the rainfall on Earth.

     TRMM is a U.S.-Japanese mission and part of NASA's Earth 
Science Enterprise, a long-term research program designed to study 
the Earth's land, oceans, air, ice and life as a total system.   
Information and images from the TRMM mission are available on the 
Internet at URL:    http://trmm.gsfc.nasa.gov/

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