DROUGHTS
AGGRAVATED BY DUST IN THE WIND Windblown
desert dust can choke rain clouds, cutting rainfall hundreds of miles away. This
new discovery, made with the help of NASA satellites, suggests that droughts over
arid regions, such as central Africa, are made worse by damaging land and livestock
management that expand the desert. The
findings, reported in the Proceedings of the National Academy of Sciences, present
a new view of the decades-long drought in the African Sahel, which has been accompanied
by increasing levels of airborne dust during the rainy season. The
higher dust frequency is not necessarily a result of the decreased rainfall, but
rather its cause, according to scientists from Israel's Hebrew University and
the Weizmann Institute. "This impact of desert dust on rainfall was not known
before," says lead author Daniel Rosenfeld, Hebrew University, Jerusalem.
"Due to the large sizes of some of these dust particles, it had been assumed
that desert dust would enhance precipitation rather than decrease it." Scientists
had expected that the largest dust particles would form giant cloud condensation
nuclei, which produce larger cloud droplets that speed the formation of rain.
"Our laboratory analysis of the desert dust, however, showed that the particles
contained very little water-absorbing matter," says co-author Yinon Rudich
of the Weizmann Institute, Rehovot. "As a result, even large dust particles
form relatively small cloud droplets." Normal
rainfall droplet creation involves water vapor condensing on particles in clouds.
The droplets eventually coalesce together to form drops large enough to fall to
Earth. However, as more and more dust particles (aerosols) enter a rain cloud,
the same amount of water becomes spread out. These smaller water droplets scatter
and are prevented from coalescing and growing large enough for a raindrop. Thus,
the cloud yields less rainfall over the course of its lifetime. (image right). The
image on the left compares a normal rain producing cloud, with the lack of rain
produced from a cloud full of aerosols from biomass burning. The
research shows dust actually amplifies the process of creating deserts. Activities
that expose and disrupt topsoil, such as grazing and agricultural cultivation,
can increase the amount of dust blown into the air. More dust reaching rain clouds
produces less rainfall, which exacerbates the drought conditions and contributes
to the desertification of the landscape. Dust
and other types of aerosol particles blowing into clouds act as nuclei where water
vapor can condense to form cloud droplets. If a lot of dust enters a cloud, the
available water is spread over many small droplets. These small droplets grow
more slowly through collisions with one another to the size of a raindrop, and
the cloud yields less rainfall over the course of its lifetime. What
the researchers saw in two separate cases, using different satellite observations,
was that cloud droplets were smaller as dust concentrations increased. NASA's
Tropical Rainfall Measuring Mission (TRMM) spacecraft captured images of clouds
over the Atlantic Ocean off the coast of northern Africa during a major dust storm
last March. Droplet sizes steadily increased the farther the clouds were from
dust-filled air. Rain was falling only from the dust-free clouds even though all
the clouds contained equal amounts of water. The
researchers also observed similar behavior in clouds over the eastern Mediterranean
Sea in March 1998, using data from aircraft and a U.S. weather satellite. TRMM
is a joint 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. Each day, the TRMM spacecraft observes the Earth's
equatorial and tropical regions, including the southernmost United States and
all of Africa. Rosenfeld
has used TRMM observations in two other recent studies to show that aerosols from
biomass-burning smoke and urban air pollution also reduce rainfall. Combined with
the negative impact of desert dust, Rosenfeld believes the aerosol rainfall suppression
effect can have a major impact on regional and global climate. "The
recent observations of the impact on precipitation of all kinds of aerosols, each
with a major human contribution, show a major climate change issue that has nothing
to do with greenhouse gases," says Rosenfeld. "Still, this is perhaps
the climate-change effect with the greatest socio-economic impact on water-scarce
areas." Back
to Top |