Geoscience Reference
In-Depth Information
1990, was tracked across North America using satellite
imagery. It arrived in the French Alps ten days later -
having traveled a distance of 20 000 km.
For mation
Dust storms commonly form as the result of the
passage of cold fronts linked to mobile polar highs
across arid or drought-affected plains. The passage of
these fronts can give rise to dust storms lasting for
several days. In the northern Sahara region, dust
storms are mainly produced by complex depressions
associated with the westerlies. The depressions origi-
nate in winter in the eastern Mediterranean Sea or the
Atlantic Ocean. In the southern Sahara, low-pressure
fronts are responsible for harmattan winds. These
depressions track south during the northern hemi-
sphere winter and easterly during the summer. A
particularly common feature of dust storms in North
Africa is the haboob , generated by cold downbursts
associated with large convective cells, and linked to the
advance of the intertropical convergence. These types
of winds also account for many of the dust storms in
the Gobi and Thar deserts in central Asia, in the
Sudan, and in Arizona. The seasonal movement of
the monsoon into the Sudan region of the Sahara is
responsible for dust storms in eastern Africa, while the
Indian monsoon controls the timing of dust storms on
the Arabian Peninsula. Dust transport out of northern
Africa to the Mediterranean Sea and the Atlantic Ocean
is correlated to stages in the North Atlantic Oscillation
(NAO) described in the previous chapter. When the
NAO index is high in winter, dust transport out of
Africa is greater. East of the Mediterranean, depres-
sions moving across Turkey and northern Iraq generate
most dust storms. In Australia, dust storms are gener-
ated by the passage of intense, cold fronts across the
continent, following the desiccating effects of hot, dry
winds produced by the subtropical jet stream in the lee
of high-pressure cells. The Victorian dust storm of
8 February 1983 (Figure 3.29), and the devastating
bushfires of Ash Wednesday exactly one week later,
both originated via this mechanism. In mountainous
regions such as western North America, katabatic
winds can generate dust storms in places such as
California and along Colorado's Front Ranges.
Similarly, cold, dense winds flowing off the mountains
in the Himalayas and the Hindu Kush generate dust
storms southward over the Arabian Sea. Such localized
winds also produce dust storms in the valleys leading
Fig. 3.29 Dust storm from the mallee country of western Victoria about
to bear down on the city of Melbourne for only the second
time in a century, 8 February 1983 (photograph courtesy
the Australian Bureau of Meteorology).
from the Argentine foothills. There is a correlation
worldwide between dust storms and rainfall. Where
rainfall exceeds 1000 mm yr -l , dust storms occur on less
than one day per year, mainly because vegetation
prevents silt entrainment by winds into the atmos-
phere, and convective instability tends to give rise to
thunderstorms rather than dust storms. Dust storm
frequency is greatest where rainfall lies between
100 and 200 mm. Except under unusual conditions,
dust storms in both hemispheres tend to have their
greatest frequency of occurrence in late spring
and early summer. This pattern corresponds to the
changeover from minimum rainfall in early spring to
intense evaporation in summer. In Australia, however,
the pattern also represents the most likely period of
drought associated with the Southern Oscillation.
Where cold fronts form dust storms, cold air forces
warm air up rapidly, giving a convex, lobed appearance
to the front of the dust storm. As the air is forced up, it
sets up a vortex that tends to depress the top of the
cold front and further enhance the lobe-like appear-
ance of the storm. The classic image is one of a sharp
wall of sediment moving across the landscape with
turbulent overturning of dense clouds of dust (see
Figure 3.29). Wind speeds can obtain consistent
velocities exceeding 60 km hr -1 or 30 m s -1 . Material
the size of coarse sand can be moved via saltation in
the first tens of centimeters above the ground surface.
Fine sand can be moved within 2 m of the ground,
while silt-sized particles can be carried to heights in
excess of 1.5 km. Clay-sized particles can be suspended
throughout the depth of the troposphere and carried
thousands of kilometers. Most of the dust is suspended
 
Search WWH ::




Custom Search