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b
a
c
d
Fig. 6.2 Different generic types of synoptic-scale dust storms (here depicted for the northern
hemisphere): ( a ) Type I: Continental desert depression without a clear frontal structure; ( b ) Type
II: Well developed cyclone with dust emission behind (and sometimes also ahead of) the cold
front; ( c ) Type III: Weak cyclonic disturbance with long trailing cold front and strong post-frontal
ridge formation causing highly isallobaric flow; ( d ) Type IV: Intensified subtropical (or sometimes
mid-latitude) high-pressure system. Black lines are isobars with 'L' and 'H' marking low- and
high-pressure centres. Grey arrows show areas of strong surface winds with resulting dust plumes
in grey shading. Fronts are marked with standard symbols
Many case studies of significant dust events and statistical analyses of long-term
records have contributed to our understanding of the dominant synoptic types and
their seasonal variations. The different mechanisms of dust lifting and transport
documented in the literature can sometimes occur in isolation, but are often part of a
succession of events, leading to prolonged and widespread dust episodes, which can
last several days and affect areas of almost continental scale. Figure 6.2 categorises
the different types/stages of synoptic-scale dust events. Most of these occur from
late winter to early summer with spring being the most active season in large parts
of northern Africa, Asia, Australia and North America. This seasonality is related to
the transition from the winter to the summer circulation, which is often accompanied
by episodes of rapid equatorward transport of cold airmasses from high latitudes into
the subtropics, where most dust sources are located (Chap. 3 ) . In spring, the mid and
high latitudes are still covered with snow and sea ice, and sea surface temperatures
are at a minimum, while the dry subtropical landmasses heat up quickly in response
to the increased solar radiation, leading to large temperature contrasts. The resulting
baroclinicity provides energy for strong synoptic-scale disturbances that can lead
to cyclonic storms with sharp cold fronts. These often generate an eastward and
sometimes poleward transport of dust (Fig. 6.2 a, b).
The advection of air from high-latitude origin behind the cold front can lead to
rapid pressure increase and anticyclogenesis. These events have been termed cold
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