Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 125

intrusions, cold surges or even pressure surges with dust typically being lifted and
transported in northerly or northwesterly winds (southerly or southwesterly in the
southern hemisphere) (Fig. 6.2 c). In addition waves on the upper-level subtropical
jet can intensify and shift subtropical anticyclones into nearby continental regions
and increase winds there without a surface cyclogenesis. These situations tend
to enhance the trade wind circulations creating equatorward and westward dust
transport (Fig. 6.2 d). The following subsections provide more detail and regional
examples of these four types.

6.3.1 Cyclone Dominated Dust Events

Cyclonic dust storms are most prominent near stormtrack areas, usually mainly
affecting the poleward parts of hot deserts. Figure 6.3 a shows an example of an
intense cyclonic dust storm over India and Pakistan in April 2005 (Badarinath
et al. 2007 ; Prasad and Singh 2007 ). The associated low-pressure system forms
on the eastern side of an upper-trough extending equatorwards from a more
intense midlatitude system (Fig. 6.3 b). There are sharp pressure gradients on the
northwestern and northeastern flanks of the low, both associated with dust activity
(Fig. 6.3 a). The formation of such a cyclone over hot and dry land is typically
associated with frontal and cloud structures that are much less pronounced than
those of a typical midlatitude cyclone (Fig. 6.3 a). A similar phenomenon is the
Khamsin (Libya and Egypt) or Sharav cyclone (Middle East) that forms along the
northern margin of the Sahara (Alpert and Ziv 1989 ;BouKarametal. 2010 ). These
systems are often related to lee cyclogenesis to the south of the Atlas Mountains and
then track eastward along the Mediterranean coast. Many tracks turn northwards
into Turkey (Hannachi et al. 2010 ), but some systems continue moving eastwards
(Saeed and Al-Dashti 2011 ). Incorporating moisture from the Mediterranean Sea
can lead to a rapid intensification of the systems, associated with significant rainfall.
Khamsin cyclones often resemble Type I (Fig. 6.2 a) in their early stages. Other
examples are heat troughs over Australia (Strong et al. 2011 ).
Many low-latitude cyclones, however, develop sharp cold fronts with dust
emission in the westerly or even northwesterly (southwesterly in the southern
hemisphere) flow behind them (Type II, see Fig. 6.2 b). Khamsin cyclones in their
late stages are often of this type as the example shown in Fig. 6.3 c with its core
over Turkey and widespread dust emission behind the cold front over Libya and
Egypt. In a similar way, dust storms from the northern Negev to Iraq are associated
with cold fronts from lows between Cyprus and the Turkish-Syrian border (Offer
and Goossens 2001 ; Liu et al. 2007 ; Saeed and Al-Dashti 2011 ). These situations
are sometimes referred to as winter Shamal in Iraq. Mobile cold fronts with dust
emission and transport to the west and south also dominate dust sources in the
interior parts of southern and central Australia in spring and summer (Ekström et al.
2004 ; McTainsh et al. 2005 ; Leslie and Speer 2006 ;Wainetal. 2006 ;Shaoetal.
2007 ; Baddock et al. 2009 ;Gabricetal. 2010 ; Strong et al. 2011 ). Dust storms over