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the tropical low-pressure trough without a clear low-pressure centre (Type IV,
Fig. 6.2 d). For the central and southern Sahara and Sahel, for example, ridging
of the Azores and Libyan Highs increases the south-north pressure gradient and
leads to a surge in the northeasterly harmattan or Etesian winds that amongst other
areas affect the Bodélé Depression, the most important single dust source region on
the planet (Washington and Todd 2005 ; Washington et al. 2006 ; Klose et al. 2010 ;
Knippertz et al. 2011 ). Figures 6.4 c, d shows a typical case with multiple local dust
emission events across the entire Sahel as revealed by both satellite data and station
observations.
6.3.3
Dynamics and Character of Dust Fronts
The passage of dust-generating cold fronts discussed in the previous subsections
(particularly Types II and III) is typically associated with a marked drop in
temperature and visibility and increases in wind speed and pressure (e.g. Knippertz
and Fink 2006 ; Maghrabi et al. 2011 ). Depending on the season and region as well
as the evolution of the cyclone, however, the depth and shape of the frontal zone
can vary significantly. Smirnov et al. ( 1993 ) categorised two different types of cold
fronts in central Asia, which are common in many other dust regions around the
world: While some fronts have a more typical midlatitude character with a rearward
sloping frontal surface and deep frontal circulation associated with cloud formation
(Fig. 6.5 b), others develop into shallow density current-like features that may not
generate any clouds at all (Fig. 6.5 a; see cloud distributions in Figs. 6.3 and 6.4 ).
The deep frontal type may generate precipitation and it has been suggested that
evaporative cooling from moderate rain can help to sharpen the front, while not
suppressing dust generation through wet soils (Gläser et al. 2012 ). The shallow type
is frequently found in the equatorward portion of cold fronts or fronts in very dry
continental regions. Dust storms with a characteristic density-current shape as in
Fig. 6.5 a have been termed 'haboobs' (from the Arab word for wind), which is
also used for dust fronts generated by moist convection (see Sect. 6.4 ). Numerous
examples have been documented in the literature, particularly for Australia (Garratt
1984 ; Smith et al. 1995 ;Gibson 2007 ;Gabricetal. 2010 ).
Due to the slow geostrophic adjustment at low latitudes, the rapid increase of
pressure behind the cold front can lead to highly isallobaric winds, blowing almost
perpendicular to the isobars (see schematic in Fig. 6.2 c; Knippertz and Fink 2006 ;
Wain et al. 2006 ; Kaplan et al. 2011 ; Lewis et al. 2011 ). For dust storms in the
western USA Lewis et al. ( 2011 ) document a high degree of flow imbalance and
complex adjustment processes around strong upper-level jets streaks and troughs
with marked secondary circulations (see also Pauley et al. 1996 ), which ultimately
create the near-surface gust front (Fig. 6.6 ). In addition, recent modelling case
studies suggest that dust-radiative effects can modify the temperature, stability and
winds in synoptic-scale dust outbreaks and therefore alter the characteristics of
fronts (Alizadeh Choobari et al. 2012b ; Kocha et al. 2012 ).
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