Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 127

Novlan et al. 2007 ; Rivera Rivera et al. 2009 ), and southwestern USA (Brazel and
Nickling 1986 ; Bach et al. 1996 ). Dust events associated with mobile cyclones
and associated cold fronts are observed as far north as the northern Great Plains
(Godon and Todhunter 1998 ) and Canadian Prairies (Wheaton and Chakravarti
1990 ), and as far south as Mexico City (Jauregui 1989 ). Dust from Type II cyclones
is often transported polewards, particularly if the surface low is connected to a
midlevel trough that provides the extensive meridional flow for long-range transport.
Example cases have been documented for Europe (DeSouza-Machado et al. 2010 ;
Wiegand et al. 2011 ; Cabello et al. 2012 ; White et al. 2012 ) or the Asian states of
the former USSR (Middleton 1986 ).
During summertime, when monsoon circulations are strongest (see Sect. 6.2 ),
the midlatitude stormtrack is weak and shifted polewards, and only dust sources
located at higher latitudes are occasionally affected by cyclonic dust storms. For
example, Orlovsky et al. ( 2005 ) describe intrusions of cooler air into the thermal
depression over southeastern Turkmenistan. Pre-frontal troughs can occasionally
affect coastal areas in south and west Australia during summer and autumn in
drought conditions (Garratt 1984 ; Ekström et al. 2004 ;Gibson 2007 ; Strong et al.
2011 ). Weak summer cyclones also affect the Canadian Prairies (Wheaton and
Chakravarti 1990 ). Tropical cyclones are frequent during this time of year but only
rarely and indirectly affect dust source regions (e.g. Brazel and Nickling 1986 ), as
they are generally associated with too much precipitation to allow dust generation.
Interesting examples of tropical synoptic-scale weather systems causing sum-
mertime dustiness are the cyclonic surface vortices associated with AEWs over the
Sahel/southern Sahara. AEWs also contribute to dust emission through convective
cold pools (see Sect. 6.4 ), favoured by the moist southerlies to the east of the
AEW trough. These are sometimes enhanced through interactions with subtropical
upper-level troughs (Knippertz and Todd 2010 ), which have been proposed to
create mobile, dust generating, cyclonic surface disturbances called Soudano-
Saharan Depressions, but this concept has recently been questioned on the basis of
modern re-analysis and satellite data (Schepanski and Knippertz 2011 ). In addition,
AEWs modify dust transport over the continent and downstream tropical Atlantic
(Karyampudi and Carlson 1988 ).

6.3.2 Anticyclone Dominated Dust Events

Some of the cyclonic storms affecting dust regions develop long, trailing cold fronts
with marked post-frontal anticyclogenesis, which dominates low-level pressure
patterns, winds and dust emission, typically taking place in equatorward and
eastward flow in these cases (Type III, Fig. 6.2 c). Through the deep penetration of
cold air and high winds into lower latitudes, such situations can create long-lasting
and extensive events with high dust amounts. Prominent examples of this type over
the Sahara occurred during 02-07 March 2004 (Knippertz and Fink 2006 ; Min et al.
2009 ; Mangold et al. 2011 ;Shaoetal. 2010 ) and 07-13 March 2006 (Slingo et al.