Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 132

Fig. 6.7 Schematic depiction of a convective dust storm in the foothills of the Atlas Mountains in
northwestern Africa. The mountains serve as a trigger for deep moist convection. Evaporation on
the dry Saharan side leads to evaporative cooling and the generation of a dust density current. (Fig.
2 from Knippertz et al. 2007 )
Convective haboobs have been documented for all major dust source regions of
the planet. In particular the squall lines of the Sudan have long been connected to
this kind of dust storm during spring and summer (Sutton 1925 ; Farquharson 1937 ;
Freeman 1952 ), but haboobs frequently occur throughout the entire Sahel/southern
Sahara, where convective organisation is supported by shear underneath a midlevel
jet (Lawson 1971 ; Marsham et al. 2008a ; Williams et al. 2008 ; Knippertz and Todd
2010 ; Marsham et al. 2013a ; Roberts and Knippertz 2014 ). Other active haboob
regions during the pre-monsoon and monsoon seasons include northwestern India
(Andhi dust storms, see Middleton 1986 and reference to Joseph 1982 ), the Middle
East (Membery 1985 ; Offer and Goossens 2001 ; Miller et al. 2008 ) and the southern
Great Plains of North America (Orgill and Sehmel 1976 ; Chen and Fryrear 2002 ;
Novlan et al. 2007 ) as well as central Mexico (Jauregui 1989 ). Once the monsoon
has fully set in, the soil is typically too wet for dust generation, which can create a
negative correlation between dust and the frequency of thunderstorms on a seasonal
basis (Littmann 1991 ). Away from main monsoon rains, haboobs tend to occur in
the foothills of mountain ranges, which provide a trigger for convective initiation,
as for example along the Atlas chain in northern Africa (Fig. 6.7 ; Knippertz et al.
2007 ; Emmel et al. 2010 ) and in the southwestern USA (Orgill and Sehmel 1976 ;
Brazel and Nickling 1986 ; Bach et al. 1996 ). Convective dust storms appear to be
more localised and infrequent in China (Takemi 2005 ), Australia (Leslie and Speer
2006 ; Strong et al. 2011 ) and the northern Great Plains of the USA (Godon and
Todhunter 1998 ).
There is an ongoing debate on the relative importance of convective haboobs
(Engelstaedter and Washington 2008 ; Williams 2008 ). Recent modelling studies