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a
b
c
d
Fig. 6.4
Examples of synoptic dust storm Types III (
top
)andIV(
bottom
). (
a
) Meteosat dust
product (as in Fig.
6.3
c) at 1800 UTC 24 April 2013. (
b
) Simulated mean sea-level pressure
(contour interval of 4 hPa) and surface PM
10
concentrations (up to 7,500 mg m
3
) at 1600 UTC
on 22 September 2009 (Fig. 2a in Alizadeh Choobari et al.
2012a
). (
c
) Mean sea-level pressure and
(
d
) Meteosat dust product overlaid with station reports of 'dust in suspension' (
black
) and “dust
raised by the wind” (
orange
) at 1200 UTC on 21 January 2008 (Figs. 7a, d from Knippertz et al.
2011
)
2006
; Tulet et al.
2008
; Cavazos et al.
2009
; Mallet et al.
2009
; Kocha et al.
2012
;
Stanelle et al.
2010
). The satellite image of a recent case in April 2013 (Fig.
6.4
a)
shows a well-developed cyclonic cloud feature near the coasts of Algeria, Tunisia
and Libya, while an elongated cloud-free band of enhanced dust loadings stretches
far into the Sahara along the trailing cold front.
Examples of cold northerlies associated with winter anticyclones and fronts
have been documented for Central Asia (Orlovsky et al.
2005
),theMiddleEast
and Arabian Peninsula (de Villiers and van Heerden
2011
; Maghrabi et al.
2011
;
sometimes referred to as Nashi dust storms), and North America (Novlan et al.
2007
). Being surrounded by ocean on the poleward side, Australia is not as strongly
affected by wintertime continental high-pressure systems as the African-Asian dust
belt and North America, but still many cyclonic storms generate marked high-
pressure ridges behind the cold front with dust being lifted and transported in
post-frontal southerlies like in the extreme cases of October 2002 and September
2009 (Fig.
6.4
b; Leslie and Speer
2006
;Shaoetal.
2007
; Baddock et al.
2009
;
Alizadeh Choobari et al.
2012a
).
In source regions equatorward of the mean subtropical anticyclones dust can
be generated through an intensification of the trade wind circulation towards
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