Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 134

South America (Kurgansky et al. 2011 ), Australia (Hess and Spillane 1990 ;Oke
et al. 2007 ; Lyons et al. 2008 ) and the northern fringes of the Sahara (McGinnigle
1966 ; Mattsson et al. 1993 ; Ansmann et al. 2009 ). Dust devils are a fascinating
hydrodynamical problem, which has been investigated by many authors from a
theoretical (e.g. Renno et al. 1998 ; Kurgansky 2006 ) or more recently also from
a high-resolution modelling point of view (Kanak 2005 ; Ohno and Takemi 2010 ;
Raasch and Franke 2011 ; Sullivan and Patton 2011 ; Ito et al. 2013 ). Figure 6.8 b
shows an example of a model-generated dust devil indicated by the abruptly
increasing cyclonic winds and hot core. Both theoretical and observational studies
show large sensitivities of dust devil frequency, intensity and characteristics to
aspects such as sensible heat flux, background wind, PBL depth, surface roughness
and inhomogeneities, vegetation and terrain slope. The contribution of dust devils
and dusty plumes to the global dust emission (in particular of dust available for
long-range transport) is unknown. While extrapolations of limited observations in
North America by Koch and Renno ( 2005 ) suggest values of up to 35 %, Balme and
Greeley ( 2006 ) regard dust devils as a “nuisance level phenomenon”.

6.6 Diurnal Variations

Luo et al. ( 2004 ) show that 35-70 % of dust mobilisation in the world's major dust
source regions is associated with diurnal variability, dominating over synoptic-scale
variability in all major dust regions but East Asia and Australia, where cyclones
and fronts are of greater importance. Diurnal variations in the meteorological
mechanisms described in previous subsections have different causes, but are mostly
related to the build-up and decay of the PBL. This is most obvious for dust devils
and dusty plumes, which only occur during the hottest hours of the day, typically
from10to16LT(e.g.Ansmannetal. 2009 ). Deep moist convection typically
peaks in the afternoon hours, but the organisation into mesoscale convective systems
or regional scale circulations can extend the occurrence of haboobs well into the
night (Knippertz et al. 2007 ;Emmeletal. 2010 ; Marsham et al. 2011 , 2013a ;
Heinold et al. 2013 ). Daytime heating can also set up land-sea or mountain-valley
circulations that can be important in certain regions.
A large body of work discusses the importance of nocturnal low-level jets
(NLLJs) for dust emission (e.g. Knippertz 2008 ; Todd et al. 2008a,b ). These form
due to a decoupling of the air above the radiative surface inversion from surface
friction, which then oscillates around the geostrophic wind (Blackadar 1957 ;vande
Wiel et al. 2010 ). The amplitude of this inertial oscillation depends on the magnitude
of the ageostrophic component at the time of decoupling and therefore on the back-
ground pressure gradient, the latitude and the roughness of the underlying surface.
The oscillation period depends on the Coriolis parameter, favouring highly super-
geostrophic NLLJs around 25 ı latitude (Todd et al. 2008b ). After sunrise, the grow-
ing PBL mixes momentum from the jet level to the surface, which leads to gusty
winds and potentially dust emission (Fig. 6.9 a; Engelstaedter and Washington 2007 ;