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Knippertz 2008 ). The degree of decoupling depends on the nighttime radiative
cooling and therefore on cloud cover, column water vapour and dust loading. In
situations of very large pressure gradients (or weak stability), vertical shear can
drive mechanical mixing and lead to a breakdown of the NLLJ before sunrise
(for a more detailed discussion, see Knippertz and Todd 2012 ). In addition, the
diurnal cycle in surface heating, pressure and turbulence leads to a continental-scale
pulsation of the monsoon circulation (Parker et al. 2005 ; Abdou et al. 2010 )that
affects winds around the Saharan heat low. In higher latitudes, where NLLJs are less
prominent and where the strongest winds tend to be near the tropopause, the largest
downward momentum transport occurs when the PBL is deepest in the afternoon,
but such effects have been observed over Africa as well (e.g. Marsham et al. 2008b ;
Todd et al. 2013 ).
The mechanisms described above suggest a daytime maximum in dustiness in
most source regions. This has been confirmed by station observations around the
world (Mbourou et al. 1997 ; Offer and Goossens 2001 ; Natsagdory et al. 2003 ;
Orlovsky et al. 2005 ; Marticorena et al. 2010 ). While North American stations tend
to show an afternoon maximum when the PBL is deepest (Orgill and Sehmel 1976 ;
Hahnenberger and Nicoll 2012 ), many areas in northern Africa show morning peaks
related to the breakdown of the NLLJ, in particular in winter and spring (Mbourou
et al. 1997 ; Washington et al. 2006 ), while summer has an important contribution
from afternoon convective storms (Marticorena et al. 2010 ; Marsham et al. 2013a ).
Recently, satellite products have become available whose time resolution allows an
assessment of the diurnal cycle. Twice-daily time series of MODIS data revealed
a strong diurnal pulsing of emission from the Bodélé Depression during winter,
indicating discrete 'packets' of emission during daytime hours (Koren and Kaufman
2004 ). The beginning of dust plume trajectories from 15-min Meteosat imagery
suggests a dominance of the mid-morning period in all North African source regions
in all seasons (Schepanski et al. 2007 , 2009 ), while estimates based on re-analysis
data suggest a fairly small contribution of NLLJs along the Mediterranean Coast in
winter and spring (Fig. 6.9 b). The NLLJs over northern Africa are typically related
to the harmattan and Etesian winds in winter and the Saharan heat low in summer
(Fiedler et al. 2013 ).
However, satellite retrievals struggle to detect very shallow dust layers, dust over
bright surfaces or dust below clouds, leading to some important biases (Kocha
et al. 2013 ; see also Chap. 7 ) . Therefore the diurnal cycle in dust emission
has also been investigated in high-resolution models. These reveal an important
contribution from afternoon and nighttime convective systems (Marsham et al.
2011 ;seeFig. 6.10 ), which can even support NLLJ formation through the creation
of pressure gradients around ageing cold pools from the previous day (Heinold et al.
2013 ).
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