Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 342

positive TOA forcing by more absorbing minerals. A similar magnitude of the
precipitation response occurs despite much smaller atmospheric heating (i.e., a
weaker “heat pump”) associated with reflective aerosols.

13.4 Feedback of Climate Anomalies upon the Dust Cycle

13.4.1 Surface Wind Speed and Dust Mobilization

The entrance of dust into the atmosphere is a sensitive function of surface wind
speed (Chap. 5 ) . The circulations raising dust are described in Chap. 6 and by
Knippertz and Todd ( 2012 ). Here, we consider how dust perturbs the surface wind
that feeds back upon dust mobilization.
Several studies have found that dust radiative forcing reduces the mobilization
on the order of 10 % compared to a model where the dust distribution is calculated
but has no radiative effect (e.g., Perlwitz et al. 2001 ; Yue et al. 2011 ). Miller et al.
( 2004a )andPérezetal.( 2006 ) related the decrease in emission to a reduction of
eddy mixing within the boundary layer. Negative radiative forcing at the surface
reduces the flux of sensible heat from the ground that powers eddy mixing in the
arid regions that are dust sources. These eddies normally accelerate the surface wind
each morning by mixing down fast air that accelerated above the stable stratification
of the nocturnal boundary layer. The reduction of the surface wind speed by dust
radiative forcing is largest at midmorning when the boundary layer deepens most
rapidly, following warming of the surface. This is the time of day when dust
events are most frequently initiated in North Africa (N'Tchayi Mbourou et al. 1997 ;
Schepanski et al. 2009 ). Nocturnal warming of the surface by dust also contributes
to the slowing of the surface winds the following morning by reducing stratification
and the nocturnal acceleration of the winds aloft. The dependence of wind speed
upon surface forcing predicts that dust emission should increase in models with
positive forcing (Miller et al. 2004a ). An increase in emission has been found by
Woodage et al. ( 2010 ), although the precise mechanism has not been identified.
A few models calculate the intermittent increase of surface wind speed and
mobilization by eddies whose amplitude is parameterized in terms of the surface
flux of sensible heat (e.g., Lunt and Valdes 2002 ;Cakmuretal. 2004 ). A reduction
of eddy wind speed by negative surface forcing has been shown to cause a reduction
in dust emission that is comparable to the decrease by the explicitly resolved
model surface wind (Miller, unpublished results). Other studies have shown that
dust radiative forcing alters the regional distribution of surface pressure, increasing
mobilization where the surface wind is augmented (Heinold et al. 2007 ; Ahn et al.
2007 ).