Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 327

the surface longwave forcing (Fig. 13.2 c, f). Yoshioka et al. ( 2007 ) calculate such a
large longwave value that the net surface forcing is small and even positive when the
regional average is restricted to the Sahara (not shown). In this region, dust increases
the net radiative flux incident upon the ground (Figures 2b and c of Yoshioka et al.
2007 ). As a consequence of the longwave forcing, the atmospheric forcing (equal to
the difference in forcing between TOA and the surface) has contrasting signs in the
two models (Fig. 13.2 b, e). The dust layer is heated according to Miller et al. ( 2006 ),
while this layer is cooled in Yoshioka et al. ( 2007 ). The contrasting estimates of the
sign of atmospheric forcing by dust have implications for the dynamical response,
as described in the following sections.
Shortwave forcing at the surface is smaller in the GISS ESM (Fig. 13.2 c, f), partly
as a result of its brighter desert surface (Yoshioka et al. 2007 ). The net solar flux
at this level is small due to reflection, so that any perturbation by aerosols is also
small (Liao and Seinfeld 1998 ). The warm, dry climate of the Sahara exaggerates
differences in surface longwave forcing between the models. The low humidity
allows dust particles to absorb longwave radiation with reduced competition from
water vapor, while high temperatures within the boundary layer increase downward
longwave emission by dust (Liao and Seinfeld 1998 ).
Near dust sources, aerosol radiative forcing is intermittently much larger than the
annual averages illustrated by Fig. 13.2 . Dust was mobilized along a cold front over
North Africa on March 8, 2006. Surface radiometers at Niamey, Niger, measured
a dimming of 250 Wm 2 , compared to an unperturbed value near 1;000 Wm 2
(Slingo et al. 2006 ). The sun was almost completely obscured, but the surface
remained illuminated by diffuse radiation scattered within the dust layer. Such large
episodic forcing results in an initial temperature response that is large compared to
the seasonal average, as described in the next section.

13.3 Dust Radiative Impacts upon Climate

13.3.1 Temperature

A dust outbreak interrupts the daily cycle of solar heating by dimming the surface.
Over land, the ground cools with the passage of the dust layer overhead, reducing
the upward transfer of heat by the longwave and sensible fluxes, as documented
by measurements in the Sahel (Miller et al. 2009 ). Measured temperature anomalies
can be attributed to dust using models. Within the Bodélé Depression, a prolific dust
source at the southern margin of the Sahara, midday temperature is reduced by a few
degrees K during dust outbreaks, while warmer nights result from the inhibition of
longwave cooling by the surface (Tegen et al. 2006 ).
Pérez et al. ( 2006 ) simulated the regional cooling accompanying a dust outbreak
that extended across North Africa during April 2002. Figure 13.3 a shows midday
dimming of the surface as large as 700 Wm 2 beneath the dust plume. Dust reduces