Geoscience Reference
In-Depth Information
Forcing:
Nort
h Africa (5°-35°N, 18°
W-40
°E)
a
b
c
SW
-8.08
SW
7.38
SW
-15.47
LW
3.35
LW
-11.39
LW
14.74
Net
-4.73
Net
-4.00
Net
-0.73
TOA
Atm
Srf
-
2
0
-10
0
10
2
0
-2
0
-10
0
10
2
0
-2
0
-10
0
10
2
0
d
e
f
SW
-3.39
SW
6.99
SW
-10.38
LW
1.63
LW
-3.60
LW
5.23
Net
-1.76
Net
3.39
Net
-5.14
TOA
Atm
Srf
-20
-10
0
10
20
-20
-10
0
10
20
-20
-10
0
10
20
Fig. 13.2
Annual dust radiative forcing (Wm
2
) over North Africa at the (
left
) top of atmosphere
(
TOA
)and(
right
) surface (
Srf
), with the atmospheric flux divergence (
Atm
) calculated as their
difference (
center
). Forcing is calculated by Yoshioka et al. (
2007
) and using the dust concentration
of Miller et al. (
2006
) with optical properties compiled by Sinyuk et al. (
2003
)
sensitivity of both the forcing and response to the particle optical properties, but
may also be realistic in some regions due to the mineral composition of the dust
source. Longwave absorption for all three cases is based upon measurements by
Vo l z (
1973
).
Figure
13.1
illustrates the dependence of TOA forcing upon the surface
albedo. Forcing is positive over the Sahara desert, where the bright surface
reflects sunlight back into the dust plume, resulting in additional absorption
(Liao and Seinfeld
1998
). The region of positive forcing extends across the Sahel
to the Guinea coast for the cases with greater solar absorption (Fig.
13.1
b, c),
presumably due to the presence of dust among the bright clouds that accompany
the monsoon precipitation during this season. TOA forcing is negative where the
dust plume extends downwind over the subtropical Atlantic Ocean and Arabian
Sea, where the reflectivity of the aerosol particles is in contrast to the low albedo
of the ocean surface, especially for the case of small solar absorption (Fig.
13.1
a).
At the surface, the forcing is negative and generally marks the region of high dust
concentration (Fig.
13.1
d-f). Both local and global dimming increase by roughly a
factor of 2 for the range of shortwave absorption considered in the figure.
Figure
13.2
compares two calculations of annual forcing over North Africa,
based upon either Yoshioka et al. (
2007
) or the Sinyuk et al. (
2003
) compila-
tion illustrated in Fig.
13.1
. The prescribed particle index of refraction is nearly
identical in both calculations, so that forcing differences indicate the influence of
contrasting particle size distributions and environmental factors including surface
albedo, temperature, and humidity. In both cases, dust aerosols reduce the energy
captured at TOA, resulting in net forcing that is negative but of different magnitude
(Fig.
13.2
a, d). The compensation between scattering of insolation on the one hand
and solar absorption and reduced OLR on the other results in net TOA forcing that
is the small residual of opposing effects, magnifying the contrast in net forcing
between the two calculations that results from differences between the separate
shortwave and longwave fluxes. The largest difference between the models occurs in
Search WWH ::
Custom Search