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is not straightforward. Table
13.1
shows that evaporation actually increases as
increasing particle absorption causes further dimming of the surface, a dependence
corroborated by a previous version of the climate model and dust distribution (Miller
et al.
2004b
).
Xian (
2008
) describes one mechanism by which evaporation can increase with
shortwave absorption, noting that ocean evaporation depends upon forcing at TOA
in addition to the surface value. The evaporative anomaly ıE can be written using a
bulk formula:
ı
q
.SST/
q
S
ıE
D
C
D
j
u
0
j
dq
!
ˇ
ˇ
ˇ
ˇ
SST
ˇ
ˇ
ˇ
ˇ
T
S
dT
r
dq
dT
D
C
D
j
u
0
j
ıSST
ıT
S
dq
!
ıT
S
;
ˇ
ˇ
ˇ
ˇ
SST
ˇ
ˇ
ˇ
ˇ
SST
ˇ
ˇ
ˇ
ˇ
T
S
dq
dT
dT
r
dq
dT
D
C
D
j
u
0
j
ı.SST
T
S
/
C
C
D
j
u
0
j
(13.5)
where is the atmospheric density, C
D
is a drag coefficient, and
u
0
is the surface
wind speed, all of which we assume constant for purposes of discussion. The
quantities q
S
and q
.SST/ represent the surface specific humidity and the saturation
specific humidity at the sea surface temperature SST, respectively. Equation
13.5
is
derived by expressing each specific humidity in terms of the unperturbed temper-
ature and linearizing. Xian (
2008
) notes that TOA forcing influences evaporation
through its effect upon the anomalous surface air temperature, the final term in
(
13.5
). In convecting regions, ıT
S
is directly related to the TOA forcing according
to (
13.1
)and(
13.3
), so that positive F
T
increases evaporation.
The sensible and longwave fluxes can be parameterized according to the differ-
ence in temperature between the surface and near-surface air (a representation that
is conventional for the sensible flux but more approximate for longwave radiation).
Then, from (
13.5
) it can be shown that
D
D
F
T
C
ı.LE/
ı.LP/
F
S
;
(13.6)
where ı.LP/ is the global precipitation anomaly multiplied by L.Both and are
positive (with
0:5 for typical tropical values), so that evaporation is reduced
by dimming of the surface (F
S
<0) but increased by positive TOA forcing.
Equation (
13.6
) applies only over the ocean or else vegetated regions with abun-
dant reservoirs of soil moisture, where the evaporative anomaly can be represented
by (
13.5
). However, ESMs show that evaporation in arid regions like the Sahel can
also increase with shortwave absorption, given an increase in precipitation (Miller
et al.
2004b
). Soil moisture supplied by a precipitation anomaly allows the surface
to respond to incident radiation with a greater proportion of evaporation. Thus,
evaporation may increase beneath the aerosol layer, despite the reduction of the
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