aerosols through decreased low-level cloud cover
and other studies found that cooling effects
through increased cloud coverage [81, 82]. The
height of the absorbing layer seems to impact on
the sign of the effect.
Other effects include the cloud glaciation effect
where an increase in anthropogenic ice nuclei can
lead to an increase in cloud glaciation [83], the
deactivation effect where anthropogenic sulfate
coats ice nuclei making them inactive [84], and
the thermodynamic effect where smaller droplets
freeze less efficiently than larger droplets [85].
From the IPCC AR4, the most recent consensus on the first indirect effect of warm clouds [86]
estimated a range between À0.22 and
À1.85 W m
À2 , with a best estimate of
À0.7 W m
À2 . The combined first and second
indirect effect was estimated to have a radiative
forcing between À0.2 and À2.3 W m
À2 , illustrating the difficulties in achieving convergence of
these models as the level of complexity increases.
Aerosol–Cloud–Precipitation Interactions
Although the second indirect effect, through
changes in cloud microphysics, has been demonstrated to reduce precipitation with increased
anthropogenic aerosol, this effect typically applies
to warm stratiform clouds and shallow cumulus
clouds. Similarly, the radiative effects generally
act to suppress precipitation through decreasing
the amount of solar radiation reaching the surface
thereby reducing the amount of energy available
to evaporate water and energize convective
clouds. The radiation not reflected back to space
is partly absorbed in the atmosphere in the presence of absorbing carbonaceous aerosol leading to
a warming of the air above the surface. This
warming stabilizes the lower atmosphere and suppresses convection and the generation of convective clouds.
Some studies have also shown [87] that
increases in cloud nuclei can invigorate deep convective rain clouds with warm cloud bases
(T > 15
C). In such clouds, the auto-conversion
processes (coalescence of droplets into raindrop
sizes) are delayed and hence the onset of precipitation in the cloud is also delayed. This results in
more water ascending to higher altitudes and
colder temperatures (T < 0
C). Through the
delay of precipitation, the additional latent heat
release invigorates the cloud. In addition, by not
raining early, the condensed water can form ice
precipitation particles that release the latent heat
of freezing aloft and these reabsorb heat at lower
levels where they melt after falling [88]. This
process leads to consumption of more convective
available potential energy (CAPE) which would
then be converted to an equally greater amount of
released kinetic energy that could invigorate convection and lead to a greater convective overturning, more precipitation, and deeper depletion
of the static instability.
The increased amount of aerosol has two competing effects: one radiative, which suppresses
convection (and consequently rainfall) through
absorption and warming of atmospheric layers
relative to the surface, and the other, which can
lead to invigoration of updrafts, leading to an
increase in precipitation. These two processes
have typically been treated separately leading to
opposing views on the impact of aerosols on precipitation. This competing effect was elucidated
recently [89] by looking at the combined effect of
increasing aerosol and cloud condensation nuclei
concentrations on both microphysical and radiative properties. They modeled typical warm tropical convective clouds and found that the
microphysical invigoration effect was at a maximum for moderate cloud condensation nuclei concentrations, above which the impact of additional
nuclei was reduced. However, as the cloud condensation nuclei concentrations were linearly correlated to AOD, AOD progressively increased
with increasing cloud condensation nuclei concentrations which reduced the solar flux which in
turn energized convection reaching the surface.
As a result, with increasing aerosol loads beyond
the optimum, the weakening of the microphysical
invigoration is reinforced by the suppressive
effect of reduced surface heating. This conceptual
model is illustrated in Fig. 18 where it is seen that
starting out with clean (low) nuclei concentrations
(N ~ 100 cm
À3 ), the addition of more nuclei has
the effect of increasing the released convective
energy up to a maximum point corresponding to
a cloud condensation nuclei concentration of the
268
Aerosol in Global Atmosphere
and other studies found that cooling effects
through increased cloud coverage [81, 82]. The
height of the absorbing layer seems to impact on
the sign of the effect.
Other effects include the cloud glaciation effect
where an increase in anthropogenic ice nuclei can
lead to an increase in cloud glaciation [83], the
deactivation effect where anthropogenic sulfate
coats ice nuclei making them inactive [84], and
the thermodynamic effect where smaller droplets
freeze less efficiently than larger droplets [85].
From the IPCC AR4, the most recent consensus on the first indirect effect of warm clouds [86]
estimated a range between À0.22 and
À1.85 W m
À2 , with a best estimate of
À0.7 W m
À2 . The combined first and second
indirect effect was estimated to have a radiative
forcing between À0.2 and À2.3 W m
À2 , illustrating the difficulties in achieving convergence of
these models as the level of complexity increases.
Aerosol–Cloud–Precipitation Interactions
Although the second indirect effect, through
changes in cloud microphysics, has been demonstrated to reduce precipitation with increased
anthropogenic aerosol, this effect typically applies
to warm stratiform clouds and shallow cumulus
clouds. Similarly, the radiative effects generally
act to suppress precipitation through decreasing
the amount of solar radiation reaching the surface
thereby reducing the amount of energy available
to evaporate water and energize convective
clouds. The radiation not reflected back to space
is partly absorbed in the atmosphere in the presence of absorbing carbonaceous aerosol leading to
a warming of the air above the surface. This
warming stabilizes the lower atmosphere and suppresses convection and the generation of convective clouds.
Some studies have also shown [87] that
increases in cloud nuclei can invigorate deep convective rain clouds with warm cloud bases
(T > 15
C). In such clouds, the auto-conversion
processes (coalescence of droplets into raindrop
sizes) are delayed and hence the onset of precipitation in the cloud is also delayed. This results in
more water ascending to higher altitudes and
colder temperatures (T < 0
C). Through the
delay of precipitation, the additional latent heat
release invigorates the cloud. In addition, by not
raining early, the condensed water can form ice
precipitation particles that release the latent heat
of freezing aloft and these reabsorb heat at lower
levels where they melt after falling [88]. This
process leads to consumption of more convective
available potential energy (CAPE) which would
then be converted to an equally greater amount of
released kinetic energy that could invigorate convection and lead to a greater convective overturning, more precipitation, and deeper depletion
of the static instability.
The increased amount of aerosol has two competing effects: one radiative, which suppresses
convection (and consequently rainfall) through
absorption and warming of atmospheric layers
relative to the surface, and the other, which can
lead to invigoration of updrafts, leading to an
increase in precipitation. These two processes
have typically been treated separately leading to
opposing views on the impact of aerosols on precipitation. This competing effect was elucidated
recently [89] by looking at the combined effect of
increasing aerosol and cloud condensation nuclei
concentrations on both microphysical and radiative properties. They modeled typical warm tropical convective clouds and found that the
microphysical invigoration effect was at a maximum for moderate cloud condensation nuclei concentrations, above which the impact of additional
nuclei was reduced. However, as the cloud condensation nuclei concentrations were linearly correlated to AOD, AOD progressively increased
with increasing cloud condensation nuclei concentrations which reduced the solar flux which in
turn energized convection reaching the surface.
As a result, with increasing aerosol loads beyond
the optimum, the weakening of the microphysical
invigoration is reinforced by the suppressive
effect of reduced surface heating. This conceptual
model is illustrated in Fig. 18 where it is seen that
starting out with clean (low) nuclei concentrations
(N ~ 100 cm
À3 ), the addition of more nuclei has
the effect of increasing the released convective
energy up to a maximum point corresponding to
a cloud condensation nuclei concentration of the
268
Aerosol in Global Atmosphere