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Flood or Drought: How Do Aerosols Affect Precipitation?

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Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.
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Flood or Drought: How Do Aerosols
Affect Precipitation?
Daniel Rosenfeld,
1
*Ulrike Lohmann,
2
Graciela B. Raga,
3
Colin D. ODowd,
4
Markku Kulmala,
5
Sandro Fuzzi,
6
Anni Reissell,
5
Meinrat O. Andreae
7
Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on
cloud properties and the initiation of precipitation. Large concentrations of human-made
aerosols have been reported to both decrease and increase rainfall as a result of their radiative
and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain
out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds
evaporate much of their water before precipitation can occur, if they can form at all given the
reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model
that explains this apparent dichotomy.
Cloud physicists commonly classify the
characteristics of aerosols and clouds
into maritimeand continentalregimes,
where continentalhas become synonymous
with aerosol-laden and polluted.Indeed, aero-
sol concentrations in polluted air masses are
typically one to two orders of magnitude greater
than in pristine oceanic air (Fig. 1) (1). How-
ever, before humankind started to change the
environment, aerosol concentrations were not
much greater (up to double) over land than
over the oceans (1,2). Anthropogenic aerosols
alter Earths energy budget by scattering and
absorbing the solar radiation that energizes the
formation of clouds (35). Because all cloud
droplets must form on preexisting aerosol par-
ticles that act as cloud condensation nuclei (CCN),
increased aerosols also change the composi-
tion of clouds (i.e., the size distribution of cloud
droplets). This, in turn, determines to a large ex-
tent the precipitation-forming processes.
Precipitation plays a key role in the climate
system. About 37% of the energy input to the
atmosphere occurs by release of latent heat
from vapor that condenses into cloud drops
and ice crystals (6). Reevaporation of clouds
consumes back the released heat. When water
is precipitated to the surface, this heat is left in
the atmosphere and becomes available to ener-
gize convection and larger-scale atmospheric
circulation systems.
The dominance of anthropogenic aerosols
over much of the land area means that cloud com-
position, precipitation, the hydrological cycle,
and the atmospheric circulation systems are all
affected by both radiative and microphysical im-
pactsofaerosols,andarelikelytobeinadiffer-
ent state relative to the pre-industrial era.
The Opposing Effects of Aerosols
on Clouds and Precipitation
The radiative effects of aerosols on clouds most-
ly act to suppress precipitation, because they de-
crease the amount of solar radiation that reaches
the land surface, and therefore cause less heat to
be available for evaporating water and energiz-
ing convective rain clouds (7). The fraction of
radiation that is not reflected back to space by
the aerosols is absorbed into the atmosphere,
mainly by carbonaceous aerosols, leading to
heating of the air above the surface. This sta-
bilizes the low atmosphere and suppresses the
generation of convective clouds (5). The warmer
and drier air thus produces circulation systems
that redistribute the remaining precipitation (8,9).
For example, elevated dry convection was ob-
served to develop from the top of heavy smoke
palls from burning oil wells (10). Warming of
the lower troposphere by absorbing aerosols
can also strengthen the Asian summer monsoon
circulation and cause a local increase in precipi-
tation, despite the global reduction of evaporation
that compensates for greater radiative heating
by aerosols (11). In the case of bright aerosols
that mainly scatter the radiation back to space,
the consequent surface cooling also can alter
atmospheric circulation systems. It has been
suggested that this mechanism has cooled the
North Atlantic and hence pushed the Intertropical
Convergence Zone southward, thereby contrib-
uting to the drying in the Sahel (12,13).
Aerosols also have important microphysical
effects (14). Added CCN slow the conversion of
cloud drops into raindrops by nucleating larger
number concentrations of smaller drops, which
are slower to coalesce into raindrops or rime
onto ice hydrometeors (15,16). This effect was
shown to shut off precipitation from very shal-
low and short-lived clouds, as in the case of
REVIEW
1
Institute of Earth Sciences, Hebrew University of Jerusa-
lem, Jerusalem 91904, Israel.
2
Institute for Atmospheric
and Climate Science, ETH Zürich, 8092 Zürich, Switzerland.
3
Universidad Nacional Autónoma de México, Mexico City
04510, Mexico.
4
School of Physics and Centre for Climate
and Air Pollution Studies, Environmental Change Institute,
National University of Ireland, Galway, Ireland.
5
Depart-
ment of Physics, University of Helsinki, Post Office Box 64,
Helsinki 00014, Finland.
6
Istituto di Scienze dellAtmosfera
e del ClimaCNR, Bologna 40129, Italy.
7
Biogeochemistry
Department, Max Planck Institute for Chemistry, Post Office
Box 3060, D-55020 Mainz, Germany.
*To whom correspondence should be addressed. E-mail:
daniel.rosenfeld@huji.ac.il
y= 0.0027x0.643
R2= 0.87
0.010
0.100
1.000
10 100 1000 10,000
CCN0.4 (cm-3)
AOT
500
Remote marine
Remote continental
Polluted marine
Polluted continental
Fig. 1. Relations between observed aerosol optical thickness at 500 nm and CCN concentrations at
supersaturation of 0.4% from studies where these variables have been measured simultaneously, or
where data from nearby sites at comparable times were available. The error bars reflect the variability
of measurements within each study (standard deviations or quartiles). The equation of the regression
line between aerosol optical thickness (y)andCCN
0.4
(x) is given by the inset expression; Ris the correlation
coefficient. The aerosols exclude desert dust. [Adapted from (1)]
www.sciencemag.org SCIENCE VOL 321 5 SEPTEMBER 2008 1309
Downloaded from
smoke from ship smokestacks in otherwise
pristine clouds over the ocean (17). This created
the expectation that polluted areas would suffer
from reduced rainfall. On the other hand, it was
expected that accelerating the conversion of
cloud water to precipitation (i.e., increasing the
autoconversion rate) by cloud seeding would
enhance rainfall amounts. It turns out, however,
that polluted areas are not generally drier, and
rain enhancement by cloud seeding remains
inconclusive (18,19).
With the advent of satellite measurements,
it became possible to observe the larger pic-
ture of aerosol effects on clouds and precip-
itation. (We exclude the impacts of ice nuclei
aerosols, which are much less understood than
the effects of CCN aerosols.) Urban and in-
dustrial air pollution plumes were observed to
completely suppress precipitation from 2.5-km-
deep clouds over Australia (20). Heavy smoke
from forest fires was observed to suppress rain-
fall from 5-km-deep tropical clouds (21,22).
The clouds appeared to regain their precipitation
capability when ingesting giant (>1 mmdiame-
ter) CCN salt particles from sea spray (23)and
salt playas (24). These observations were the
impetus for the World Meteorological Organi-
zation and the International Union of Geodesy
and Geophysics to mandate an assessment of
aerosol impact on precipitation (19). This report
concluded that it is difficult to establish clear
causal relationships between aerosols and precip-
itation and to determine the sign of the precipi-
tation change in a climatological sense. Based on
many observations and model simulations the ef-
fects of aerosols on clouds are more clearly un-
derstood (particularly in ice-free clouds); the
effects on precipitation are less clear.
A recent National Research Council report that
reviewed radiative forcing of climate change
(25) concluded that the concept of radiative
forcing needs to be extended to account for (1)
the vertical structure of radiative forcing, (2) re-
gional variability in radiative forcing, and (3)
nonradiative forcing.It recommended to move
beyond simple climate models based entirely
on global mean top of the atmosphere radiative
forcing and incorporate new global and regional
radiative and nonradiative forcing metrics as they
become available.We propose such a new met-
ric below.
How Can Slowing the Conversion
of Cloud Droplets to Raindrops
Enhance Rainfall?
A growing body of observations shows that sub-
micrometer CCN aerosols decrease precipitation
Growing Mature Dissipating
Hazy Pristine
0°C
0°C
Direction of airflow
Ice and snow crystals
Graupel or small hail
Raindrop
Larger cloud droplet
Small cloud droplet
Smaller cloud droplet
Aerosol particles
Fig. 2. Evolution of deep convective clouds developing in the pristine
(top) and polluted (bottom) atmosphere. Cloud droplets coalesce into
raindrops that rain out from the pristine clouds. The smaller drops in the
polluted air do not precipitate before reaching the supercooled levels,
where they freeze onto ice precipitation that falls and melts at lower
levels. The additional release of latent heat of freezing aloft and reab-
sorbed heat at lower levels by the melting ice implies greater upward
heat transport for the same amount of surface precipitation in the more
polluted atmosphere. This means consumption of more instability for the
same amount of rainfall. The inevitable result is invigoration of the con-
vective clouds and additional rainfall, despite the slower conversion of
cloud droplets to raindrops (43).
5 SEPTEMBER 2008 VOL 321 SCIENCE www.sciencemag.org
1310
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from shallow clouds (17,20,21,2628)and
invigorate deep convective rain clouds with
warm (> ~15°C) cloud base (2933), although
the impact on the overall rainfall amount is not
easily detectable (34,35). These observations are
supported by a large number of cloud-resolving
model studies (3643). The simulations also
show that adding giant CCN to polluted clouds
accelerates the autoconversion, mainly through
nucleating large drops that rapidly grow into
precipitation particles by collecting the other
smaller cloud droplets (44). However, the auto-
conversion rate is not restored to that of pristine
clouds (42).
Fundamentally, the amount of precipitation
must balance the amount of evaporation at a
global scale. Therefore, the consequence of aero-
sols suppressing precipitation from shallow
clouds must be an increase in precipitation from
deeper clouds. Such compensation can be ac-
complished not only at the global scale (45) but
also at the cloud scale; that is, the clouds can
grow to heights where aerosols no longer im-
pede precipitation (46). All of this is consistent
with the conceptual model shown in Fig. 2. This
model suggests that slowing the rate of cloud
droplet coalescence into raindrops (i.e., auto-
conversion) delays the precipitation of the cloud
water, so that more water can
ascend to altitudes where the
temperature is colder than 0°C.
Even if the total rainfall amount
is not decreased by the increase
in aerosols, delaying the forma-
tion of rain is sufficient to cause
invigoration of cloud dynam-
ics. By not raining early, the
condensed water can form ice
precipitation particles that release
the latent heat of freezing aloft
(6,29,30) and reabsorb heat at
lower levels where they melt
after falling.
The role of ice melting below
the 0°C isotherm level in invig-
oration has been successfully
modeled (47), although models
also predict invigoration through
increased aerosol loads even with-
out ice processes (43). These
model simulations suggest that
the delay of early rain causes
greater amounts of cloud water
and rain intensities later in the
life cycle of the cloud. The en-
hanced evaporative cooling of
the added cloud water, mainly
in the downdrafts, provides part
of the invigoration by the mech-
anism of enhanced cold pools
near the surface that push up-
ward the ambient air. The greater
cooling below and heating above
lead to enhanced upward heat
transport, both in absolute terms
and normalized for the same amount of sur-
face precipitation. The consumption of more
convective available potential energy (CAPE)
for the same rainfall amount would then be con-
verted 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 (6). Simulations have shown that greater
heating higher in the troposphere enhances the
atmospheric circulation systems (48).
In clouds with bases near or above the 0°C
isotherm, almost all the condensate freezes, even
if it forms initially as supercooled raindrops in a
low-CCN environment. Moreover, the slowing
of the autoconversion rate by large concentra-
tions of CCN can leave much of the cloud drop-
lets airborne when strong updrafts thrust them
above the homogeneous ice nucleation level of
~38°C, where they freeze into small ice parti-
cles that have no effective mechanism to coag-
ulate and fall as precipitation. This phenomenon
was observed by aircraft (49) and simulated
for convective storms in west Texas (50) and
the U.S. high plains (51). When the same sim-
ulation (50) was repeated with reduced CCN
concentrations, the calculated rainfall amount
increased substantially. The same model showed
that adding small CCN aerosols in warm-base
clouds has the opposite effect to that of cold-
base clouds: increasing the precipitation amount
by invigorating the convective overturning, while
keeping the precipitation efficiency (i.e., sur-
face precipitation divided by total cloud con-
densates) lower (52).
The invigoration due to aerosols slowing
the autoconversion can be explained according
to fundamental theoretical considerations of
the pseudo-adiabatic parcel theory (Fig. 3). The
CAPE measures the amount of moist static en-
ergy that is available to drive the convection. Its
value is normally calculated with reference to a
pseudo-adiabatic cloud parcel that rises while
precipitating all its condensate in the form of
rain, even at subfreezing temperatures.
Consider the case of a tropical air parcel that
ascends from sea level with initial conditions
of cloud base pressure of 960 hPa and temper-
ature of 22°C. When not allowing precipitation,
all the condensed water remains in the parcel
and requires 415 J kg
1
to rise to the height of
the 4°C isotherm (point d
1
in Fig. 3), which is
the highest temperature at which freezing can
practically occur in the atmosphere. Freezing
all the cloud water would warm the air and add
thermal buoyancy by an amount that would
almost exactly balance the condensate load (d
2
).
When the ice hydrometeors precipitate from a
parcel, it becomes more positively buoyant be-
cause of its reduced weight (d
3
), so that the re-
leased convective energy at the top of the cloud
(d
4
) is the largest. Specifically, it is greater by
~1000 J kg
1
relative to the case where cloud
water is precipitated as rain below the 4°C iso-
therm and as ice above that level (c
1
). However,
further delaying the conversion of cloud water
into precipitation to greater heights above the
0°C level weakens the convection. In the ex-
treme case of extending the suppression from
the 4°C to the 36°C isotherm level (a
1
), ad-
ditional energy of 727 J kg
1
is invested in lifting
the condensates. There is no effective mecha-
nism for precipitating cloud water that glaciated
homogeneously into small ice particles. This
would prevent the unloading of the parcel, tak-
ing up even more convective energy and further
suppressing the convection and the precipita-
tion. In reality, cloud parcels always mix with
the environment, but this applies equally to all
the scenarios in Fig. 3, so that qualitatively the
contrasting aerosol effects remain the same.
Although the idealized calculations here are
useful to establish the concepts, the exact cal-
culations require running three-dimensional
models on the full life cycle of convective cloud
systems, followed by validation with detailed
observations.
The importance of the aerosol control of the
released convective energy by adding as much
as 1000 J kg
1
can be appreciated by consider-
ing that CAPE averages ~1000 to 1500 J kg
1
in the Amazon (30). Simulations of aerosols in-
vigorating peak updrafts by 20% (37,52)are
d1
c Unload > 0 freeze unloada Water load
d Load > 0 freeze unloadb Water unload
abcd
1890
868 d4
c1
-415
-1142
d1d2d3
a1
0
Buoyancy (g/kg)
Temperature (°C)
-20 -10 0 10203040
-80
-60
-40
-20
20
0
Fig. 3. The buoyancy of an unmixed adiabatically raising air par-
cel. The zero-buoyancy reference is the standard parcel: liquid water
saturation, immediately precipitating all condensates without freez-
ing (vertical line b). Cloud base is at 22°C and 960 hPa. The buoy-
ancy of the following scenarios is shown: (a) suppressing rainfall and
keeping all condensed water load, without freezing; (b) precipitat-
ing all condensed water, without freezing; (c) precipitating all con-
densates, with freezing at T<4°C; (d) Suppressing precipitation until
T=4°C, and then freezing and precipitating all condensed water
above that temperature. The released static energy (J kg
1
)with
respect to reference line bis denoted by the numbers.
www.sciencemag.org SCIENCE VOL 321 5 SEPTEMBER 2008 1311
REVIEW
consistent with an increase of released convec-
tiveenergybynearly50%.
Role of Radiative Versus
Microphysical Aerosol Effects
Until now, the radiative and microphysical im-
pacts of aerosols on the climate system have
been considered separately and independently;
their various, often conflicting, influences have
not been amenable to quantitative weighting on
the same scale. Given the opposing microphys-
ical and radiative effects on the vigor and rainfall
amounts of deep warm-base convective clouds,
there is a need to assess the combined effects of
these two factors (25).
A quantitative comparison between the
strengths of the radiative and microphysical ef-
fects of the aerosols is presented in Fig. 4. Be-
cause optically active aerosols are larger than
0.05 mm in radius, and because mature pollu-
tion aerosols of this or larger size can act as
CCN (53), CCN concentrations generally in-
crease with aerosol optical thickness (AOT)
(Fig. 1). The empirical relationship between AOT
andCCNisshowninFig.4byAOT=0.002
(CCN
0.4
)
0.64
(1), where CCN
0.4
is the concen-
tration of CCN active at a supersaturation of
0.4%. The cloud droplet concentration N
c
is
proportional to (CCN
0.4
)
k
, where kis typically
smaller than 1. Using k= 0.825 relates 2000
cloud drops cm
3
to 10
4
CCN
0.4
cm
3
, which
corresponds to ΑΟΤ = 1. The value of kwas
inferred from Ramanathan et al.(7), although
Freud et al.(54) imply that kis closer to 1. In
turn, N
c
was shown to be related to the depth
above cloud base (D) required for onset of rain
(54). This depth determines the thermodynamic
track of the rising parcel (Fig. 3) and hence the
vigor of the convection and the extent of con-
vective overturning, which determines the rainfall
amount produced by the cloud system throughout
its life cycle. The cloudy parcel ascends along
curve ain Fig. 3 as long as the cloud top has not
reached D, and shifts to a track between curves c
and daccording to the amount of condensed
wateratthatheight.
The dependence of DonCCNisobtainedbya
compilation of aircraft measurements (27,54,55)
that provides an approximate relation of D=
80 + (4 × CCN
0.4
). According to this relation,
CCN
0.4
should reach ~1200 cm
3
for preventing
rainout from typical tropical clouds before reach-
ing the practical freezing temperature of 4°C,
which is at D5 km. At this point the in-
vigoration effect is at its maximum, where the
cloud parcel follows curve din Fig. 3. Adding
CCN beyond this point suppresses the vigor of
the convection by shifting the cloud parcel grad-
ually from curve dto curve ain Fig. 3. This means
that the microphysical effect on invigorating the
convection has a maximum at moderate CCN con-
centrations. This maximum becomes smaller for
cooler-base clouds, where the distance to the freez-
ing level is shorter, so that fewer CCN are suffi-
cient to suppress the onset of rain up to that level.
At the point of strongest microphysical in-
vigoration, AOT is still at the modest value of
~0.25. Added aerosols increase the AOT and
reduce the flux of solar energy to the surface,
which energizes convection. As a result, with
increasing aerosol loads beyond the optimum,
the weakening of the microphysical invigora-
tion is reinforced by the suppressive effect of
reduced surface heating.
The interplay between the microphysical and
radiative effects of the aerosols may explain the
observations of Bell et al.(33), who showed
that the weekly cycle of air pollution aerosols
in the southeastern United States is associated
with a weekday maximum and weekend min-
imum in the intensity of afternoon convective
rainfall during summer. This was mirrored by
a minimum in the midweek rainfall over the
adjacent sea areas, reflecting an aerosol-induced
modulation of the monsoonal convergence of
air and its rising over land with return flow
aloft to the ocean. This is a remarkable find-
ing, as it suggests that the microphysical im-
pacts of aerosols on invigorating warm-base
deep clouds are not necessarily at the expense
of other clouds in the same region, but can
lead to changes in regional circulation that lead
to greater moisture convergence and regional
precipitation.
This weekly cycle emerged in the late 1980s
and strengthened through the 1990s, along with
the contemporary reversal of the dimming trend
of solar radiation reaching the surface, which
took place until the 1980s (56). This was likely
caused by the reversal in the emissions trends
of sulfates and black carbon (57). It is possible
that the weekly cycle emerged when the over-
all aerosol levels decreased to the range where
the microphysical impacts are dominant, as shown
in Fig. 4.
Measuring Radiative and Microphysical
Aerosol Effects with the Same Metric
The precipitation and the radiative effects of
the aerosols (both direct and cloud-mediated)
can be integrally measured when considering
the combined changes in the energy of the at-
mosphere and the surface. The commonly used
metrics are the radiative forcing at the top of
the atmosphere (TOA) and at the BOA (bottom
of the atmosphere, i.e., Earths surface), measured
in W m
2
. The atmospheric radiative forcing
is the difference between TOA and BOA forc-
ing (7). Here we propose a new metric, the aero-
sol thermodynamic forcing (TF)
(58), representing the aerosol-
induced change in the atmo-
spheric energy budget that is
not radiative in nature. In con-
trast to TOA radiative forcing,
TF does not change the net Earth
energy budget, but rather redis-
tributes it internally; hence, TF
can affect temperature gradi-
ents and atmospheric circula-
tion. The main source of TF is
the change in the amount of latent
heat released by aerosol-induced
changes in clouds and precipi-
tation. It can be expressed as a
change in latent heat flux (in
units of W m
2
) in the atmo-
spheric column.
The vertical distribution of
the atmospheric heating is crit-
ically important because it de-
termines the vertical lapse rate
and hence the CAPE, which
quantifies the ability to produce
convective clouds and precipita-
tion. Atmospheric radiative heat-
ing due to absorbing aerosols
tends to reduce CAPE and there-
by suppress the development
of convective clouds, whereas the
microphysical effects of aero-
sols allow a deeper exploitation
of CAPE and hence invigora-
tion of convection and associated
precipitation.
All the components of the
aerosol radiative (direct and cloud-
CCN0.4 aerosol concentration (cm-3)
100 1000 10,000
-1000
-500
0
500
1000
0
0.2
0.4
0.6
0.8
1
Aerosol optical thickness
Aerosol transmission
Released convective energy
(J kg-1)
CAPE Transmission
AOT
Fig. 4. Illustration of the relations between the aerosol micro-
physical and radiative effects. The aerosol optical thickness
(AOT) is assumed to reach 1 at CCN
0.4
=10
4
cm
3
(dashed red
line), which corresponds to nucleation of 2000 cloud drops cm
3
.
The related transmission of radiation reaching the surface is shown
by the solid red line. The vigor of the convection is shown by
the blue line, which provides the released convective available
potential energy (CAPE) of a cloud parcel that ascends to the
cloud top near the tropopause. The calculation is based on the
scheme in Fig. 3, with respect to curve cas the zero reference.
Note that a maximum in CAPE occurs at CCN
0.4
1200 cm
3
,
which corresponds to the maximum cloud invigoration accord-
ing to curve doftheschemeinFig.3.TheAOTcorrespondingto
the CCN
0.4
at the microphysical optimum is only 0.25. Adding
aerosols beyond this point substantially decreases the vigor of the
cloud because both microphysical and radiative effects work in the
same direction: smaller release of convective energy aloft and less
radiative heating at the surface.
5 SEPTEMBER 2008 VOL 321 SCIENCE www.sciencemag.org
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mediated) and thermodynamic forcing and the
resulting changes in CAPE can now be quan-
tified as energy flux perturbations in units of
Wm
2
. Consider the example of smoke chang-
ing tropical convection from thermodynamic
path cto path din Fig. 3. At the end of the
convective cycle, an additional 1000 J kg
1
are
depleted from CAPE relative to convection un-
der pristine conditions. The resultant increased
convective overturning is likely to produce more
rainfall and increase the temperature by con-
verting more latent heat into sensible heat, at a
rate of 29 W m
2
for each added millimeter of
rainfall during 24 hours. This can be consid-
ered as a cloud-mediated TF of aerosols, which
works to enhance rainfall and accelerate the
hydrological cycle, resulting in a positive sign
for TF. On the other hand, if the smoke be-
comes very thick, its radiative impact would
be to reduce surface latent and sensible heat-
ing and warm the mid-troposphere. For ex-
ample, an AOT of 1 induces a BOA forcing of
45 W m
2
in the Amazon (5). This stabiliza-
tion of the atmosphere would cause less con-
vection and depletion of CAPE, less rainfall,
and a resulting deceleration of the hydrolog-
ical cycle (7). Furthermore, too much aerosol
can suppress the precipitation-forming processes
to the extent of changing from thermodynamic
path dto path ain Fig. 3 (see also Fig. 4), hence
reversing the cloud-mediated TF of aerosols from
positive to negative, adding to the negative radia-
tive forcing.
Thermodynamic forcing can occur even with-
out changing the surface rainfall: The energy
change when polluted clouds develop along track
din Fig. 3, with respect to the pristine reference
state shown in track c, would be defined as TF.
In this case, the TF solely due to added release
of latent heat of freezing is 2.44 W m
2
mm
1
day
1
of heating above the freezing level and
the same amount of cooling due to melting be-
low the melting level. This is a net vertical re-
distribution of latent heat. For an area-average
rainfall of 20 mm day
1
, the TF scales to 48.8 W
m
1
. In addition, we should consider the thermo-
dynamic consequences of the aerosol-induced
added rainfall due to increased convective over-
turning. This would convert latent heat to sen-
sible heat at a rate of 29 W m
2
mm
1
day
1
.Such
deeper consumption of CAPE would require a
longer time for the atmosphere to recover for the
next convective cycle, representing a temporal
redistribution of heating and precipitation.
Concluding Thoughts
The next challenge will be to map the radiative
and cloud-mediated thermodynamic forcing of
the aerosols in the parameter space of AOT ver-
sus CCN. The good correlation between AOT
and CCN means that, at least at large scales, the
radiative and microphysical effects of aerosols
on cloud physics are not free to vary indepen-
dently (1), and hence mainly the diagonal of the
parameter space is populated.
According to Fig. 4, there should be an op-
timum aerosol load in the tropical atmosphere
that should lead to the most positive aerosol
thermodynamic forcing, manifested as the most
vigorous convection. This optimum probably oc-
curs at AOT 0.25 and CCN
0.4
1200 cm
3
.
Remarkably, these fundamental considerations
for AOT 0.25 for optimal cloud development
were matched recently by observations in the
Amazon (59).
This hypothesis reconciles the apparent con-
tradictory reports that were reviewed in two
major assessments (18,19) as impeding our
overall understanding of cloud-aerosol impacts
on precipitation and the climate system. The
main cause for the previous uncertainties was
the nonmonotonic character of competing ef-
fects, which is inevitable in a system that has an
optimum. The new conceptual model outlined
here improves our understanding and ability
to simulate present and future climates. It also
has implications for intentional weather and
climate modification, which are being consid-
ered in the context of cloud seeding for pre-
cipitation enhancement and geoengineering.
Testing this hypothesis is planned within the
Aerosol Cloud Precipitation Climate (ACPC)
initiative (60,61).
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International Teams Program.
10.1126/science.1160606
www.sciencemag.org SCIENCE VOL 321 5 SEPTEMBER 2008 1313
REVIEW
... This effect Heymsfield et al., 2010 [29] and 2011 [30] is partly compensated by the additional weight of the condensed water that reduces the buoyancy inside the cloud. The net increase in buoyancy in polluted as compared to clean clouds has been made responsible for the invigoration of deep convective clouds [81] and been referred to as the thermodynamic indirect effect [15]—see the " Aerosol Effects on Deep Convective Clouds " section. In addition to changes in the number concentration and size of cloud droplets, changes in MPCs can be triggered by a change in the number concentration of anthropogenic aerosols can act as ice nucleating particles (INP). ...
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1] Applications of new retrieval methods to old satellite data allowed us to study the effects of smoke from the Kuwait oil fires in 1991 on clouds and precipitation. The properties of smoke-affected and smoke-free clouds were compared on the background of the dust-laden desert atmosphere. Several effects were observed: (1) clouds typically developed at the top of the smoke plume, probably because of solar heating and induced convection by the strongly absorbing aerosols; (2) large salt particles from the burning mix of oil and brines formed giant cloud condensation nuclei (CCN) close to the source, which initiated coalescence in the highly polluted clouds; (3) farther away from the smoke source, the giant CCN were deposited, and the extremely high concentrations of medium and small CCN dominated cloud development by strongly suppressing drop coalescence and growth with altitude; and (4) the smaller cloud droplets in the smoke-affected clouds froze at colder temperatures and suppressed both the water and ice precipitation forming processes. These observations imply that over land the smoke particles are not washed out efficiently and can be transported to long distances, extending the observed effects to large areas. The absorption of solar radiation by the smoke induces convection above the smoke plumes and consequently leads to formation of clouds with roots at the top of the smoke layer. This process dominates over the semidirect effect of cloud evaporation due to the smoke-induced enhanced solar heating, at least in the case of the Kuwait fires.
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