scispace - formally typeset
Search or ask a question
Journal ArticleDOI

Smoking Rain Clouds over the Amazon

27 Feb 2004-Science (American Association for the Advancement of Science)-Vol. 303, Iss: 5662, pp 1337-1342
TL;DR: Heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation, which affects the water cycle, the pollution burden of the atmosphere, and the dynamics of atmospheric circulation.
Abstract: Heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation from 1.5 kilometers above cloud base in pristine clouds to more than 5 kilometers in polluted clouds and more than 7 kilometers in pyro-clouds. Suppression of low-level rainout and aerosol washout allows transport of water and smoke to upper levels,where the clouds appear “smoking” as they detrain much of the pollution. Elevating the onset of precipitation allows invigoration of the updrafts,causing intense thunderstorms,large hail,and greater likelihood for overshooting cloud tops into the stratosphere. There,detrained pollutants and water vapor would have profound radiative impacts on the climate system. The invigorated storms release the latent heat higher in the atmosphere. This should substantially affect the regional and global circulation systems. Together,these processes affect the water cycle,the pollution burden of the atmosphere,and the dynamics of atmospheric circulation. Several hundred thousand deforestation and agricultural fires burn in Amazonia during the dry season each year, covering vast areas with dense smoke (1, 2). The smoke’s radiative impact suppresses surface heating and evaporation and stabilizes the lower troposphere. In turn, this suppresses the formation of convective clouds and precipitation and thus slows down the hydrological cycle (3). The microphysical effects of the aerosols on clouds and precipitation are no less important but have until now only been inferred from modeling and satellite observations. Convective clouds forming in smoky air show substantially reduced droplet size compared to that of similar clouds in clean air (4), with a mean satelliteretrieved effective droplet radius of 9 m in smoky clouds compared to 14 mi n clean clouds (5). This reduction of cloud droplet size by smoke is associated with an inhibition of the onset of precipitation radar echoes up to heights of 6.5 km, compared to 3 km in smoke-free clouds (6, 7). Here, we report in situ measurements for

Summary (2 min read)

Jump to: [Introduction][The SMOCC Campaign][Results][Discussion] and [REPORTS]

Introduction

  • Heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation from 1.5 kilometers above cloud base in pristine clouds to more than 5 kilometers in polluted clouds and more than 7 kilometers in pyro-clouds.
  • This should substantially affect the regional and global circulation systems.
  • The microphysical effects of the aerosols on clouds and precipitation are no less important but have until now only been inferred from modeling and satellite observations.
  • Therefore, clouds over the Amazon during the rainy season are predominantly microphysically maritime, hence the term “green ocean” (9). (iii) Smoky clouds: Vegetation burning produces high concentrations of aerosols, a large fraction of which are capable of nucleating cloud droplets.
  • The lack of early precipitation allows updrafts to accelerate and transport cloud water in deep convection to the high and supercooled regions, where it can release additional latent heat of freezing, which it would not have delivered in the maritime case of early rainout.

The SMOCC Campaign

  • The need to validate this conceptual model motivated the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia– Smoke, Aerosols, Clouds, Rainfall, and Climate) campaign from September to November 2002.
  • 6Departamento de Ciencias Atmosfericas, Instituto de Astronomia, Geofisica e Ciências Atmosfericas, Universidade de Sao Paulo, Rua do Matao, 1226 CEP 05508-900, Sao Paulo, Brazil.
  • To investigate the role of smoke aerosols with minimal influence of changes in meteorological conditions, the authors conducted a set of flights on 4 and 5 October 2002, which went from a moderately polluted region in Rondonia to an area with very clean air over the western Amazon (Fig. 1).

Results

  • The fires emit smoke particles, which the authors quantified as condensation nuclei (CN) concentrations.
  • These values are slightly higher than the lowest campaign-average values observed previously over the Amazon [380 to 390 cm 3 (8, 13)] and about twice as high as typical values in the marine boundary layer (MBL) over the Atlantic off Brazil.
  • Cloud-processed or aged (days) particles in the regional haze and the aerosols in the clean (green ocean) BL are larger, with modal diameters around 130 to 170 nm.
  • According to the CDSD (Fig. 4, A and B), the large-drop tail at cloud base appears to have played a role in creating the raindrops over the ocean but not over the almost similarly pristine land and definitely not over the smoky land (Fig. 4C), at least up to the aircraft operational height limitation of 4.5 km.
  • They show a polluted BL below cloud base (at 1000 m) and a somewhat less polluted CDL.

Discussion

  • In spite of the different source mechanisms and compositions of the aerosol particles in smoky and clean regions and their vastly different concentrations, they are strikingly similar in their ability to nucleate cloud droplets.
  • The similarity in size distribution suggests that it is similar to the smoky BL aerosol shown in Fig. 27 FEBRUARY 2004 VOL 303 SCIENCE www.sciencemag.org1340the authors.
  • The suppressed precipitation below H 6 km can also explain the observation that smoky Cb in the tropics enrich the lower stratosphere with water vapor (36) by allowing a greater amount of cloud condensates in the form of smaller particles to detrain from cloud tops.
  • In the green ocean regime, low CCN concentrations favor efficient precipitation scavenging, which in turn reduces CCN concentrations, until a balance between natural CCN production rates and precipitation removal is achieved.

REPORTS

  • Measurement of the Effect of Amazon Smoke on Inhibition of Cloud Formation Ilan Koren,1,2* Yoram J. Kaufman,1 Lorraine A. Remer,1 Jose V. Martins1,3.
  • Aerosols can counteract regional greenhouse warming by reflecting solar radiation to space or by enhancing cloud reflectance (2) or lifetime (3, 4).
  • Here, using data from the MODIS-Aqua space instrument, the authors report measurements of the effect of smoke on cloud formation over the Amazon basin during the dry season (August–September) of 2002—namely, the reduction of the fraction of scattered cumulus clouds with the increase in smoke column concentration.
  • 2National Research Council, Washington, DC 20001, USA.

Did you find this useful? Give us your feedback

Figures (3)

Content maybe subject to copyright    Report

Smoking Rain Clouds over
the Amazon
M. O. Andreae,
1
* D. Rosenfeld,
2
* P. Artaxo,
3
A. A. Costa,
4
G. P. Frank,
1
K. M. Longo,
5
M. A. F. Silva-Dias
6
Heavy smoke from forest fires in the Amazon was observed to reduce cloud
droplet size and so delay the onset of precipitation from 1.5 kilometers above
cloud base in pristine clouds to more than 5 kilometers in polluted clouds and
more than 7 kilometers in pyro-clouds. Suppression of low-level rainout and
aerosol washout allows transport of water and smoke to upper levels, where
the clouds appear “smoking” as they detrain much of the pollution. Elevating
the onset of precipitation allows invigoration of the updrafts, causing intense
thunderstorms, large hail, and greater likelihood for overshooting cloud tops
into the stratosphere. There, detrained pollutants and water vapor would have
profound radiative impacts on the climate system. The invigorated storms
release the latent heat higher in the atmosphere. This should substantially
affect the regional and global circulation systems. Together, these processes
affect the water cycle, the pollution burden of the atmosphere, and the dy-
namics of atmospheric circulation.
Several hundred thousand deforestation and
agricultural fires burn in Amazonia during
the dry season each year, covering vast
areas with dense smoke (1, 2). The smoke’s
radiative impact suppresses surface heating
and evaporation and stabilizes the lower
troposphere. In turn, this suppresses the
formation of convective clouds and precip-
itation and thus slows down the hydrolog-
ical cycle (3). The microphysical effects of
the aerosols on clouds and precipitation are
no less important but have until now only
been inferred from modeling and satellite
observations. Convective clouds forming in
smoky air show substantially reduced drop-
let size compared to that of similar clouds
in clean air (4 ), with a mean satellite-
retrieved effective droplet radius of 9 m
in smoky clouds compared to 14 min
clean clouds (5). This reduction of cloud
droplet size by smoke is associated with an
inhibition of the onset of precipitation radar
echoes up to heights of 6.5 km, compared
to 3 km in smoke-free clouds (6, 7).
Here, we report in situ measurements for
validating the satellite inferences, comprising
quantitative information on aerosols, cloud
drop size distribution, and precipitation under
a wide spectrum of conditions, from very
clean air masses over the Equatorial Atlantic
and the Amazon, through smoky air masses,
to the extreme of pyro-clouds (i.e., clouds
that form in the smoke plume over an active
fire). Our results portray the following con-
ceptual model of the various cloud and pre-
cipitation regimes:
(i) Blue ocean: Low concentrations of
cloud condensation nuclei (CCN) in the clean
atmosphere over the ocean produce clouds
that are microphysically “maritime,” i.e.,
have relatively few but large drops that coa-
lesce rapidly into raindrops. In addition, the
typically weaker updrafts over the ocean al-
low more time for raindrops to grow and
precipitate before reaching the freezing level.
Early precipitation further suppresses the up-
draft and vigor of the convection.
(ii) Green ocean: Over the unpolluted
Amazon, especially in the rainy season, the
aerosol concentration is almost as low as over
the ocean (8). This is a result of effective
aerosol washout by precipitation and of the
feedback of the cleaned air to form clouds
with even faster drop coalescence, precipita-
tion, and aerosol washout. Therefore, clouds
over the Amazon during the rainy season are
predominantly microphysically maritime,
hence the term “green ocean” (9).
(iii) Smoky clouds: Vegetation burning
produces high concentrations of aerosols, a
large fraction of which are capable of nucle-
ating cloud droplets. This results in high con-
centrations of small cloud droplets that are
slow to coalesce and precipitate. The lack of
precipitation, except from the deepest clouds,
keeps the particles in the air and creates a
positive feedback for maintaining the smoky
and rainless conditions. The lack of early
precipitation allows updrafts to accelerate
and transport cloud water in deep convection
to the high and supercooled regions, where it
can release additional latent heat of freezing,
which it would not have delivered in the
maritime case of early rainout. The added
water is available for production of intense
ice precipitation, hail, and lightning, creating
more violent convective storms.
(iv) Pyro-clouds: These, the most extreme
form of smoky clouds, feed directly on the
smoke and heat from fires. They receive con-
flicting impacts: On one hand, extreme con-
centrations of CCN suppress the onset of
precipitation, and the fire-generated heat in-
vigorates the updrafts and further suppresses
warm rain processes. On the other hand, large
ash particles can serve as giant CCN and
initiate large precipitation particles (10). This
potential interaction of opposing effects is
particularly pertinent in view of recent simu-
lations (11) suggesting that there is a satura-
tion to the suppression effects of aerosols
with enhanced concentration, and that be-
yond a certain threshold very high concentra-
tions result in recovery of drop size. Howev-
er, the invigorated updrafts probably led to
the observed enhancement of cloud drop con-
centrations in the pyro-clouds beyond that of
the clouds growing from the smoky back-
ground.
Stronger atmospheric instability and a dri-
er boundary layer can induce effects on
clouds similar to those of the addition of
aerosols, i.e., increasing updraft strength and
cloud electrification (9). Separating the two
effects represents a major challenge. To elim-
inate this factor as a major source of variabil-
ity, we focus here on analyzing cases that had
very similar thermodynamic structure of the
atmosphere (Fig. 1 and fig. S1).
The SMOCC Campaign
The need to validate this conceptual model
motivated the LBA-SMOCC (Large-Scale Bio-
sphere-Atmosphere Experiment in Amazonia–
Smoke, Aerosols, Clouds, Rainfall, and Cli-
mate) campaign from September to November
2002. At a ground site in Rondonia, Brazil, we
made detailed measurements of the physical
and chemical properties of the aerosol from the
middle of the smoky dry season to the begin-
1
Biogeochemistry Department, Max Planck Institute
for Chemistry, Post Office Box 3060, D-55020 Mainz,
Germany.
2
Institute of Earth Sciences, Hebrew Uni-
versity of Jerusalem, Jerusalem 91904, Israel.
3
Insti-
tuto de Fisica, Universidade de Sao Paulo, Rua do
Matao, Travessa R, 187, CEP 05508-900, Sao Paulo,
Brazil.
4
Universidade Estadual do Ceara´, Avenida
Paranjana, 1700, Campus do Itaperi, Fortaleza, CE,
CEP 60740-000, Brazil.
5
Centro de Previsa˜o de Tempo
e Estudo Clima´ticos Instituto Nacional de Pesquisas
Espaciais (CPTEC-INPE), Rodovia Presidente Dutra,
Km 40, SP-RJ, CEP 12630-000, Cachoeira Paulista, Sao
Paulo, Brazil.
6
Departamento de Ciencias Atmosferi-
cas, Instituto de Astronomia, Geofisica e Cieˆncias
Atmosfericas, Universidade de Sao Paulo, Rua do
Matao, 1226 CEP 05508-900, Sao Paulo, Brazil.
*To whom correspondence should be addressed.
E-mail: andreae@mpch-mainz.mpg.de (M.O.A., gener-
al issues); daniel.rosenfeld@huji.ac.il (D.R., cloud phys-
ics issues)
RESEARCH ARTICLE
www.sciencemag.org SCIENCE VOL 303 27 FEBRUARY 2004 1337

ning of the wet season, when pollution levels
approach background conditions. We used two
instrumented aircraft to characterize the range
of aerosol and cloud ratios from pyro-clouds to
pristine air (12). To investigate the role of
smoke aerosols with minimal influence of
changes in meteorological conditions, we con-
ducted a set of flights on 4 and 5 October 2002,
which went from a moderately polluted region
in Rondonia to an area with very clean air over
the western Amazon (Fig. 1). The air across the
entire transect had originated from trade wind
inflow across the northeastern coast of Brazil.
Along the northern streamlines, this airmass
remained without much contact with fires,
whereas it received substantial regional pol-
lution from fires in the southeastern part of
our transect.
Results
The fires emit smoke particles, which we quan-
tified as condensation nuclei (CN) concentra-
tions. The size distribution and composition of
the CN determine the CCN spectrum, i.e., the
CCN number concentration as a function of
supersaturation. The CCN spectrum and the
cloud base updraft velocity determine the cloud
droplet size distribution (CDSD) at cloud base.
The CDSD at cloud base controls most of the
vertical evolution of CDSD in the growing con-
vective elements, which in turn determines the
height above cloud base (H) for onset of precip-
itation. The measured values for the four regimes
(from blue ocean to the pyro-clouds) are sum-
marized in Table 1.
Aerosols. Aerosol concentrations during
LBA-SMOCC fell into three distinct ranges (Ta-
ble 1): CN concentrations were near 500 cm
3
in
the westernmost Amazon (Fig. 2) and at our
ground site at the beginning of the rainy season.
These values are slightly higher than the lowest
campaign-average values observed previously
over the Amazon [380 to 390 cm
3
(8, 13)] and
about twice as high as typical values in the
marine boundary layer (MBL) over the Atlantic
off Brazil. The slight enhancement observed dur-
ing our campaign as compared to previous ob-
servations over Amazonia may be because of
reduced washout during the dry season (previous
data were from the wet season) or from minor
smoke inputs.
In the smoky boundary layer (BL), mean
CN levels were 2000 to 8000 cm
3
, with
smoke plumes and haze layers often exceed-
ing this range. These concentrations usually
decreased into the cloud detrainment layer
(CDL) and free troposphere (FT), but plumes
and layers detrained from clouds gave rise to
Fig. 1. Smoke aerosol distribution (D 2.5 m; in gm
2
) and wind field in the BL over South
America during the transect flights from Rondonia to the western Amazon. The aerosol distribution
was obtained with the use of the Geostationary Operational Environmental Satellites–Automated
Biomass Burning Algorithm (GOES ABBA) Fire product to estimate smoke emissions and the RAMS
model to simulate their transport and removal (38). The flight track is indicated as a red line; the
study area off Fortaleza, by a blue rectangle; and letters L and F represent the locations of the LET
and FNS sounding sites, respectively (fig. S1).
Table 1. Representative values of CO, CN, CCN (1%), and cloud droplet
number concentrations in the different regimes sampled during LBA-SMOCC
(except where indicated otherwise). Also shown are the drop diameter of
modal liquid water content, the height above cloud base to the onset of
precipitation, and the cloud base height. Absolute cloud drop concentrations
are obviously too high, at least in the clean conditions, for still-unexplained
reasons, but we reported here the instrumental values. On the other side,
quality control data of the SPP-100 show a severe undercounting because of
coincidence of droplets in the measurement volume in the high aerosol
situations. We report here the nominal values before attempting any correc-
tions. D
L
, drop diameter of modal liquid water content, is close to the
effective radius but is not affected by truncation of the large drops because
of instrument limitations. In situ onset of precipitation was defined as
aircraft-radar rain echo and visible drops on the windshield. It occurred when
LWC modal droplet diameter exceeded 24 m. Numbers in brackets pertain
to results from experiments in Thailand. Satellite onset of precipitation is on
the basis of the 14-m threshold for effective radius, from same day and area
of the clouds sampled by the aircraft. Dash entries indicate data not available.
CO
(ppb)
CN (cm
3
)
CCN at 1%
SS (cm
3
)
Cloud
droplets
(cm
3
)
D
L
(m) at
H 1500
m
In situ H
(m)
Satellite H
(m)
Cloud base
height (m)
Blue ocean 120
*
100–350 320 600 30 1000 1100 400
Green ocean 140 500 340 1000 23–26 1500 [1400] 1500 1500 [1500]
Smoky Cb 200650 2000 8000 10004000 2200 15–17 [5200] 6700 1700 [2800]
Pyro-Cb 1 10
4
2.2 10
4
§ 20 10
4
–44 10
4
10 10
4
–23 10
4
2400 12 7800 1700
*
From the Model of Atmospheric Transport and Chemistry (MATCH) for the region off the Brazilian coast, at the time and place of the blue ocean flights. Data collected in the
MBL over the Atlantic off Natal, Brazil, September 1989. Because of instrument malfunction, CCN measurements were not possible during this period. The value shown is from
the measured CN (500 cm
3
) and the CCN (1%)/CN ratio of 0.68 for the green ocean obtained during the LBA-CLAIRE-98 experiment. §Maxima observed during smoke plume
passes. Scaled from CO measurements with the use of observed CN/CO 20 cm
3
ppb
1
. Scaled from CN with the use of observed CCN/CN 0.52.
R ESEARCH A RTICLE
27 FEBRUARY 2004 VOL 303 SCIENCE www.sciencemag.org1338

elevated CN concentrations up to the highest
accessible altitude (4300 m) (Fig. 2). The
highest particle concentrations were present
inside plumes over active fires and in pyro-
clouds (Table 1).
The particle-size spectra of fresh (age of
minutes to hours) smoke in plumes and in the
BL with recent smoke inputs show little dif-
ference, having number-modal diameters
around 100 nm. Cloud-processed or aged
(days) particles in the regional haze and the
aerosols in the clean (green ocean) BL are
larger, with modal diameters around 130 to
170 nm. This shift in size distributions is
consistent with the CCN behavior of the aero-
sol. CCN efficiency spectra (the ratio CCN/
CN as a function of supersaturation, SS; Fig.
3) taken in the freshly polluted boundary
layer show that about 40 to 60% of CN are
able to nucleate cloud droplets at 1% SS,
whereas the larger particles in aged smoke
and in the clean BL have a distinctly higher
efficiency (60 to 80%). Roberts et al.(14, 15)
have shown that the CCN properties of both
pyrogenic and biogenic aerosols can be ex-
plained on the basis of their size-dependent
chemical composition, which is characterized
by a mixture of soluble inorganic and partial-
ly soluble organic constituents.
Cloud drop size distributions. The cloud
physics aircraft made vertical cross sections
near the tops of growing convective elements
that grew in isolation or as well-defined feed-
ers to cumulonimbus (Cb) clouds. The mea-
surements were limited to altitudes below the
zero isotherm (4.8 km above mean sea
level) because of aircraft constraints. The
measured CDSD for the four regimes are pro-
vided in Fig. 4, A to D. A common feature is
the widening of the CDSD with H, which can
be depicted as the dependence of the drop
diameter of modal liquid water content (D
L
)on
H (Fig. 5). When D
L
exceeded 24 m, warm
rain was formed in the updraft in sufficient
quantity to form radar echoes on the aircraft
radar and make visible drops on the windshield.
Often, rainfall was observed this way before
being recorded by the X and Y probes, proba-
bly because of their small sampling volume.
Cloud base D
L
shows small values for all
cases. However, the shape of the CDSD does
differ. Over the blue ocean, a wide tail of
large drops occurs already at cloud base (Fig.
4A), apparently because of the large sea salt
aerosols (16) that are present in the trade
wind BL in sufficient quantities to induce
such an effect (17). The surface wind during
that flight was 8ms
1
, just enough to
produce white caps and sea spray. The tail is
much smaller in the green ocean (Fig. 4B),
forming probably on large biogenic aerosols
(13). The tail is smallest in clouds growing on
the regional haze (Fig. 4C) but shows up in
the pyro-clouds (Fig. 4D), apparently on the
large ash particles. These ash particles range
in size from sub-millimeter to a few centime-
ters and can remain airborne for relatively
long times (minutes to hours) because of their
low density and convoluted shape (18, 19).
They contain substantial amounts of soluble
material and have been observed under the
microscope to deliquesce at humidities
90%. This suggests that they are able to
nucleate cloud drops quite readily. It is worth
noting that the large-droplet tail is much
smaller and appears at greater H in the pyro-
clouds compared to the oceanic clouds. Fur-
thermore, a pyro-Cb must reach H 7kmfor
precipitating (Table 1). This shows that the
ash particles play a less important role as
giant CCN in the smoky clouds than do sea
salt particles that are entrained into polluted
clouds (16).
Substantial differences appear higher in
the clouds. D
L
at H 1000 m is 10, 13, 23,
and 25 m for the four regimes, from the
pyro to the blue, respectively (Fig. 5). Pre-
cipitation developed at H equal to 1000 and
1500 m over the blue and green oceans,
respectively. According to the CDSD (Fig. 4,
A and B), the large-drop tail at cloud base
appears to have played a role in creating the
raindrops over the ocean but not over the
almost similarly pristine land and definitely
not over the smoky land (Fig. 4C), at least up
to the aircraft operational height limitation of
4.5 km. Satellite retrievals of cloud top par-
ticle effective radius (20) suggested that the
precipitation threshold of 14 m was exceed-
ed at 22°C and 28°C, which correspond to
H of 6600 and 7600 m for the smoky and
pyro-clouds, respectively (Fig. 5).
The D
L
for onset of precipitation was 24
m for the green ocean, consistent with the
value over the blue ocean. We could not
reach altitudes sufficiently high to establish
D
L
for the onset of precipitation in the smoky
clouds. Instead, we used cloud physics air-
craft measurements that were done identical-
ly, except for reaching H 7 km, by the
same flight scientist (D. Rosenfeld) as in
SMOCC with similar aircraft instrument
package during the Thailand cloud-seeding
experiment (21). On 2 May 1998, the air was
smoky from agricultural fires during the pre-
monsoon in northwestern Thailand. The
H-D
L
relation on this day coincides at lower
levels with the smoky curve of 4 October
2002 in Brazil and extends it to H 6400 m
(Fig. 5). Cloud liquid water content was 2g
m
3
up to the aircraft ceiling at the 31°C
isotherm. Ice hydrometeors in the form of
Fig. 2. Vertical distri-
bution of CN over the
smoky region in Ron-
donia and the clean
region in the western
Amazon sampled dur-
ing the transect flights
on 4 and 5 October
2002. The green ocean
values are a compos-
ite of two soundings
over the western Am-
azon and show little
horizontal or vertical
variability. The data
from Rondonia also
contain two soundings
as well as data ob-
tained during horizon-
tal legs. They show a
polluted BL below
cloud base (at 1000
m) and a somewhat
less polluted CDL. Nu-
merous plumes and
haze layers are em-
bedded in both the BL and the CDL. The layer at 4.2-km altitude had a minimal horizontal extent
of about 12 km.
Fig. 3. Cloud droplet nucleating efficiency (ex-
pressed as the ratio of CCN to CN) as a function
of supersaturation. Data are from flight
SMOCC05 on 29 September 2002 for the smoky
air and from CLAIRE-98 (8) for the green ocean.
R ESEARCH A RTICLE
www.sciencemag.org SCIENCE VOL 303 27 FEBRUARY 2004 1339

frozen raindrops appeared at 22°C(H
5200 m), where D
L
reached 22 m. Assum-
ing a D
L
24 m threshold for warm rain,
the cloud would have had to exceed H
6400 m for producing warm rain under such
smoky conditions. Obviously, any raindrop in
such a low temperature would readily freeze
and continue growing as hailstone or graupel.
The preferential freezing of the larger drops
probably curbed the rate of growth of D
L
with
H above the onset of ice precipitation at H
5200 m.
In order to extrapolate the green ocean
conditions to greater altitudes in a similar
way, we used the Thai data from 16 July
1997, a typical continental monsoon day. The
aircraft measurements showed the onset of
warm rain at D
L
24 m. D
L
kept growing
up to 35 matH 4200 m and 9°C, where
these large drops started to freeze and left
progressively smaller liquid droplets at great-
er heights and colder temperatures. Liquid
cloud water decreased to less than 0.5 g m
3
at 16°C and was not detectable above the
27°C isotherm, even in the strongest up-
drafts (20ms
1
).
At the extreme end, pyro-clouds have the
smallest D
L
for the same H and reach only 16
matH 3000 m, well below the size
required for the onset of warm rain. Accord-
ing to analysis of MODIS (22) data (fig. S2),
the onset of precipitation in these clouds oc-
curs at the extremely large H of 7600 m.
Discussion
In spite of the different source mechanisms
and compositions of the aerosol particles in
smoky and clean regions and their vastly
different concentrations, they are strikingly
similar in their ability to nucleate cloud drop-
lets. The aerosols over the green ocean are
largely of biogenic origin (primary particles
from the vegetation and gas-to-particle con-
version from biogenic gaseous precursors). In
contrast, the smoke is a mixture of ash parti-
cles, soot, organic materials, and inorganic
salts (2326). Both biogenic and pyrogenic
particles consist predominantly of organic
material [some 80% (14)], of which about
60% is water-soluble (24). Soluble inorganic
salts (NH
4
,K
,SO
4
2
, and NO
3
) represent
most of the rest of the mass. Therefore, con-
version of biological material to aerosol,
whether through combustion or by cold
processes, yields a material that is rather sim-
ilar in its gross chemical composition and
solubility in spite of considerable differences
in the actual organic compounds present.
This results in similar CCN properties for
comparable size distributions and explains
the similarity in CCN efficiency spectra be-
tween the green ocean and the cloud-pro-
cessed smoke aerosols in the CDL (Fig. 3).
The smoke aerosols in the BL, with lower
modal diameter, are somewhat less effi-
cient CCN, i.e., have lower CCN/CN ratios.
The age of these particles must be in the
range of hours (given the abundance of fires
in the region) to 1 to 2 days (the transit time
across the region of burning). We observed
only minor differences in size distribution
between smoke sampled in the smoky BL and
that in fresh plumes (age 30 min). Although
we were not able to determine directly the
CCN efficiency of smoke in fresh plumes, the
similarity in size distribution suggests that it
is similar to the smoky BL aerosol shown
in Fig. 3.
Cloud properties are quite similar between
the blue and green ocean regimes. With in-
creasing aerosol concentrations, the clouds at
first react very sensitively, through changes
in the CDSD. When moving toward the pyro
conditions, a given fractional addition in
aerosol concentration is manifested as a
smaller addition in cloud droplet concentra-
tion or a decrease in cloud drop size, but it
does not completely saturate, in contrast to
previous suggestions (11, 27). This apparent
contradiction may result from the fact that
previous studies were concentrated in shal-
low clouds, whereas the current study is
aimed at deeper, potentially precipitating
clouds with stronger updrafts. Our findings
show that the sensitivity to aerosols increases
substantially with H (Fig. 5). This leads to
profound differences in the precipitation pro-
cesses, along the lines inferred by satellite
observations (6, 7, 20).
Fig. 4. The evolution of cloud drop diameter distribution (DSD) with height in growing convective
clouds, in the four aerosol regimes of (A) blue ocean, 18 October 2002, 11:00 UT (universal time),
off the northeast Brazilian coast (4S 38W); (B) green ocean, 5 October 2002 20:00 UT, in the clean
air at the western tip of the Amazon (6S 73W); (C) smoky clouds in Rondonia, 4 October 2002,
15:00 UT (10S 62W); and (D) pyro-clouds, composite where clouds at height 4000 m are from
1 October, 19:00 UT (10S 56W), and clouds above 4000 m are from 4 October, 19:00 UT (10S
67W). The lowest DSD in each plot represents conditions at cloud base, except in (D), where a size
distribution for large ash particles outside of the cloud is also shown. Note the narrowing of CDSD
and the slowing of its rate of broadening with height for the progressively more aerosol-rich
regimes from (A) to (D).
R ESEARCH A RTICLE
27 FEBRUARY 2004 VOL 303 SCIENCE www.sciencemag.org1340

The shift of onset of precipitation to large
H under smoky conditions leads to formation
of large ice hydrometeors that were reported
on the ground as large hail (up to 2 cm) and
also produced small dents on the aircraft nose
cone after flying in smoky conditions at cloud
base level on 23 September 2002. This en-
counter occurred very near to the FNS
sounding of 18 GMT (fig. S1), which did
not show greater instability than the other
soundings. It is remarkable that vigorous
convection leading to intense thunder-
storms and hail occurred in the smoky con-
ditions (three cases were observed by the
SMOCC team) in spite of the substantial
reduction of surface solar heating by the
smoke, with a seasonal average of 62.5 W
m
2
(28). No reports of hail on the ground
could be found for smoke-free conditions.
Highly suppressed surface convective flux-
es were also indicated by the smooth flying
conditions experienced, even during mid-
day in the smoky BL, to the extent of no
discernible bumpiness. Cloud bases were
poorly defined, and the clouds seemed to be
merging in the vertical from ragged pieces
at the lower levels. Clouds that rose above
about 4 km became quite vigorous and,
even when isolated, grew strongly and pro-
duced isolated intense showers and thun-
derstorms. Late in the day, these isolated
clouds occasionally became organized into
intense squall lines. Here, we see that, al-
though the radiative effects of the smoke
suppress the BL clouds, the microphysical
effects overpower this radiative suppres-
sion and produce clouds that are more vi-
olent than can be found in microphysically
maritime environments. These observations
support suggestions (9, 29) that aerosols
play a major role in the determination of
the dynamic, microphysical, and cloud
electrification properties that distinguish
continental from maritime convection (30).
Given that the suppression of initiation of
precipitation is compensated by increased
vigor of the storms, the net effect on the area
amount of precipitation remains unknown.
Owing to the decreased surface evaporation
and the negative radiative forcing of 11.9 W
m
2
at the top of the atmosphere (28), the
overall hydrological cycle should be slowed
down and regional precipitation should de-
crease under smoky conditions (3). However,
feedbacks from the global circulation can
change the local precipitation even more than
the primary effect (31). In any case, the
change from warm rain to ice precipitation
should result in greater latent heat release
higher in the clouds for the same rainfall
amounts. This shift has the effect of enhanc-
ing planetary-scale upper-level waves that
affect global climate (32, 33).
An independent test of the importance of
coagulation and precipitation in clouds can be
made by examining the CN concentration
(normalized to the conservative tracer, CO) in
air detrained by clouds. Fresh plumes in
SMOCC had a CN/CO ratio of 10 to 30
cm
3
parts per billion (ppb)
1
, comparable
to values of 20 to 36 cm
3
ppb
1
reported
from a savanna fire (34). Detrained haze lay-
ers in the CDL had CN/CO ratios in the
same range, suggesting that little coagulation
or scavenging had occurred. Even the highest
layers encountered, such as the CN maximum
at 4.2 km (Fig. 2), still showed CN/CO
13 cm
3
ppb
1
. This indicates that, because
of suppressed coalescence and precipitation,
no more than a moderate amount of coagula-
tion and scavenging could have occurred dur-
ing convective transport, making it an effi-
cient means to transport smoke aerosol at
least to midtropospheric levels. Particle loss-
es in deep tropical convection (35) might be
greater (75% of CN and 80 to 95% of
accumulation mode particles), but, given the
large amount of biomass smoke emitted in
the tropics, even the 5 to 15% of the aerosol
that survives transport to the upper tropo-
sphere is still a large contribution to this
otherwise clean region.
The suppressed precipitation below H 6
km can also explain the observation that
smoky Cb in the tropics enrich the lower
stratosphere with water vapor (36) by allow-
ing a greater amount of cloud condensates in
the form of smaller particles to detrain from
cloud tops. Furthermore, this suppressed re-
moval of water and smoke is coupled with
invigoration of the updrafts and therefore
greater likelihood for overshooting cloud tops
into the stratosphere.
The positive feedback between aerosol
concentration and reduced efficiency of pre-
cipitation scavenging may lead to a similar
bistability of continental CCN levels and
thermodynamic regimes over the Amazon, as
has been proposed previously for the marine
BL (37). In the green ocean regime, low CCN
concentrations favor efficient precipitation
scavenging, which in turn reduces CCN con-
centrations, until a balance between natural
CCN production rates and precipitation re-
moval is achieved. Interestingly, this balance
occurs at CCN levels not very different from
those over the blue ocean. In contrast, high
CCN concentrations suppress wet removal, at
least in the lower and middle troposphere,
and thus stabilize the pollution burden. This
favors the large-scale dispersal and upward
transport as the dominant sink balancing
regional pollutant emissions.
References and Notes
1. M. O. Andreae et al., J. Geophys. Res. 107, 10.1029/
2001JD000524 (2002).
2. M. A. Silva Dias et al., J. Geophys. Res. 107, 10.1029/
2001JD000335 (2002).
3. V. Ramanathan, P. J. Crutzen, J. T. Kiehl, D. Rosenfeld,
Science 294, 2119 (2001).
4. R. C. Eagan, P. V. Hobbs, L. F. Radke, J. Appl. Meteorol.
13, 553 (1974).
5. Y. J. Kaufman, R. S. Fraser, Science 277, 1636 (1997).
6. D. Rosenfeld, Geophys. Res. Lett. 26, 3105 (1999).
7. D. Rosenfeld, W. L. Woodley, in Cloud Systems, Hur-
ricanes, and the Tropical Rainfall Measuring Mission
(TRMM), W.-K. Tao, R. Adler, Eds. (American Meteo-
rological Society, Boston, 2003), vol. 51, pp. 5980.
8. G. C. Roberts, M. O. Andreae, J. Zhou, P. Artaxo,
Geophys. Res. Lett. 28, 2807 (2001).
9. E. Williams et al., J. Geophys. Res. 107, 10.1029/
2001JD000380 (2002).
10. D. B. Johnson, J. Atmos. Sci. 39, 448 (1982).
11. G. Feingold, L. A. Remer, J. Ramaprasad, Y. J. Kaufman,
J. Geophys. Res. 106, 22907 (2001).
12. Materials and methods are available as supporting
material on Science Online.
Fig. 5. The depen-
dence of drop diame-
ter of modal cloud liq-
uid water content, D
L
,
on distance above
cloud base, H, for the
four aerosol regimes
and transition situa-
tions. The vertical line
at D
L
24 m de-
notes the onset of
warm rain.
R ESEARCH A RTICLE
www.sciencemag.org SCIENCE VOL 303 27 FEBRUARY 2004 1341

Citations
More filters
Journal ArticleDOI
TL;DR: In this article, the authors proposed a new approach to global sustainability in which they define planetary boundaries within which they expect that humanity can operate safely. But the proposed concept of "planetary boundaries" lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development.
Abstract: Anthropogenic pressures on the Earth System have reached a scale where abrupt global environmental change can no longer be excluded. We propose a new approach to global sustainability in which we define planetary boundaries within which we expect that humanity can operate safely. Transgressing one or more planetary boundaries may be deleterious or even catastrophic due to the risk of crossing thresholds that will trigger non-linear, abrupt environmental change within continental- to planetary-scale systems. We have identified nine planetary boundaries and, drawing upon current scientific understanding, we propose quantifications for seven of them. These seven are climate change (CO2 concentration in the atmosphere <350 ppm and/or a maximum change of +1 W m-2 in radiative forcing); ocean acidification (mean surface seawater saturation state with respect to aragonite ≥ 80% of pre-industrial levels); stratospheric ozone (<5% reduction in O3 concentration from pre-industrial level of 290 Dobson Units); biogeochemical nitrogen (N) cycle (limit industrial and agricultural fixation of N2 to 35 Tg N yr-1) and phosphorus (P) cycle (annual P inflow to oceans not to exceed 10 times the natural background weathering of P); global freshwater use (<4000 km3 yr-1 of consumptive use of runoff resources); land system change (<15% of the ice-free land surface under cropland); and the rate at which biological diversity is lost (annual rate of <10 extinctions per million species). The two additional planetary boundaries for which we have not yet been able to determine a boundary level are chemical pollution and atmospheric aerosol loading. We estimate that humanity has already transgressed three planetary boundaries: for climate change, rate of biodiversity loss, and changes to the global nitrogen cycle. Planetary boundaries are interdependent, because transgressing one may both shift the position of other boundaries or cause them to be transgressed. The social impacts of transgressing boundaries will be a function of the social-ecological resilience of the affected societies. Our proposed boundaries are rough, first estimates only, surrounded by large uncertainties and knowledge gaps. Filling these gaps will require major advancements in Earth System and resilience science. The proposed concept of "planetary boundaries" lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development. Planetary boundaries define, as it were, the boundaries of the "planetary playing field" for humanity if we want to be sure of avoiding major human-induced environmental change on a global scale.

4,771 citations

Journal ArticleDOI
13 Jun 2008-Science
TL;DR: Interdisciplinary science that integrates knowledge of the many interacting climate services of forests with the impacts of global change is necessary to identify and understand as yet unexplored feedbacks in the Earth system and the potential of forests to mitigate climate change.
Abstract: The world's forests influence climate through physical, chemical, and biological processes that affect planetary energetics, the hydrologic cycle, and atmospheric composition. These complex and nonlinear forest-atmosphere interactions can dampen or amplify anthropogenic climate change. Tropical, temperate, and boreal reforestation and afforestation attenuate global warming through carbon sequestration. Biogeophysical feedbacks can enhance or diminish this negative climate forcing. Tropical forests mitigate warming through evaporative cooling, but the low albedo of boreal forests is a positive climate forcing. The evaporative effect of temperate forests is unclear. The net climate forcing from these and other processes is not known. Forests are under tremendous pressure from global change. Interdisciplinary science that integrates knowledge of the many interacting climate services of forests with the impacts of global change is necessary to identify and understand as yet unexplored feedbacks in the Earth system and the potential of forests to mitigate climate change.

4,541 citations

Journal ArticleDOI
TL;DR: The second most important contribution to anthropogenic climate warming, after carbon dioxide emissions, was made by black carbon emissions as mentioned in this paper, which is an efficient absorbing agent of solar irradiation that is preferentially emitted in the tropics and can form atmospheric brown clouds in mixture with other aerosols.
Abstract: Black carbon in soot is an efficient absorbing agent of solar irradiation that is preferentially emitted in the tropics and can form atmospheric brown clouds in mixture with other aerosols. These factors combine to make black carbon emissions the second most important contribution to anthropogenic climate warming, after carbon dioxide emissions.

3,060 citations

Journal ArticleDOI
24 Apr 2009-Science
TL;DR: What is known and what is needed to develop a holistic understanding of the role of fire in the Earth system are reviewed, particularly in view of the pervasive impact of fires and the likelihood that they will become increasingly difficult to control as climate changes.
Abstract: Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.

2,365 citations

References
More filters
Journal ArticleDOI
07 Dec 2001-Science
TL;DR: Human activities are releasing tiny particles (aerosols) into the atmosphere that enhance scattering and absorption of solar radiation, which can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.
Abstract: Human activities are releasing tiny particles (aerosols) into the atmosphere. These human-made aerosols enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to availability and quality of fresh water, a major environmental issue of the 21st century.

3,469 citations

Journal ArticleDOI
TL;DR: In this article, satellite observations of the tropical rainfall measuring mission (TRMM) showed that warm rain processes in convective tropical clouds infected by heavy smoke from forest fires are practically shut off.
Abstract: Although it has been known that smoke from biomass burning suppresses warm rain processes, it was not known to what extent this occurs. The satellite observations of the Tropical-Rainfall-Measuring-Mission (TRMM), presented here, show that warm rain processes in convective tropical clouds infected by heavy smoke from forest fires are practically shut off. The tops of the smoke-infected clouds must exceed the freezing level, i.e., grow to altitudes colder than about −10°C, for the clouds to start precipitating. In contrast, adjacent tropical clouds in the cleaner air precipitate most of their water before ever freezing. There are indications that rain suppression due to air pollution prevails also in the extra-tropics.

776 citations

01 Jan 1991
TL;DR: The 1990 American Geophysical Union's Conference on Biochemical burning as discussed by the authors was attended by more than 175 participants representing 19 countries and discussed remote sensing data concerning biomass burning, gaseous and particle emissions resulting from BB in the tropics, BB in temperate and boreal ecosystems, the historic and prehistoric perspectives on BB, BB and global budgets for carbon, nitrogen, and oxygen, and the BB and the greenhouse effect.
Abstract: Topics discussed at the March 1990 American Geophysical Union's Conference on biomass burning which was attended by more than 175 participants representing 19 countries are presented. Conference highlights include discussion of remote sensing data concerning biomass burning (BB), gaseous and particle emissions resulting from BB in the tropics, BB in temperate and boreal ecosystems, the historic and prehistoric perspectives on BB, BB and global budgets for carbon, nitrogen, and oxygen, and the BB and the greenhouse effect. Global estimates of annual amounts of biomass burning and of the resulting release of carbon to the atmosphere and the mean gaseous emission ratios for fires in wetlands, chaparral, and boreal ecosystems are given. An overview is presented of some conference discussions including global burning from 1850-1980, the global impact of biomass burning, the great Chinese/Soviet fire of 1987, and burning and biogenic emissions.

646 citations

Journal ArticleDOI
TL;DR: In this article, multispectral analyses of satellite images are used to calculate the evolution of the effective radius of convective cloud particles with temperature, and to infer from that information about precipitation forming processes in the clouds.
Abstract: Multispectral analyses of satellite images are used to calculate the evolution of the effective radius of convective cloud particles with temperature, and to infer from that information about precipitation forming processes in the clouds. Different microphysical processes are identified at different heights. From cloud base to top, the microphysical classification includes zones of diffusional droplet growth, coalescence droplet growth, rainout, mixed-phase precipitation, and glaciation. Not all zones need appear in a given cloud system. Application to maritime clouds shows, from base to top, zones of coalescence, rainout, a shallow mixed-phase region, and glaciation starting at −10°C or even warmer. In contrast, continental clouds have a deep diffusional growth zone above their bases, followed by coalescence and mixed-phase zones, and glaciation at −15° to −20°C. Highly continental clouds have a narrow or no coalescence zone, a deep mixed-phase zone, and glaciation occurring between −20° and −30°C. Limit...

592 citations

Journal ArticleDOI
TL;DR: In this article, optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil were measured for gas and particle measurements.
Abstract: Gas and particle measurements are described for optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil. The hazes tended to be evenly mixed from the surface to the trade wind inversion at 3–4 km in altitude. The properties of aged gases and particles in the regional hazes were significantly different from those of young smoke (<4 min old). As the smoke aged, the total amount of carbon in non-methane hydrocarbon species (C<11) was depleted by about one third due to transformations into CO2, CO, and reactive molecules, and removed by dry deposition and/or by conversion to particulate matter. As the smoke particles aged, their sizes increased significantly due to coagulation and mass growth by secondary species (e.g., ammonium, organic acids and sulfate). During aging, condensation and gas-to-particle conversion of inorganic and organic vapors increased the aerosol mass by ∼20–40%. One third to one half of this mass growth likely occurred in the first few hours of aging due to the condensation of large organic molecules. The remaining mass growth was probably associated with photochemical and cloud-processing mechanisms operating over several days. Changes in particle sizes and compositions during aging had a large impact on the optical properties of the aerosol. Over a 2 to 4 day period, the fine particle mass-scattering efficiency and single-scattering albedo increased by 1 m2 g−1, and ∼0.06, respectively. Conversely, the Angstrom coefficient, backscatter ratio, and mass absorption efficiency decreased significantly with age.

450 citations

Frequently Asked Questions (9)
Q1. What have the authors contributed in "Smoking rain clouds over the amazon" ?

In this paper, heavy smoke from forest fires in the Amazon was observed to reduce cloud droplet size and so delay the onset of precipitation from 1.5 kilometers above cloud base in pristine clouds to more than 5 kilometers in polluted clouds and more than 7 kilometers in pyro-clouds. 

The measurements were limited to altitudes below the zero isotherm ( 4.8 km above mean sea level) because of aircraft constraints. 

Given that the suppression of initiation of precipitation is compensated by increased vigor of the storms, the net effect on the area amount of precipitation remains unknown. 

This response to the smoke radiative effect reverses the regional smoke instantaneous forcing of climate from –28 watts per square meter in cloud-free conditions to 8 watts per square meter once the reduction of cloud cover is accounted for. 

In contrast, high CCN concentrations suppress wet removal, at least in the lower and middle troposphere, and thus stabilize the pollution burden. 

CCN efficiency spectra (the ratio CCN/ CN as a function of supersaturation, SS; Fig. 3) taken in the freshly polluted boundary layer show that about 40 to 60% of CN are able to nucleate cloud droplets at 1% SS, whereas the larger particles in aged smoke and in the clean BL have a distinctly higher efficiency (60 to 80%). 

This shows that the ash particles play a less important role as giant CCN in the smoky clouds than do sea salt particles that are entrained into polluted clouds (16). 

On the other side, quality control data of the SPP-100 show a severe undercounting because of coincidence of droplets in the measurement volume in the high aerosolsituations. 

At the extreme end, pyro-clouds have the smallest DL for the same H and reach only 16 m at H 3000 m, well below the size required for the onset of warm rain.