GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 20, PAGES 3105-3108, OCTOBER 15, 1999
TRMM
ObservedFirst Direct Evidenceof Smokefrom Forest
Fires Inhibiting Rainfall
Daniel Rosenreid
Inst.of EarthSciences,
The HebrewUniversityof Jerusalem,
Israel
2. The TRMM
Abstract. Althoughit hasbeenknownthat smokefrom biomass burning suppresses
warm rain processes,it was not
Observations
Actualobservations
of bothprecipitationandcloud-dropre
over large areas,encompassing
cloudsin and out of smoke
the Tropical-Rainfall-Measuring-Mission
(TRMM), presented
plumes,becamepossiblejust recently,with the launchof the
here, showthat warm rain processes
in convectivetropical
TropicalRainfall MeasuringMission(TRMM) satelliteon 28
cloudsinfectedby heavy smokefrom forestfires are practi- November 1997. The TRMM satellite instruments used here
callyshutoff. The topsof the smoke-infected
cloudsmustex- are:
ceed the freezinglevel, i.e., grow to altitudescolder than
a. Rainfallmeasuringradar(PR), whichdetectsonly precipiabout-10øC,for thecloudsto startprecipitating.
In contrast,
tation-sizedparticlesin clouds.The sub-satellite
resolution
adjacenttropicalcloudsin the cleanerair precipitatemostof
is 4 km horizontallyby 250 m vertically;
their water before ever freezing.There are indicationsthat
microwave
radiometer
(TMI), whichis sensitive
to
rain suppression
due to air pollutionprevailsalso in the ex- b.Passive
theclouddroplets
aswell asto precipitation
particles.
The
tra-tropics.
known to what extent this occurs. The satellite observations of
sub-satellitehorizontal resolutionof the 85 GHZ channels
is 4 km;
1. Introduction
Vegetation burning emits large concentrationsof small
Cloud CondensationNuclei [Hobbsand Radke, 1969; Kaufman and Fraser, 1997] (CCN), which modify the clouddrop
size distribution(DSD) so that the sameamountof water is
redistributedover a largernumberof smallerdrops.The coalescenceefficiencyof clouddropletsinto raindropsis greatly
reducedwhen the radius of the largest cloud dropletsis
smallerthanabout25 gtm[Masonand Jonas,1974], which is
equivalentto an effectiveradius(re) thresholdof 14 gtm[RosenfeM and Gutman, 1994]. Effective radius is the cloud
droplet size distributionparameter,which is observableby
satellites.It has alreadybeenobservedby satellitethat re decreasedbelow the precipitationthresholdof re=14 gtm in
clouds infectedby smoke from burning vegetationin the
Amazon [Kaufmanand Fraser, 1997] and Indonesia[RosenfeM and Lensky,1998]. However,thesestudiesdid not provide direct evidenceshowinghow the smokeactuallysuppressesprecipitation,suchas presentedin the currentstudy.
Theymerelystrengthened
the suspicions
thatarosemorethan
30 years ago, based on laboratoryexperiments[Gunn and
Phillips, 1957] andon observations
thatprecipitation
was reduced downwind of seasonalagriculturalburning of sugar
canefieldsin Australia[Warner, 1968].
c.Visibleandinfraredsensor
(VIRS),whichcanbe usedto
inferthecloudtopdropletsizes,re.The sub-satellite
horizontalresolution
is2 km.Thevertical
distribution
of recan
becomposed
fromcloudtopsatdifferent
heights.
The impact of smoke from fires in the rain forest on
warm-rain
processes
is analyzed
here,usinga TRMM overpass
overKalimantan
(Borneo),
Indonesia,
depicted
in Figure
1. The southeastern
portionof the overpass
overthe island
wasengulfed
byheavysmoke
whilethenorthwestern
portion
wasrelatively
smokefree.Theoverpass
occurred
on 1 March
1998,10:50localtime,whensurface
heating
drivengrowth
of
the convective
cloudtopshadnot yet exceeded
the -12øC
isotherm
levelin theobservation
area,asmeasured
by the
VIRS.Thesmoke
canbeseenin theright(southeast)
sideof
theTRMM image,presented
in Fig.2. TheTRMM radardetected
precipitation
in smoke-free
clouds
ontheleft(area2),
whilealmost
nonewasobserved
inthesmoke-plagued
clouds
(area1).Thevertical
cross
section,
presented
in Fig.3, providesmoreinsights:
The precipitating
cloudsin the cleanerareadid not have
coldertops,thusnotreaching
higheraltitudes
thantherainless
cloudsin thesmokyarea.
Theclouds
thatdidnotprecipitate
hadcolder
(mapped
as
higheraltitude)
85 GHzvertical
polarization
brightness
temperature
(T85)thantheprecipitating
clouds.
TheTsswasmuch
colderfor theclouds
in thesmoke,
approaching
theactual
l TRMM(Tropical
Rainfall
Measuring
Mission)
waslaunched
on28 cloudtoptemperature.
November1997 asa cooperative
projectof the NationalAeronautics
and Spaceand Administration(NASA), and the National SpaceDeTheT85for non-precipitating
watercloudsoverlandis developmentAgencyof Japan(NASDA).
terminedmainlyby thermalradiationemittedfromthe cloud
The TRMM
data are available at
droplets,
thusrelated
to theamount
andtemperature
of the
cloud
water,
especially
nearitstop.Drops
thatarelarger
than
theRayleigh
scattering
regime
at 85 GHz,i.e.,raindrops,
backscatter
part of the upwelling
radiation[Kummerow,
1993].Thismanifests
itselfaslowerT85thanthetemperature
thatwouldhavebeenproduced
by a blackbodyemittance
http://daac.gs
fc.nasa.gov/CAMPAIGN_DOCS/hydrology/hd_trmm_i
ntro.html
Copyright1999by theAmericanGeophysical
Union.
Papernumber1999GL006066.
0094-8276/99/1999GL006066505.00
only.
3105
3106
ROSENFELD.' TRMM OBSERVED FIRST DIRECT EVIDENCE...
The contrast between the clouds in the two areas is even
largerwhenconsidering
the factthatmostof the precipitating
smoke-freecloudsare much smallerand lessdevelopedthan
the non-precipitatingcloudsaffectedby the smoke,and that
cloudsnormallydevelopmore precipitationwith greatervertical development.
Theseobservations
are consistentwith the expectedeffect
of the smoke,causingcloud dropletsto be smallerand thus
havingsmallercoalescence
efficiency.Justhow muchsmaller
the clouddropletsbecomeis illustratedin Fig. 4, whichpresentstherelationbetweenthetemperature
(T) andeffectiveradius (re) of the clouddropletsin areas1 and 2. The calculationsof the evolutionof reasa functionof T weredoneby the
methodologyof Rosenfeld and Lensky [1998]. The re of
cloudsin smokeremainswell belowthe precipitation-forming
Figure 1. The TOMS aerosolindex[Hsu et al., 1996] over
Indonesiaon 1 March 1998. The red color(aerosolindex>2.7)
indicatesthe highestaerosolconcentrations.
The box delimits threshold
of 14 gm up to a temperature
of-12øC.The median
the TRMM observationarea,shownin Fig. 2.
reof the clouddropletsin the cleanerareaexceededthe 14 gm
threshold
alreadyat +8øC,whichis at a heightof about3 kin,
in agreementwith the height of shallowestprecipitating
The lackof radardetectedprecipitation
andthe coldTs5of cloudsshownin the verticalcrosssectionshownin Fig. 3.
the smokycloudsprovidesstrongevidencethat theseclouds
containlargeamountof waterin clouddroplets.The droplets
are so small that they do not coalesceinto precipitationat 3. The significanceof the results
leastduringthe growingstages
of the cloudup to -10øC,or
heightof 6.5 km. In contrast,the abundance
of precipitation
in the relativelysmokefree cloudsshowsthat much of the
cloudwater is convertedinto precipitationsize drops,which
are detectableby the radar.That shouldhave causedcolder
indicatedTs5for the precipitatingclouds.However,the observationsshowthe inversesituationin mostcloudswith pre-
cipitationradar echoes.That meansthat these clouds lost
muchof theirwater,probablyinto precipitation,which fell to
theground.Furthermore,
theonlypossibleexplanation
for T85
of precipitating
cloudsnot lowerthanTssof cloudsreaching
the samealtitudebut not precipitatingis that the cloudslost
much of their water to precipitationalreadywhile growing,
beforehavingreachedthataltitude.This is consistent
with the
suggestion[Rosenreidand Leasky, 1998] that tropical convectivecloudswith largeclouddropletsrainoutwhile growing.
,
!
Thisis nota uniquecase.Severalothersimilarcases(not
shownhere)were observed
to have comparable
behavior.
Furthermore, much less severe situationswere found to have
similarimpacts
on clouds,suchastheburningof agricultural
vegetationin the pre-monsoon
and monsooncloudsof Thai-
land[Rosen/bid
and Wooalley,
1999].Thesefindings
validate
the indirectinferences
of Rosenreid
andLensky[1998],who
showedsimilarbehaviorof theT vs. re relationsfor cloudsin
other cases of forest fires, based on analysesof
NOAA/AVHRR (AdvancedVery High ResolutionRadiometer onboardthe NationalOceanographic
and Atmospheric
Administration
seriessatellites)
data.Thisputsa new light
alsoon theirfindingthaturbanair pollution.Rosenreid
and
Lensky[1998] showedthat air pollutionfrom Manila in the
Philippines
hasa similareffectonreastheforest-firesmoke.
In summary,the findingthatsmokefromburningvegetationhasbeenfoundto shutoff warmrain-forming
processes
!16E
Figure2. TheTRMM VIRSimageof fires,smoke
andclouds
overKalimantan,
Indonesia,
from1 March1998,02:50UT. Thecoloris
composed
of:redforvisiblereflectance,
greenfor3.7gmreflectance
(approximating
re),andbluefortheinverse
of 10.8gmbrightness
temperature.
Thenorthwest
coastof theislandis denoted
bytheyellowline.Thesmallorange
areasontheupperright(east)comerare
hotspotsindicating
thefires.Thesmoke,
streaming
fromthehotspotssouth-westward,
is indicated
by thefuzzypurplecolorof the
background.
Thesmoke-free
background
is blue.Thiscolorscheme
(seefull detailin Rosenfeld
andLensky,1998)shows
clouds
with
smalldroplets
(re<10gtm)as white,becoming
yellowat the supercooled
temperatures.
Cloudswith largerdroplets
(re>15gtm)are
colored
pink,andcoldiceclouds
appear
red.Theblackhatching
markstheareasinwhichtheTRMM radardetected
precipitation.
ROSENFELD.'
TRMM OBSERVED
FIRST DIRECT EVIDENCE...
3107
H [k.] dBZ
613
1O
51]
45'
40
5
.
'---
30
25;
20
15
Figure
3. Vertical
cross
section
along
thelineABinFig.2,where
theleftend
ispoint
Aand
theright
end
correspond
topoint
BinFig.
2.Thegrayareaistheclouds,
asmeasured
bytheirtoptemperature.
Thecolors
represent
theprecipitation
reflectivity,
indBZ,as
measured
bytheTRMMradar.
Thewhite
lineisthebrightness
temperature
oftheTRMMMicrowave
Imager
85GHzvertical
polarization,
plotted
atthealtitude
ofthattemperature.
Please
note
thatthe85GHzbrightness
temperature
andactual
cloud
top
temperature
havedifferentphysical
meaning.
in tropicalclouds,demonstrates
that naturaland man-madeair
pollutionis producingprofoundchangesin the weather.This
inadvertentweathermodificationdoesnot stopwith shallow
warm clouds.The precipitationin deeptropicalclouds,which
extendto temperatures
well below freezing,are also likely to
-20
-10
be affected, because of the less efficient accretion of the
smaller,supercooled,
clouddropsby the growingice precipitationparticles[Johnson,1987, Pinskyet al., 1998]. Suppression of precipitationdue to air pollution might have been
contributingto a trend of decreasingconvectiverainfall in
many areasof the world. Changingthe precipitationand the
10
resultant released latent heat, which is one of the most ener-
getic componentspropellingthe global circulation,is likely
alsoto havea respectiveimpacton globalweathersystem.
20
0
5
10
15
20
25
30
Acknowledgements:
Theseresultsare amongthe first scientificpayoffs of TRMM. The author is gratefulto all the membersof the
TRMM team,too numerousto mentionindividually,for all their hard
workto makethe satellitea reality andthe dataof suchhigh quality.
r eft
-20
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3108
ROSENFELD.:
TRMM
OBSERVED
FIRST DIRECT
EVIDENCE...
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(ReceivedMay 28, 1999;revisedJuly20, 1999;
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