JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106,NO. D8, PAGES 7955-7966,APRIL 27, 2001
Ice crystal shapesin cirrus cloudsderived from
POLDER/ADEOS-1
H. Chepfer,
1P.Goloub,
2J.Riedi,
2J.F.DeHaan,
3J.W.Hovenier?andP.H. Flamant
•
Abstract. This paperdiscusses
the retrievalof ice crystalshapesof cirruscloudson a globalscale
usingobservations
collectedwith POLDER-1 (POLarizationandDirectionalityof the Earth
Reflectance)onboardthe ADEOS-1 platform.The retrievalis basedon polarizedbidirectional
observations
madeby POLDER. First,normalizedpolarizedradiancesare simulatedfor cirrus
cloudscomposed
of ice crystalsthatdiffer in shapeandarerandomlyorientedin space.Different
valuesof cloudopticaldepths,viewinggeometries
andsolarzenithanglesare usedin the
simulations.
This sensitivitystudyshowsthatthe normalizedpolarizedradianceis highly sensitive
to the shapeof the scatterers
for specificviewinggeometries,andthatit saturatesafter a few
scatteringevents,whichmakesit rapidlyindependent
of the opticaldepthof the cirrusclouds.
Next, normalizedpolarizedradianceobservations
obtainedby POLDER havebeenselected,based
on suitableviewinggeometriesandon the occurrenceof thick cirruscloudscomposedof particles
randomlyorientedin space.For variousice crystalshapestheseobservations
are comparedwith
calculatedvaluespertainingto the samegeometry,in orderto determinethe shapethatbest
reproduces
themeasurements.
The methodis testedfully for thePOLDER datacollectedon
January12, 1997.Thereafter,it is appliedto six periodsof 6 daysof observations
obtainedin
January,February,March, April, May, andJune1997. This studyshowsthatthe particleshapeis
highlyvariablewith locationandseason,andthatpolycrystals
andhexagonalcolumnsare
dominantat low latitudes,whereashexagonalplatesoccurmorefrequentlyat highlatitudes.
1. Introduction
Cirrus cloudscover permanentlymore than 20% of the Earth
[Warren 1986, 1988; Liao et al., 1995], and their impacton the
Earth-atmosphere-ocean
radiation balance is still an open
question.They affect the radiativebudgetthroughtwo opposite
effects:(1) they partly reflect the solarradiationtherebycooling
the atmosphere,
and (2) they partly block the terrestrialradiation,
and this greenhouseeffect tends to heat the atmosphere.
Currently,neither of thesetwo effectsis correctlyquantifiedfor
all regionsof the Earth. Hencethe impactof cirruscloudson the
global radiative budget is still poorly known [Liou, 1986 ;
Stephenset al., 1990]. Severalparametersneed to be studiedto
improve the radiative transfer propertiesof cirrus clouds in
numericalmodelsusedfor climatestudies.Theseparametersare,
for example,globalcirruscloudcover,cloudheight,temperature,
geometricalstructure,optical depth, ice water content, and
microphysicalproperties (i.e., ice crystal size, shape, and
orientationin space).
During the last two decades, several intensive field
experiments
havebeenorganizedto studycirruscloudproperties
on a local scale:FIRE I (FirstISCCP RegionalExperimentFIRE)
1
2
3
Laboratoirede M6t6orologieDynamique,Palaiseau,France.
Laboratoired'OptiqueAtmosph6rique,
Villeneuved' Ascq,France.
Department of Physics and Astronomy, Free University of
Amsterdam,The Netherlands.
4
Also at AstronomicalInstitute"Anton Pannekoek", Universityof
Amsterdam, The Netherlands.
Copyright
2001by theAmericanGeophysical
Union.
Papernumber2000JD900285
0148-0227/01/2000JD900285 $09.00
7955
and II (seethe specialissuesof Monthly WeatherReview, 1990,
and Journal of AtmosphericScience,1995), [Arnott et al., 1991]
took place in the United States in 1986 and 1991, ICE
(International
CloudExperiment)[Franciset al., 1994; Brogniez
et al., 1995] and EUCREX (European Cloud Radiation
Experiment) [Raschke 1996, Raschke et al., 1998] were
performedin Europein 1991, 1993, and 1994, CEPEX (Central
EquatorialPacific Experiment) [McFarquar and Heymsfield
1996] was executedin the Pacific Ocean in 1993, and SUCCESS
(SUbsonicAircraft Contrailand Cloud EffectsSpecialStudy)
[specialsectionGeophysical
Research
Letter1998]tookplacein
the United Statesin 1996. These experimentsincreasedour
understanding
of cirruscloudprocesses
on a mesoscale
andthey
pointedout a strongconnectionbetweenthe microphysical
propertiesandtheradiativeimpactof cirrusclouds.
On the basisof the resultsobtainedduringthoseintensive
field programs,
andwork conducted
in parallel,severalgroups
havestartedto considerthe retrievalof cirruscloudproperties
fromspaceobservations.
As cirruscloudproperties
varyspatially
(latitudeand longitude)and temporally(lifetimeand seasons),
a
completedescriptionof the cirruspropertieson a globalscale
requiressatelliteobservations.
Minnis [1998] has summarized the needs and the state of the
art of the retrieval of cirrus cloud parametersfrom satellite
observations.Several authors [lnoue 1985; Parol et al., 1991;
Minnis et al., 1993; Giraud et al., 1997] havederivedice crystal
sizeusingthe splitwindowtechniqueappliedto IR channelsand
apriori ice crystal shape.More recently,Baran et al. [1999]
deducedthe particlesizeand shapeusinga dual view instrument,
namely ATSR observationsat 3.7 •tm and 10.8 •tm. Moreover,
Rolland and Liou [1998] have studied the possibilitiesof
deriving microphysicalparametersof cirrus clouds using the
MODIS instrumentthatwasrecentlylaunchedon EOS-AM.
7956
CHEPFER
ET AL.: ICE CRYSTAL
SHAPES
IN CIRRUS
CLOUDS
FROM
POLDER
In the presentpaperwe presentresultsof retrievingice crystal
shapeson a global scaleusingPOLDER-1/ADEOS-1 data. This
work is a first attempt to retrieve the particle shape from
polarized bidirectional observationsin the visible domain. The
POLDER instrumentcharacteristics
are presentedin section2. In
section 3 the ice crystal shapesconsideredin this study are
discussed,
as well as the radiativetransfercode usedto compute
the normalizedpolarized radiance.Section4 is devotedto the
definition
of criteria
that will
be used to select POLDER
data
suitablefor the retrievalof ice crystalshape.In section5, results
of 1 day of observationsare first presentedas a testcase,and then
results of analyzing six periods of 6 days of POLDER
observationsare presented.Section6 summarizesthe resultsand
discusses the limitations
of the method used. The conclusions
as
well aspossiblefutureimprovementsare given in section7.
2. POLDER
Instrument
The POLDER instrument[Deschampset al., 1994] has been
designed to measure normalized total radiances (L,•) and
normalizedpolarized radiances(L,•,p)at the top of the
atmosphere.
The POLDER- 1 instrumenton ADEOS- 1 performed
measurementsfor 8 months, from November 1, 1996 to June 30,
1997, and the POLDER-2 instrument is scheduled for launch on
ADEOS-2
in 2001.
POLDER
contains a CCD
matrix
of 242 x
274 pixels.The pixel resolutionin the nadir directionis equal to
6.2 km x 6.2 km. Each fixed target is seen severaltimes in
different viewing geometriesby the POLDER matrix, and the
maximumnumberof differentdirectionsin which one targetcan
be seenduringan ADEOS overpass
is 14.
The viewing directionsobservedwith POLDER correspondto
scatteringanglesbetween60ø and 180ø, dependingon the solar
zenith angle and the position of ADEOS with respect to the
positionof the target. POLDER measuresnormalizedradiances
in eight differentwavelengthchannels,someof which havebeen
b
dedicated
to the studyof variousatmospheric
constituents,
such
km:•)of a cirruscloud
as aerosols[Herman et al., 1997; Deuzd et al., 1998], clouds Figure1. POLDERimage(1700x1500
[Buriezet al., 1997], andwatervapor[Vesperiniet al., 2000], over the North PacificOcean(May 9, 1997) in (a) radianceand
while others are used for studyingground propertieslike (b) polarizedradiance.The wavelengthis 864 nm. The line
corresponds
to the solarprincipalplaneandthecurvesdenotethe
vegetation[Leroy et al., 1997] and ocean color. The state of
scatteringangle with a 10ø incrementbetweeneach line. (c)
polarization
of the light reflectedby the atmosphere
is measured Viewing geometryof the POLDER instrument.
in threedifferentchannels(443 nm, 670 nm, and 865 nm). The
accuracyof thenormalizedpolarizedradianceis 0.001.
The importanceof polarization measurementsfor remote
3. Simulations
of Bidirectional
Polarized
sensing
application
hasbeenpointedoutby Vande Hulst [1957]
Reflectances
and Hansen [1971] and, more recently,with regardto the
POLDER instrument,by Herman et al. [1997]. The POLDER 3.1. Models
instrument
providesnot only, as do otherinstruments
[Diner at
al., 1999], multiangleobservations
but alsopolarizedradiance
In contrast to low-level liquid clouds, ice clouds can be
data.On theotherhand,thespatialresolution
(6.2x6.2km2) is composedof particleswith shapesmuch more complex than
comparatively
coarse.Figuresla andlb showa POLDERimage spheres.In situ observations,collectedduring intensivefield
of a cirruscloudoverthe oceanin totalradianceandpolarized experimentssuch as FIRE, ICE, EUCREX, CEPEX, SUCCESS,
radiance,
respectively.
Studiesof cirruscloudproperties
using haveshowna high variabilityand sometimes
a high complexity
POLDER data started with observations collected with an
of ice crystalshapes[e.g. Heymsfield,1975 ; Krupp, 1991 ;
airbornesimulatorduringintensivefield experiments.
The first Miloshevichand Heymsfield,1996). They also showedthat the
resultsconcerned
thediscrimination
betweeniceandliquidwater shapeof the crystalsdependson latitude, altitude, and on the
clouds by Goloub et al. [1994] and the determinationof ice conditionsduringformationof the cirrusclouds.Theseice crystal
crystalshapeand orientationin space[Chepferet al., 1998]. shapesare sometimessimple, like plates or columnswith a
Morerecently,thesestudies
havebeenextended
to a globalscale hexagonalbase, and sometimesvery complicated,like bullet
with POLDER-1/ADEOS-1 observations[Goloub et al., 2000
and Chepferet al., 1999].
rosettes or dendritic particles. The hexagonal structure is
regularlyobserved,which is consistent
with the fact that liquid
CHEPFER ET AL.' ICE CRYSTAL
SHAPES IN CIRRUS CLOUDS
waternaturallysolidifiesin a hexagonalstructure.On thebasisof
these local observationswe have selectedvarious typical ice
crystalshapesto studythe sensitivityof polarizedradiancewith
respectto the particleshape.The selectedshapesare (1) simple
ice crystals,like columnsandplates,with a shaperatio Qsr=L•R
(L is the length of the crystal, and R is the radius of the
circumscribed
hexagonalbase) which rangesfrom 0.05 to 2.5
[Wendlinget al., 1979; Takanoand Liou, 1989], and (2) more
complexpolycrystallineparticles[Macke et al., 1996] which
wereusedfor ISCCP ice cloudopticaldepthretrieval.It should
be notedthatwe usedthe symbolQ insteadof Qsrin our figures.
The complete scatteringmatrix has been computedfor
randomlyorientedparticleshavingtheseshapes,by usinga raytracing method supplemented
by Fraunhoferdiffraction (cf.
Mackeet al. [1996] for the polycrystals
andBrogniez[1988] and
Chepfer[1997] for the hexagonalparticles).At the wavelength
used(865 nm), the absorptionby ice is very low [Warren et al.,
1986, 1988], hencethe single-scattering
albedois closeto 1. The
scatteringmatrix is employedin a doubling-addingradiative
transfercode [De Haan et al., 1986] in order to take into account
FROM POLDER
7957
0.010
0.009
0.008
•
0.007 -
._
0.006 -
'• 0.005
• 0.004
z
0.0030.002 0.001 -
0.000
60
•
I
80
,
100
I
120
i
I
140
,
I
160
]
Scattering
angle
Figure 2. Simulatedvaluesof the normalizedpolarizedradiance
multiple
scattering
intheatmosphere.
These
radiative
transfer
• reflected
byacirrus
cloud
asafunction
ofthe
cloud
optical
depth
computations
yield
theStokes
parameters
oftheemergent
lightat •5.Thecloud
iscomposed
ofpolycrystalline
particles.
the top of the plane-parallel atmosphere,(I,Q,U,V). The
normalizedpolarizedradianceLn,p(Ov,½v)is
derivedfrom these
Stokesparameters
asfollows
wavelength of 865 nm is used because the contribution of
scatteringby moleculeslocatedabovecirruscloudsis low at this
wavelength.As an example,for a cirruscloud top locatedat 530
Ln,
p(0v,½s-Ov)-Es
hPa, the contributionof Rayleigh scatteringto the normalized
whereEs is the incidentsolarflux at 865 nm at the top of the polarized radiancereachesa maximum of 0.0022 for a solar
atmosphere,0v and ½v are, respectively,the zenith and the zenith angle equal to 40ø. This contributiondecreaseswith the
azimuth viewing angles, and ½sis the azimuth angle of the cloud pressure.As we do not have reliable informationon the
incidentsunlight.Accordingto the usualconvention,(½s-½v)
is cirrus cloud top pressureobservedwith POLDER, we have
equal to 0ø and 180ø for forward and backward directions, chosento neglect the contributionof the Rayleigh scatteringin
respectively.The normalizedtotal radiance(Ln) is derived from our computations
andconsiderit asan uncertainty.
thefirstelement(I) of the Stokesvectorin thefollowingmanner
Figure 2 depictsthe normalizedpolarizedradiance(compare
equation (1)) as a function of the scatteringangle for cirrus
L•(0v,0s
_•r/
-0•)-•s
s
(2) cloudscomposedof polycrystals.The albedoof singlescattering
is one, and the underlyingsurfaceis black. Resultsfor different
Ir•/Q2+u2+v
2
The quantitiesusedin this studyare the normalizedpolarized valuesof thecloudopticaldepth•5,between0.5 and4, havebeen
radiances
(Ln,
p) andthenormalized
totalradiances
(Ln),whichare plotted.Further,a constantsolarzenith angleequalof 60ø and a
directlymeasuredby POLDER. The normalized polarized viewing directionin the solar principal plane (0s-½u= 0ø) have
radianceLn,
p(Ov,
Os-Ov)
and the normalizedtotal radiance been assumed.Figure 2 shows that the normalizedpolarized
are, respectively, linked to the polarized reflectance radianceincreaseswith the cloud optical depthuntil it saturates
Pp(Ov,
0s-0v)
andthetotalreflectance
pp(Ov,
0•-0v)asfollows: for opticaldepthshigherthan 4. For thickercloudsthe polarized
pp(
Ov,½s
-rpv)=.Ln'
p(0v,½s
-•pv)
p (0v,½•
_&):.
L,(0v,½•
-&)
(3)
(4)
radianceis independent
of the opticaldepth.
The normalizedpolarized radiancehas been computedfor
differentparticle shapesin order to studythe sensitivityof the
signalto the particletype.The cloudopticaldepthwas chosento
be 5 to ensurea saturatedsignal.Plate 1 showspolardiagramsof
thesaturated
normalized
polarized
radiance
(Ln,
psat)
computed
for
hexagonalcompactcrystalswith a shaperatio Qsr=l, hexagonal
columnswith Qsr=2.5,polycrystallineparticles,hexagonalplates
where,u•is thecosineof thesolarzenithangle.
with Qsr=0.1, and hexagonalplates with Qsr=0.05.This plate
illustrates the high sensitivity of the saturatednormalized
3.2. Simulations
polarizedradianceto the ice crystalshape.The high sensitivityof
In this sub-section,
simulatedvaluesof normalizedpolarized the polarizationto the particle shapewas also revealedby lidar
radiancesat 865 nm are presented
for cirrusclouds.Resultsfor lineardepolarization
observations
[Sassen,1991]. That Ln,pis
various ice crystal shapes,different viewing geometries,and much more sensitiveto particle shapethan the normalizedtotal
severalsolar zenith angles are given in order to describethe radiance (Ln) was also found with the airborne version of
sensitivity of the polarized signal to these parameters.A
POLDER duringEUCREX'94 [Chepferet al., 1998]. The space-
7958
CHEPFER
ET AL.: ICE CRYSTAL
SHAPES
IN CIRRUS
CLOUDS
FROM
POLDER
Compact: Q=I
0.024
1200
1500
1800
Polycrystals
Column: Q=2.5
0.024,
0.024
I
I
1200 /
1200
•0 ø
I
I
0o
1500
1500
to
1800
1800
0v
Plates: Q=0.05
Plates: Q=O.1
0.070
. to
0.070
600
1200
•
---
* to
-.
1200
600
v
'0{•o
v 0'0001500
1500
•ø
1800
0v
c•
to
1800
c•
0v
Plate 1. Calculatedvaluesof the saturatednormalizedpolarizedradiancefor a cirruscloud composedof various
particleshapes'
(a) hexagonal
compacts
(Qsr=l),(b) hexagonal
columns
(Qsr=2.5),
(c) polycrystalline
particles,
(d)
hexagonal
plates(Qsr=0.1),
and(e) hexagonal
plates(Qsr=0.05).
Thesolarzenithangleis 57.5ø.
CHEPFER
ET AL.: ICE CRYSTAL
SHAPES IN CIRRUS
a 0.045
_
Hexagonal
plates
Q=0.05
•
...... Hexagonal
plates
Q=0.1•
0.035
------ Polycrystals
/•_.
co/•y•ctsQ•l_
O.O30_---- Hexagonal
0.040
_
0.025
_
0.020
0.015
_
ß
...-'
.-.....-
0v=50
o
q>s-q>v=30ø/330
ø _
0.010 -
0.005
•5
0.000l'
20 25 30 35 40 45 50 55 60 65
Solarzenithangle
0.040
-•
Hexagonal
plates
Q=0.050v=400
0.035
....
Polycrystals
......Hexagonal
plates
Q---0.1q>s-q>v
=0ø
Hexagonal
compacts
Q=I•
- --
0.015
0.010 -
0.005
t
0.000/5
20
25
30
35
40
45
50
55
60
65
Solarzenithangle
c 0.045
0.030 -
<50ø and in the corresponding
directionsthat are symmetricwith
respectto the solarprincipalplane:35ø< 0v <55ø and 310ø<qbv
<360ø. Measurementsof the normalizedpolarizedradiancesin
theseviewing directionsare best suitedto discriminatebetween
the shapesof the ice crystals.
Figure 3 shows the variation of the saturatednormalized
polarizedradiancewith solarzenithanglefor theparticleshapes
consideredin this study.Figures 3a, 3b, and 3c pertain to the
viewingdirections(0v= 50; 0s-½v
= 30ø or 330ø),(0v=40ø; 0s-½v
=
0ø), and (0v=40ø; 0s-½v=50
ø or 310ø), respectively.Thesethree
examplesillustratethat the normalizedpolarizedradiancecanbe
uqedto discriminatebetweenthe different ice crystal shapesin
cirruscloudsand that this discriminationis easierfor high values
of solarzenithangles(0•=60ø) thanfor low ones(0•=20ø).Figure
3 alsoshowsthat the distinctionbetweenpolycrystallineparticles
and hexagonalcolumns(Q•r=2.5)is difficult when we consider
the uncertaintyinducedby ignoringRayleighscattering,which
In the following sectionsthe POLDER-saturatednormalized
polarizedradiancesmeasuredabovecirruscloudsare compared
to calculatedvaluesto infer a globalmap of ice crystalshapes.In
comparisonto the normalizedtotal radiance(L,), the saturated
independentof the cirrus cloud optical depth. As soon as the
cloud optical depth is higher than 4, it only dependson the
viewing geometryand the solar zenith angle.Consequently,
the
particle shape obtained by comparingsaturatednormalized
polarizedradianceobservations
and simulatedvaluesconcerns
only the upperlayerof the cirrusclouds(i.e., the first few orders
of scattering).
0.020
0.035 -
7959
normalized
polarized
radiance
(Ln,
psat)
hastheadvantage
ofbeing
Hexagonal
columns-
0.025
0.040 -
FROM POLDER
yieldsanerrorAL,,/,=0.0022.
b 0.045
0.030
CLOUDS
Hexagonal
plates
Q=0.05
...... Hexagonal
plates
Q=0.1
Polycrystals
Hexagonal
compacts
Q=I
m Hexagonal
column
Q=2.5
_
_
_
0.025 0.020 0.015 -
4. Selection of POLDER
Observations
The detectionof ice cloud pixels well suitedfor ice crystal
shape determinationis based on successivetests on the
normalized total radiancesand normalized polarized radiances
collectedby POLDER. The first testconcernsthe clouddetection
(cloudy pixels), the second test concerns the cloud
thermodynamical
phasewhich in the presentstudy is used to
selectice cloudsonly, the third and fourth testsare appliedto
selectcirruscloudscomposedof ice crystalsrandomlyorientedin
space.Thesedifferenttestsarebriefly describedbelow (for more
details,seeChepferet al. [1999] ).
1. The first testaimsat selectingcloudypixelslocatedabove
oceans or sea surfaces and above land surfaces. Cloud detection
0.0100.005 i
0.005 20 25 30 35 40 45 50
Solarzenithangle
i
i
55
60
65
above oceans consistsin selecting pixels with bidirectional
reflectanceshigher than a thresholdvalue equal to 40%. This
thresholdvalueis safeenoughto rejectclearskypixelsandpixels
corresponding
to opticallythin clouds.For pixelslocatedabove
land surfaces,cloud detectionis more complex becausethe
ground
reflectance
variesstrongly
depending
onthesuiface
type
Figure 3. Saturatednormalizedpolarizedradianceas a function (snow,desert,vegetation).We useda clouddetectionschemethat
over
of the solar zenith angle for different ice crystal shapesand has been speciallydevelopedfor POLDER measurements
variousviewing directions: (a) Ov=50ø; ½s-Ov
= 30ø or 330% (b) land surfacesby Brdonet al. [1999]. The use of this schemefor
Ov=40ø; ½s-Ov
= 0ø , (c) Ov=40ø; ½s-Ov
= 50øor 310ø
the currentapplicationis describedby Chepferet al. [1999].
2. The second test consists of identifying the cloud
thermodynamical
phase(ice or liquid water)in orderto selectice
borne version of POLDER
measures normalized radiance in
cloudpixels and rejectliquid water cloudpixels.This test was
viewingdirectionsfor whichthe viewingangles(0v) are smaller developedby Goloubet al. [1994] whousedPOLDER- polarized
than60ø.For theseviewingangles,
Plate1 showsthatL",p satis radiancesmeasuredat 865 nm. It is basedon (1) the presence(or
most sensitiveto the particle shape for viewing directions absence)of a peak at scatteringanglesaround140ø and (2) the
comprised
in thefollowingangleboxes:35ø< 0v<55ø and0ø< Ov behaviourof the normalizedpolarizedradiancesfor scattering
7960
CHEPFER
ET AL.' ICE CRYSTAL
SHAPES IN CIRRUS
anglessmallerthan 110ø. The resultsof this thermodynamical
cloudphasetest [Goloubet al., 1994] havebeencomparedwith
lidar measurements
[Chepferet al., 2000] for validation.Further,
Goloub et al. [2000] showedthat this test selectsice cloudsfor
which the normalizedpolarizedradianceis saturated.
3.The third and fourth tests aim at selecting ice clouds
composedof particlesrandomlyorientedin space.The third test
aims at selectingpixels for which the POLDER views the
directionof specularreflection.POLDER observesa giventarget
in 12 to 14 different directions.To detect specularreflection,
these directions have to include (1) the specular reflection
directionitself, which corresponds
to a viewing zenithangle(0v)
equalto a solarzenith angle(0s) in the solarprincipalplane
½v=180ø),
and (2) neighboringdirectionsto detectthe presenceof
a peak. Thesegeometricalconstraintsstronglyreducethe number
of pixels available for this study. The fourth test enablesus to
detectcloudscomposedof particlesrandomlyorientedin space.
It consistsin removingpixelsfor whichthe polarizednormalized
radiancepresentsa peak in the speculardirection.The peak is
identified when the bi-directionalnormalizedpolarizedradiance
CLOUDS
0.030,
i
I
FROM POLDER
I
•
Plates
Q=0•
0.025
•.......Plates
Q=0.1 •
i -'
Polycrystals
Compacts
Q=I
0.020[•- ColumnsQ=2.5
ß * *Measurements
0.015
• *
......
•-....
0.010
0.005
'=-
98
100
102
104
'
'
I
106
108
110
I
Scattering
angle
in thespecular
(Ln,p,spec)
direction
(+/- 2ø)hasa greater
value
than the bidirectional normalized polarized radiance in the
following
(Ln,p,spec
+1) andprevious
(Ln,p,
spec_l)directions
measured with POLDER.
Hence if
(Ln,p,spec-Ln,
p,spec1) > 0 (Ln,
p,spec-Ln,p,spec+
l ) > 0, (5)
30'N
a specularreflection peak is declaredto be present.A previous
studyhasbeen devotedto the quantityof particlespreferentially
orientatedin space[Chepferet al., 1999]. In contrast,when the
inequalities given in equation (5) are not satisfied,the cirrus
cloud is consideredto be composedof particles randomly
orientedin space,and the pixel is selectedfor the particleshape
i
ß
determination.
The varioustestsdescribedabove enableselectionof pixels
corresponding
to thick ice cloudsfor which the ice crystalscanbe
assumedrandomly oriented. For those pixels the saturated
normalizedpolarizedradiancesmeasuredwith POLDER can be Figure 4. (a) Comparisonsbetween saturated normalized
comparedwith simulatedvalues(comparesection3) in order to polarizedradiances
simulated
andobserved
withPOLDERovera
characterize
the shapeof ice crystals.
thick cirruscloud. (b) Map of particleshapesrandomlyoriented
in spacefor January2, 1997.
5. Results
5.1. Test Case
Pixels pertainingto thick ice cloudswhoseice crystalsand
randomlyorientedin spacehave been selectedemployingthe
testsdescribed
above(section4) for January2, 1997.Only 4082
pixels [Chepferat al., 1999] can be used for crystalshape
retrieval.The pixelsareclassifiedin boxesof viewinganglesand
solarzenithangleswith 10øbin sizein orderto be compared
with
simulationscorresponding
to the sameviewingand solarzenith
angleconditionsat +/- 5ø (section2b). Then differencesbetween
the saturatedpolarizedradiancemeasuredwith POLDER andthe
corresponding
simulatedvaluesare computedfor eachof the five
ice crystalshapes.
Finally,thepixelis flaggedwiththeice crystal
shapefor which the differencebetween the simulatedand the
observed values is minimal. In the case of a minimal difference
largerthan0.01, the pixel is rejected.
When the POLDER instrumentseesone pixel in 14 different
viewingdirections,the shaperetrievalprocedure
is appliedto
eachsuitabledirection.Next,for a givenpixel,a qualityindicator
(at) is computedfor eachshapemodel.We use 6g=N./Ntot,
where
Ni is the numberof timesfor which the shape" i "has been
obtained,andNtot is the total numberof shaperetrievalsfor the
pixel concerned.
Finally,for a givenpixel,theretrievedshapeis
considered
reliableif Nto
t • 2 andat> 0.5. Usually,
Ntottakes
valuesbetween2 and5, andatis higherthantwo thirds.Thistest
reducesthe numberof pixelsfor whichthe ice crystalshapeis
retrieved,but it preventsnonreliableshaperetrievals.
Figure 4a shows an example of comparisonsbetween
observations
andsimulations
for onepixel observed
on January
2, where0s= 34ø. The saturated
normalizedpolarizedradiance
simulated
for the appropriate
viewingdirections
is plottedas a
functionof the scattering
anglefor the five differentshapes.In
thiscasethepixelis observed
for threedifferentdirections
(Ntot=
3) corresponding
to the big dots in Figure 4a. Each of these
observations
is comparedto simulations
and flaggedwith the
particleshapethatagreesbest.For example,Figure4a showsthat
all threeobservations
are flaggedas "plates Qsr=O.1.
". Further,
this retrievalis considered
reliablebecausethe qualityfactor
at=3/3=l.
CHEPFER ET AL.: ICE CRYSTAL SHAPES IN CIRRUS CLOUDS FROM POLDER
PlatesQ----0.05
PlatesQ=0.1
Pol'
60øN
7961
histograms correspondingto the adjacent latitude intervals
(Figures6b and 6d) arehighly similar,showingthatthe dominant
particle shape composingcirrus clouds is symmetricalwith
respectto the ITCZ. Figure 6e showsthat at high latitudesthe
hexagonal plates occur more frequently compared to low
latitudes.
30"N
The results obtained in the period June 1-6, 1997, are
presentedin Figure7a to 7e. In that periodthe ITCZ waslocated
in the 0ø-30øNinterval. The dominatingice crystalshapein that
intervalis the hexagonalcompacts(Q,,r=l) and the columnshape
is the secondmost frequent. In contrastto January,in June the
dnminnntice crystalshapeis not the samein the adjacentlatitude
bands(Figures7c and 7d). Plates(Qsr=0.1) are dominantin the
30øS
30 ø to 60øN latitude band, whereas the columns are dominant in
60øS
the 0ø to 30ø S band. As in January,the dominantparticleshapes
at high latitudes(Figure7e) are the platesandpolycrystals.
The results obtained for six different periodsof 6 days are
90øS
presentedin Table 1. The polycrystalsandhexagonalcolumnsare
Figure5. Map of particleshapes
for a 6-dayperiodin January classified in one single category since their polarization
1997.
signaturesare nearlythe same(section3.2). Table 1 showsthat in
general, the polycrystalsand hexagonal columns occur most
frequentlyglobally,whereasat high latitudesthehexagonalplates
Repeating
thesecomparisons
for eachselected
pixelyieldsa occurmore frequently.Table 2 summarizesmean occurrencesof
mapof icecrystal
shapes
forJanuary
2, 1997(Figure4b).This the different shapesfor all periodsconsideredand confirmsthe
mapshows
thatfewpixelscanbeusedfortheshape
retrieval.
As above
shownin Figure4b, closelyspaced
pixelscancorrespond
to
differentparticle shape.The heterogeneous
geographical
results.
distribution
of thesepixelsis the resultof the successive
tests 6. Discussion
applied
to thePOLDERobservations
(section
4b),namely,(1)
onlylatitudes
between
90øSand40øNwerecovered
in January,
(2) only ice cloudspixelswere selected,(3) only pixels
corresponding
to cirruscloudscomposed
of particles
randomly
oriented
in space
havebeenusedforthisstudy(i.e.,thespecular
Severalpointsconcerningthe resultsmentionedabovewill be
addressed here.
1. On the one hand, the accuracyof the polarized radiance
measured with POLDER
is better than 0.001, so measurement
directionneedsto be observed,and the specularreflectionpeak inaccuracieswill not affect the shaperetrieval significantly.On
has to be absent),(4) only pixels with viewing geometries the other hand, the POLDER spatial resolution is 6x6 km at
sensitiveto the particleshape(in polarization)have been nadir, which meansthat the retrievedshapeis an averageshape
corresponding
to an areaof at least6x6 km.
retained.
2. The observationshave been comparedto several apriori
models.In this respect,two remarksare in order. (1) The shape
5.2. Analyzingsix Periodsof 6 Days
retrieval concernsonly cirrus clouds composedof randomly
To obtainmore informationaboutthe globaldistributionof orientedice crystals,becausethe radiative transfercode that we
icecrystal
shapes,
wehaveprocessed
sixperiods
pertaining
to 6 used assumesrandom orientation in space of the scattering
dayscollected
in January,
February,
March,April,May,andJune particles. Selecting pixels with randomly oriented ice crystals
1997.
strongly reduces the number of pixels available for shape
Figure5 presentstheresultsobtainedfor theperiodJanuary1- retrieval. In the future a similar method could be used to retrieve
6, 1997, and confirms the latitudinal distributionof the pixels the shapeof ice crystalsthat arehorizontallyorientedin space,as
usedfor the particle shaperetrieval.This distributionis mainly soon as radiative transfer codes become available that are able to
handlesuchorientedparticles.(2) Five differentmodelsfor the
due to the constraintson POLDER viewing geometryimposedby
We do not pretendto
the selectionprocedure.In addition,it showsa lack of continuity ice crystalshapeshave been considered.
in nature.Nevertheless,we
betweenpixels located above land and above ocean surfaces, reproduceall the casesencountered
which is probablydue to the differentcloud detectionschemes have noted that in most casesthe polarizedsignaturecan be
used above land and sea. A main difference between the schemes
reproduced
by one of the modelsconsidered.
In the future,other
basedon cloudmicrophysical
models,couldbe takeninto
is that the cirrus cloudsretainedfor the shaperetrieval are very shapes,
thick above oceansin order to avoid sea glitter and to properly account (as soon as the complete scatteringmatrix becomes
available)in orderto obtainmorerealisticparticleshapes.
determinethe ice crystalorientationin space.
To
obtain
information
about the latitude
variation
of the
particle shape, 30ø latitude intervals are consideredand a
histogramis madeof the retrievedshapesin eachinterval.Figure
6a showsthe numberof pixels usedfor the shaperetrieval for
each latitude interval. Figures 6b to 6e show the percentageof
each shaperetrievedin each latitude interval. In Januarythe
IntertropicalConvergence
Zone(ITCZ) is locatedbetween0ø and
30ø south,and the columnsdominatethere (Figure 6c). The two
3.
Several insitu observations have shown that cirrus clouds
are often composedof ice crystalswith a mixture of particle
shapes.Suchcirruscloudshavenot beenconsidered
in thisstudy,
as it would have added an additional degree of freedom to the
calculations. Including a particle shape distribution would
increasethe numberof possiblesolutions.Hence the retrieved
shapeis an effectiveor averageparticleshapeexcept,perhaps,for
particle shapescorrespondingto the extreme curves shown in
7962
CHEPFER ET AL.' ICE CRYSTAL SHAPES IN CIRRUS CLOUDS FROM POLDER
1997 ßJanuary 1st to 6th
I
'
50
70
2000,
1600[
1200
800
400-
-90
-70
-50
-30
-10
10
30
90
Latitude
100
100
b
0o- 30oN
-
90-
0ø- 300 S
80
701-
70
60-
60
50-
40'
50
40 •
_
I
30-
20 L
20
I0;
10
Plates
Plates
Poly!ompa!ts
Columns
Plates PlatesPolyQ=0.05Q=0.1 crystals
Q=I
Q=2.5
Q=0.05 Q=0.1 crystals
Q=I
0
0
CompactsColumns
Q=2.5
I00
100[
9øtd
300- 600 S
90-
e
60ø-900 S
-
80-
70-
70-
60-
60-
5040-
40
30-
30
t
20
t
II
Plates PlatesPoly- Compacts
Columns
Q=0.05 Q=0.I crystalsQ=I
Q=2.5
Plates PlatesPoly- CompactsColumns
Q=2.5
Q=0.05Q=0.1crystals
Q=I
Figure6. Latitudinal
variationof theparticleshapeona globalscalein theperiodJanuary
1-6 1997.(a) Numberof
pixelsusedfor the shaperetrievalasa functionof latitude.(b-e)Histograms
showing
therelativeoccurrence
(in
percent)
of theretrieved
particleshape.
Results
forfourlatitudebands(width30ø) aregiven.
Figure 3, i.e., hexagonalplates (Qsr=0.05) and hexagonal 1 haveshown
a widevariability
of theparticle
shapes
in cirrus
compact(Qsr=l). For otherretrievedparticleshapes,the cirrus clouds,
including
sometimes
idealshapes
likehexagonal
plates
or
cloud might actuallyconsistof a mixtureof variousparticle columns
andpolycrystals.
Theparticleshapes
considered
in this
types.
paperareplausible,
astheycorrespond
to particle
typesthathave
4. The particle shapesretrieved with POLDER have to be beenobserved
in nature.
Nevertheless,
these
results
arecurrently
validatedin the future.The field experiments
discussed
in section difficultto validaterigorously,becauseno collocated
in situ
CHEPFER ET AL.: ICE CRYSTAL SHAPES IN CIRRUS CLOUDS FROM POLDER
7963
1997 ßJune 1st to 6th
2000
1600
1200
8OO
400
-30
0
30
60
Latitude
100
100
90
-
•o-
80
-
80-
70
-
70-
60
-
60-
50
- •-
50-
600- 900 N
40
40
30
30
20
20
10
10
0
Plates PlatesPoly Compacts
Columns
Q=0.05 Q=0.!crystals
Q=I
Q=2.5
0
30o- 600 N
Compacts
Columns
Q=O.05Q=O.
1 crystals
Q=I
Q=2.5
100
1oo
b
oø-30øN
-
90
80-
-
0
70-
-
0
60-
-
0
50-
-
0
90--
d
40-
.0
30
0
20
.0
,o
o .PlatesßIPlatesIPolyI ICompacts
Columns
0
Q=0.05Q=0.! crystals
Q=I
Q=2.5
0
_ c
0ø- 30øs
Plates Plates
Compacts;c umns
Q=2.5
Q=0.05Q=0.1 crystalsQ=I
Figure 7. SameasFigure6, but for theperiodJune1-6, 1997.
observations
havebeendonein cirruscloudduringthePOLDER1/ADEOS-1mission.Hencefuturevalidationis requiredusing
two approaches:(1) comparisonswith in situ microphysical
samplescollectedwith an aircraftin cirruscloudsduringthe
POLDER-2/ADEOS-2 mission,(2) comparisons
with retrieved
shapesdeduced from forthcomingspace-borneobservations,
usingretrievaltechniques
basedon otherphysicalmethods.
7. Conclusions
In this paper,normalized
polarizedradianceobservations
obtainedwith POLDER have been usedto retrievethe shapeof
ice crystals
thatcompose
cirrusclouds.
Themethod
appliedin
thisstudyis completely
newasit usespolarization
toretrieve
the
ice crystalshape.This methodhas threemain advantages
in
7964
CHEPFER ET AL.: ICE CRYSTAL SHAPES IN CIRRUS CLOUDS FROM POLDER
Table 1. LatitudinalVariationof the FrequencyDistributionof RetrievedParticleShapes
Numberof Pixels
PlatesQ•r=0.05
(%)
PlatesQ•r=0.1
(%)
90-60 N
0
0
60-30 N
0
0
30-0 N
0-30 S
30-60 S
60-90 S
90-60 N
924
509
1423
348
0
1
0
4
19
0
Period
Latitude
January
1-6
February
7-12
Columns(Q•r=2.5) and
Polycrystals(%)
CompactsQ•r= 1 (%)
0
0
0
0
0
0
35
6
38
(42)
0
(55)
(82)
(50)
37
0
60-30 N
0
0
0
0
30-0 N
0-30 S
30-60 S
60-90 S
1614
173
821
12
12
4
2
0
(61)
23
22
(67)
26
(56)
(72)
33
9
12
7
1
0
0
1
17
5
0
0
March
90-60 N
0
0
0
0
19-24
60-30 N
30-0 N
0-30 S
30-60 S
629
323
882
439
1
2
2
2
24
41
24
21
(74)
(53)
(60)
(74)
0
0
0
0
0
0
1008
0
5
0
(47)
0
44
15
30-0 N
0
0
0
0
0
0-30 S
30-60 S
1307
63
3
0
18
8
(65)
(89)
15
0
0
0
0
0
0
578
214
774
0
5
1
9
0
(46)
24
21
0
44
(40)
(57)
60-90
April
18-23
S
90-60 N
60-30 N
60-90
May
S
90-60 N
60-30 N
30-0 N
0-30S
June
1-6
1
4
13
3
0
3
0
6
36
14
30-60
S
0
0
0
0
0
60-90
S
0
0
0
0
0
90-60 N
60-30 N
176
1108
8
6
40
(45)
(50)
36
13
30-0 N
1078
1
14
42
(43)
0-30 S
1201
1
16
(66)
16
30-60 S
60-90 S
0
0
0
0
0
0
0
0
0
2
0
In eachlatitudeband,the dominantshapeis listedin parentheses.
comparison
with methodsbasedon normalizedtotalradiance(Ln) retrievedparticleshapepertainsto ice crystalscontainedin the
observations:
(1) thenormalized
polarizedradiance
(Ln,p)
is much highersublayersof the clouds.
This studyis basedon analyzingobservations
collectedduring
more sensitiveto a changein particleshapethanthe normalized
totalradiance,
(2) thenormalized
polarizedradiance
(Ln,p)
allows six periodsof 6 daysin January,February,March, April, May,
and June 1997. It is shownthat the shapeof ice crystalsvaries
strongly spatially. Our analysis yielded the following
conclusions:
(1) the distributionof ice particleshapeseemsto be
symmetricon both sidesof the ITCZ (at leastin January),(2) the
polycrystalsand hexagonalcolumnsseemto dominanteat low
latitudes,whereasthe hexagonalplates seemsto occur more
frequently at high latitudes. This latitudinal distributionof ice
particleshapesmay be due to the conditionsduringformationof
discriminationbetweenice and liquid clouds,without additional
information on the cloud temperatureor altitude, and (3) the
normalizedpolarizedradianceis saturatedafter a few scattering
events and therefore independentof the cloud optical depth
providedthe cirruscloudis sufficientlythick.Thusonedegreeof
freedomhasbeenremovedfrom the shaperetrievalprocedure,as
comparedto a methodbasedon total normalizedradiance(Ln)
observations.
The
main
limitation
of the method
is that the
Table2. SameAsTable1,ButAveraged
OverAll Periods
of 6 DaysConsidered
(January,
February,
March,April,Mayand
June 1997)
i
iiitt it
i ii t tlt
ii iiiiiii
i
iiiiiiiiii
i iiiii
iii
i
iii
iiiiiiiiii
.....................................
Numberof Pixels
PlatesQ•r=0.05(%)
PlateQ•r=0.1
(%)
90o-60øN
60o-30øN
176
3323
8
4.5
40
42
(50)
(47)
2
7
300-0øN
00-30øS
4156
1710
5
3
(39.5)
18
(39.5)
(64.5)
300-60ø S
600-90øS
2746
360
3
18
30
(43)
(61)
39
16
14.5
6
0
Latitude
Columns(Qsr=2.5)andPolycrystals Compacts
Qsr=l
(%)
(%)
CHEPFER ET AL.: ICE CRYSTAL
SHAPES IN CIRRUS CLOUDS FROM POLDER
the cirrus clouds: at low latitudes, cirrus are often (but not
always) the result of convection,whereasat middle and high
latitudes,they are often associatedwith fronts but can also be
generated
by convection.The resultsobtainedat low latitudesare
consistentwith the studyof Baran et al. [1999] who usedATSR
observationsand concludedthat near the top of tropical cirrus
clouds,colunmsandpolycrystalsaremostlikely to occur.
In a follow-up studythe 8 monthsof POLDER observations
might be analyzedin order to obtain more data on the natural
variabilityof the shapeof ice crystalsin cirruscloudon a global
scale.Finally, the next launchof ADEOS-2 andEOS-AM should
provide the opportunityto derive global maps of crystal shape
simultaneouslywith different instrumentsusing independent
retrieval methods,in order to validate the latitudinal shape
distributionobtainedin the presentstudy.
Acknowledgments.We are indebtedto A. Macke for providingus
with the scatteringmatrixof polycrystals.
The resultspresented
herewere
obtainedusinga subsetof the overalldata set collectedby the CNES's
POLDER
radiometer
onboard the NASDA
ADEOS.
7965
Nolin, B. Pinty, C. B. Schaaf,and J. Stroeve,New directionsin Earth
observing:Scientificapplicationsof multiangleremotesensing,Bull.
Am. Meteorol., 80, 2209-2228, 1999.
FIRE IFO II., J. Atmos. $ci., 52, 4041-4392, 1995.
Francis,P. N., A. Jones,R. W. Saunders,K. P. Shine,A. Slingo,and Z.
Sun, An observationaland theoricalstudyof the radiativeproperties
of cirrus: Some results from ICE'89, Q. J. R. Meteorol. $oc.,120,
809-848, 1994.
Giraud, V., J. C. Buriez, Y. Fouquart,and F. Parol, Largescaleanalysis
of
cirrus
clouds
from
AVHRR
data:
Assesment
of
both
a
microphysical index and the cloud top temperature,J. Appl.
Meteorol.,36, 664-675, 1997.
Goloub,P., J. L. Deuzt, M. Herman, and Y. Fouquart,Analysisof the
POLDER polarizationmeasurementsperformedover cloud covers,
IEEE Trans. Geosci. Rein. $ens., 32, 78-88, 1994.
Goloub, P. M, Herman, H. Chepfer, J. Riedi, G. Brogniez,P. Couvert,
and G. S•ze, Cloud thermodynamicalphaseclassificationfrom the
POLDER spaceborne
instrument,J. Geophys.Res., 105(11), 14,74714,759, 2000.
Hansen, J. E, Multiple scattering of polarized light in planetary
atmospheres,
part I, The doublingmethod,J. Atmos.$ci., 28, 120125, 1971.
Herman, M., J.L. Deuzt, C. Devaux, P. Goloub, F.M. Br6on, and D.
Tanrt, Remote sensingof aerosolsover land surfacesincluding
polarizationmeasurements:
Applicationto someairbornePOLDER
References
Arnott,W. P., Y. Dong,andJ. Hallet,Theroleof smallice crystals
in
radiativeproperties
of cirrus:A casestudy,FIRE II, November22,
1991,J. Geophys.Res.,99, 1371-1381,1994.
Baran, A. J., S. J. Brown, J. S. Foot, and D. L. Mitchell,Retrievalof
tropicalcirrusthermalopticaldepth,crystalsizeandshapeusinga
dual view instrumentat 3.7 mm and 10.8 mm, J. Atmos.$ci, in
press,2000.
Br6on,F.-M., and S. Colzy, Cloud detectionfrom the spaceborne
POLDERinstrument
andvalidationagainstsynoptic
observations,
J.
measurements,
J. Geophys.
Res.,102, 17,039-17,049,1997.
HeymsfieldA. J., Cirrus uncinusgeneratingcells and the evolutionof
cirriformclouds,part I, Aircraft observations
of the growthof the ice
phase,J. Atmos..$ci.,32,799-807, 1975.
Inoue,T., On the temperatureand the effectiveemissivitydetermination
of semitransparent
cirruscloudsby bi-spectralmeasurements
in the
10 [tm windowregion,J. Meteorol. $oc. Jpn.,63, 88-98, 1985.
Krupp, C.,Holographicmeasurementsof ice crystalsin cirrus clouds
during.
theInternational
CloudExperiment
ICE 1989,in Reportof
the 4"' ICE/EUCREX Workshop,Lab. d'Opt. Atmos.,USTL, Lille,
France, 1991.
Leroy, M., J.L. Deuzt, F.M. Br6on, O. Hautecoeur,M. Herman, J.C.
Buriez, D. Tanrt, S. Bouffi•s, P. Chazette, and J.L. Roujean,
Retrieval of atmospheric properties and surface bidirectional
orientedin space,paper presentedat the InternationalRadiation
reflectancesover the land from POLDER, J. Geophys.Res., 102,
Symposium,
Lille,France.18-24August,1988.
Appl. Meteorol., 38, 777-785, 1999.
Brogniez,G., Light scattering
by finite hexagonal
crystalsarbitrarily
Brogniez,G., J. C. Buriez,V. Giraud,and C. Vanbauce,Determinationof
17,023-17,037, 1997.
effectiveemittance
andradiatively
equivalent
microphysical
model Liao, X., W. B. Rossow,and D. Rind, ComparisonbetweenSAGE II and
ISCCP high-levelclouds,1, Global and zonal meancloud amounts,
of cirrusfrom ground-based
and satelliteobservations
duringthe
J. Geophys.Res.,100, 1121-1135, 1995.
International
CirrusExperiment:the 18 october1989 casestudy,
Mon. Weather.Rev., 123, 1025-1036, 1995.
Buriez,J. C., C. Vanbauce,F. Parol,P. Goloub,M. Herman,B. Bonnel,
Y. Fouquart,P. Couvert,and G. S•ze, Cloud detectionand derivation
of cloudproperties
fromPOLDER, Int. J. Remote.$ens.,18, 27852813, 1997.
Liou, K. N., Review. Influence of cirrus clouds on weather and climate
processes:A global perspective,Mon. WeatherRev., 114, 1167 1199, 1986.
Macke, A., J. Mueller, and E. Raschke,Single scatteringpropertiesof
atmospheric
ice crystal."J. Atmos.$ci., 53, 2813-1825, 1996.
Chepfer,
H., Etudeth6orique
et exptrimentale
despropritttsoptiques
et McFarquhar,G. M., and A. J. Heymsfield,Microphysicalcharacteristics
of three cirrus anvils sampledduring the CentralEquatorialPacific
radiatives
descirrus,Ph.D.thesis,197p., Lille Univ. 1997.
Experiment(CEPEX), J. Atmos.$ci., 52, 2401-2423, 1996.
Chepfer,
H., G. Brogniez,
andY. Fouquart,
Cirrusclouds
microphysical
properties
deduced
fromPOLDERobservations,
J. Quant.Spectrosc. Miloshevich,L. M., and A. J. Heymsfield,A balloon-bornecloudparticle
Radiat. Transfer,60, 375-390, 1998.
Chepfer,H., G. Brogniez,P. Goloub,F-M. Br6on,P. H. Flamant,Cirrus
cloudIce crystalshorizontallyorientedin spaceobservedwith
POLDER/ADEOS,J. Quant. Spectrosc
and Raiat. Transfer,63,
521-543, 1999.
Chepfer,H., P. Goloub,J. Spinhirne,P.H. Flamant,M. Lavorato,L.
Sauvage,
G. Brogniez,
andJ. Pelon,Cirruscloudsproperties
derived
replicator for measuringvertical profiles of cloud microphysics:
Instrument design and performance,in paper presentedat the
InternationalConferenceon Cloudsand Precipitation,Zurich, 1996.
Minnis, P., K-N.
Liou, and Y. Takano, Inference of cirrus cloud
propertiesusingsatellite-observed
visibleandinfraredradiances.part
I, Parametrization of radiance fields, J. Atmos. $ci., 50, 1279-1304,
1993.
fromPOLDER-1/ADEOS
polarized
radiances:
Firstvalidation
using Minnis, P., Satellite remote sensingof cirrus: An overview, paper
a ground-based
lidarnetwork,J. Appl.Meteor.,39, 154-168,2000.
presented at the International Conferenceon Cirrus, Baltimore,
Md., October 6-8, 1998.
De Haan,J. F., P. B. Bosma,andJ. W. Hovenier,
Theaddingmethodfor
multiple scatteringcalculationsof polarized light, Astron. Parol,F., J. C. Buriez, G. Brogniez,and Y. Fouquart,Informationcontent
Astrophys.,183,371-391, 1986.
of AVHRR channel4 and 5 with respectto the effectiveradiusof
cirruscloudparticles,J. Appl. Meteorol.,30, 973-984, 1998.
Buriez, and G. S•ze, The POLDER mission: Instrument Raschke,E., EuropeanCloud and Radiation EXperiment (EUCREX),
final report,,EV5V - CT 92 - 0130 EUCREX-2, 1996.
characteristics
andscientificobjectives,
IEEE Trans.Geosci.Remote
Sens.,32, 598-615, 1994.
Raschke,E., P. Flamant, Y. Fouquart,P. Hignett, H. Isaka, P. R. Jonas,
H. Sundquist,and P. Wendling, Cloud-radiationstudiesduring the
Deuz•, J. L., M. Herman,P. Goloub,D. Tanr•, and A. Marchand,
Characterizationof aerosolsover the oceanfrom POLDER/ADEOSEuropean
CloudRadiationExperiment(EUCREX), Surv.Geophys.,
1, Geophys.Res.Lett.,26(10), 1421-1424,1999.
19, 89-138, 1998.
First ISCCPRegionalExperiment
(FIRE), Mon. WeatherRev., 118, Rolland,P., andK. N. Liou,Remotesensingof opticalandmicrophysical
2259-2446, 1990.
propertiesof cirrus cloudsusing MODIS channels,International
Deschamps,
P. Y., F. M. Br6on,M. Leroy,A. Podaire,A. Brickaud,J. C.
Diner,D. J., G. P. Asner,R. Davies,Y. Knyazikhin,
J.P. Muller,A. W.
Conference on Cirrus, Baltimore, Md., October 6-8, 1998.
7966
CHEPFER
ET AL.: ICE CRYSTAL
SHAPES
Sassen,K., The polarizationlidar techniquefor cloudresearch:A review
and current assessment,Bull. Am. Meteorol. Soc., 72, 1848-1866,
1991.
Stephens,G. L., S.C. Tsay, P. W. StackhouseJr., and P. J. Flateau,The
relevanceof the microphysicaland radiative propertiesof cirrus
IN CIRRUS
CLOUDS
FROM
POLDER
Tech.Not, NCAR, TN-317 STR,212 pp., Natl. Cent.for Atmos.Res.,
Boulder, Colo., 1988.
Wendling,P., R. Wendling,and H. K. Weickmann,Scattering
of solar
radiationby hexagonalice crystals,Appl. Opt., 18, 2663 - 2671,
1979.
clouds to the climate and climatic feedback." J. Atmos. Sci., 47,
1742-1753, 1990.
SUCCESS,2, Geophys.Res.Lett., 25, 1327-1398, 1998.
Takano, Y., and K. N Liou, Solar radiative transferin cirrus clouds. Part I
: Single-scattering
and opticalpropertiesof hexagonalice crystals,J.
Atmos. Sci., 46, 3-19, 1989.
Van de Hulst H. C., Light Scatteringby SmallParticles,470 pp, Dover,
Mineola, N.Y., 1957.
Vesperini, M., F.M. Br6on, and D. Tanrt, Atmosphericwater vapor
contentfrom spacebornePOLDER measurements,
IEEE TGRS, 37,
1613-1619, 2000.
Warren, S. G., C. J. Hahn, J. London, R. M. Chervin, and R. Jenne,
H. ChepferandP. Flamant,Laboratoire
de M6t6orologie
Dynamique,
Ecole
Polytechnique, 91128
(chepfer
@lmd.polytechnique.fr)
Palaiseau
Cedex,
P. Golouband J. Riedi, Laboratoired'OptiqueAtmosph6rique,
Universit6
deLille I, 59655Villeneuved'AscqCedex,France.
J. De HaanandJ. Hovenier,Department
of Physics
andAstronomy,
FreeUniversityof Amsterdam,The Netherlands.
J. Hovenier,
Astronomical
Institute"AntonPannekoek",
University
of
Amsterdam, The Netherlands.
NCAR Tech. Note, NCAR, TN-273 STR, 212 pp., Natl. Cent. for
Atmos. Res., Boulder, Colo., 1986.
Warren S. G., C. J. Hahn, J. London, R. M. Chervin, and R. Jenne, NCAR
France.
(ReceivedSeptember29, 1999; revisedMarch 16, 2000;
acceptedMay 4, 2000.)