BulletinofGlaciologicalResearch22（2005）63−67（JJ◎JapaneseSocietyofSnowandIce

EffectofcIouds andl＝reewateron snow a］bedo
Punit VASHISTH

InstrumentSResearchandDevelopmentEstt，Raipur，Dehradun−248008，India（ReceivedSeptember28，2004；reVisedmanuscriptreceivedNovember17，2004）

Abstract
Study ofspectralreflectance andintegrated albedo ofsnow has been carried out withspecial
emphasisontheeffectsofcloudamountandfreewaterconcentrationonsnowsurface．Experiments
havebeenperformedinlaboratory，byilluminatingsnowsamplesusinganartificiallightsourceto
estimateeffectoffreewaterconcentrationonreflectancecharacteristicsofsnowandtheresultshavebeencomparedwithfieldmeasurementsattwodifferentstationsinGreatHimalayanrangesandone
station over Antarcticice shelf．Emplricalrelationship hasbeendeveloped toestimate variationinsnow albedo with cloud amount．

1．lntroduction
Thereflection，absorptionandtransmissionofso−

lar radiation by any geological surface govern theOfthe realrefractiveindexoficedifferssignificantly
from the refractiveindex of water．The highabsorpTtion coefhcient values of waterin this wavelength
make water much more absorptive compared toice．

Whereasin visible spectrum（0．40toO．70pm），theab−radiationbudgetandlocalclimate．Forahighlyreflec−SOrptioncoefBcientoficeissimilartothatofdistilledWater（Hobbs，1974）．
tingsurfacelikesnow，thisdependenceisstrongdue

to negative radiation budget．The variation ofsnow
albedo with meteorologicaland snow parametersis
extremelylmpOrtantininvestigating the overa11en−ergy budget across the snowpack，in predicting the
rateandtypeofsnowmetamorphismtherebyimprov−

A part of clear skyincident solarradiation getsreflected from the surface of a particulate material
like snow and the rest penetrates below the surface
depending on the absorption coefhcients．The totalreflectance of snow surfaceis dividedinto surface

present study on snow albedo and reflectance has

redradiationleadingtolowreflectancevalues．

tiononseasonalsnowpackin Himalayan ranges．The
Study，Whichincludesinvestigations on theeffectof
cloud amount and free water concentration on snow

2．Laboratoryandfield experimentation

ingavalanchepredictiontechniques，Studyoftheab− reflectance and volumetric multiple scattering（Cho−udhuryandChang，1979；1981；Warren，1982；Vashisth，
1ation rates of snowpack and climatic modelling of1994）．Duetolowabsorptioncoefncientvalues，mOStOf
SnOW bound regions．Considerableprogress hasbeen
thevisibleradiationgetsre且ectedfromsnowsurface
made to understand opticalproperties of snow，but（＞95％reflectance）．High absorption coefficients re−the effects of cloud cover and free water concentra−
ducethemultiplevolumetricscattering，1nnearinfra−tion on snow albedo are yet to be understood．The
beeninitiated to understand the effect ofsolar radia−

albedo，hasbeenusedtoevaluatethesnow metamor−

phismratesandavalancheprediction．
Ice has acomplex refractiveindex thatisoffunT
damentalimportancein determining the amount ofincident radiation absorbed orscattered byice crys−tals．Irvine and Po1lock（1968）havetabulated thereal

andimaginaryrefractiveindicesfornormalincidence

2．J．f、feJd〝ヱeαS〟γe〝1β乃J5Inthecourseofthepresentstudy，meaSurementS
have been made at two differentfieldlocations situ−

ated at3000m altitudein PirPanjalrange（Station−I）
and3800maltitudeinWesternHimalayanrange（Sta−tion−II）．Albedostudieshavealsobeenconductedover
Antarcticice shelf．At both the stations，the seasonal

SnOWCOVerbuildupstartsduringDecember−Januaryofradiation．Incaseofastronglyabsorbingmedia，the and stays ti11May．The snowpack depth varies be−
tween20cmto4．5m．Theaverageannualsnowfallat
radiationabsorbedwilldependon bothrealandima，
fieldstation−IandIIvariesbetweenlO−12mand6−8m
glnaryCOmPOnentSOfthecomplexrefractiveindex．In
nearinfraredspectralrange（0．70to3．00FLm），thevalue respectively．The standing snow cover built up atBulletinofGlaciologicalResearch64both the stations varied between ト3m during theperiod of measurement．During the observation pe−
riod，the average air temperatures atstation−Iin theyear1993−94and1994−95were 8．20c and −5．00c
respectively．Atstation−II，theaverageairtemperature
was−12．30cduringtheobservationperiodofDecem−
bertoMarch（1993−95）．TheAntarcticobservationsite
was situated nearIndian Antarctic Station（70005′s，12000 E）overice shelf and observations were madeduringJanuary−March1995．Theaverageairtempera−ture at Antarctica was T2．5Oc．
Theintegrated snow albedo，SPeCtralreflectance，Observed that clouds，Whenever present affects theandastereo−ZOOm microscope．

Kortum（1969）tabulated the reflectancefordiffe−
rent single scattering（w。）values for direct and diff−

SnOW albedoin two different manners．For an opti−

Cally thick snowpack，anincreasein theintegrated
albedovalues withincreaseincloudamounthasbeen
Observed．Thiscan directly beinferred from thefactthat under cloudy conditions，the radiation reaching
SnOW Surfaceisrestricted tovisiblespectrum．Duetohighreflectancevaluesofsnowinthisspectrum，highalbedo values are obtained．Secondly，under cloudy

COnditions a maJOr Part Of theincident radiationis

diffused．This diffused radiation has significant amo−

unt of near horizontalangles ofincident radiation．
Thesurfacereflectancefromsnow，Whichisgoverned
andtype，SnOWgrainshapeandsize havebeenmoni− StrOngly by elevation angle ofincident radiationin−
toredathourlyintervals．Integratedalbedowasmeas− CreaSeSSharplyatlowelevationanglestoyield high−ured using a thermocouple based albedo meter andintegrated albedo values as indicated by Choudhury
theoutputwascontinuouslyrecordedoveranelectro
andChang（1981）．Figure2（a）and2（b）showtheexperi−poly chart recorder．Spectralmeasurements of re−mentalresults obtained under simulatedlaboratory
flected and incident radiation were recorded using a
COnditionsfordirectanddiffusedreflectancebysnow
GER spectralradiometerin O．3−2．5pm spectralrange． for snow samples having grains diameter as O．5mm
Theamountoffreewaterinsnowpackwasmeasured
andl．Ommrespectively．Itisevidentthatthediffusedusingadielectricmoisturemeter．Snowgrainsizeand
re且ectancevaluesarehighercomparedtodirectre且e−typeweremeasuredusingamovingstagemicrometer
CtanCe Values．
globaland diffusedincidentradiation，Cloud amountZ2 エαわ0γαJoり′e二ゆgγg∽g乃fs

Forestimationofalbedoandreflectancevariation

With free waterconcentrationin snowpack，artificial
WaterSpraylngeXperimentswere performedinlabo−
ratory with snow samples maintained at a tempera−ture of −2Oc whereas the temperature of sprayedusedirradiance．Using the tabulated phase function

and w。Values，diffused and direct reflectance values
from snow surface have been evaluated．Resultsindi−cate that the reflectance from snow surface for diff−usedilluminationis much highercompared todirect
illumination．The presence of cIoudsincreases thewaterwasmaintainedatO℃．Snowsamplesof20×20

diffused radiation and hence the reflectance values

lampat負Ⅹedelevationangleof400．Thisavoidsany

ingfromcloudsofVenusobtainedsimilarresults．

×20cm with known grain sizes were prepared andilluminatedartificiallyusinga500−Wattphotographic

Variationin reflectance values due to changeininci−leading to higherintegrated albedo．Sagan and Pol−
1ack（1967）whileinvestigatingtheanisotropicscatter−

Figurel（a）−1（d）indicatesincreaseinintegrated
dentangleofradiation．Thesnowsamplewasweigh− albedovalueswithcloud amount．Eachdatapointin
tedbeforesprayingwaterandthenafiⅩedamountof thefigureis the average of eight，hourly measure−distilled water was sprayed uniformly all over theSurface ofsnow sample．The variationin reflectance
Values of snow was measured using a spectralradi−
Ometer．Theinitialdensities of snow samples were
between320kgm−3to440kgm3and afterspraylng
thewater，thefinaldensityofsamplesreachedashighas660kgm3．To avoid any variationin reflectance
Values due tochangein snowgrainsizeduetofreez−
ingofwater，a11thereadingsaretakenwithinatime
period ofthreetofiveminutes．

ments．Thefigureindicatesthatirrespectiveoftypeof

3．Results and discussion

formulated to estimateincreasein albedoforcloudy

ユノ．．・1／わぐ（わ川タイ（〜／高〃〃・州＝≠仙人

Clouds have significant effect on the solar spec−

trum reaching earths surface．A thick cloudlayer

COntainslarge amount of perceptible waterin gase−

Cloud，albedoincreasesby8LlO％asthecloudamountincreasesfromOto80Cta．Theaveragealbedovalueof

Station−Iis higherduring the year1993−94（〜75％）as
COmpared to the year1994−95（〜65％）．This can be

attributed to the fact that the average temperaturesduring1994−95 were higher compared to1993−94，

Whichresultedin highermeltingandmetamorphism
rates leading to bigger snow grain diameters andmorefreewateronsnowsurface．Basedonexperimen−
taldata，an empiricalrelationship（eq．1）has been
COnditions．

Ac＝1．03Ar＋』0，（1）

WhereAc：Integratedalbedoforcloudyskycondition（in percent）

OuS，1iquidorsolidstate，Whichmakesitbehavelikea N：Amountofcloud（octa）
black bodyforIR radiation（Rusin，1961）．It hasbeen Ao：Integrated albedoforclearsky condition（in per−

Vashisth％Avorago▲lbdo

100∂5100％AvoragoAlbodo

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Fig．1．Variation ofintegrated snow albedo with cloud amount at station−Isituated at3000m altitudein Pir Panjal
ranges，Station−IIsituatedat3800maltitudeinWestHimalayanrangesandAntarcticiceshelf（70Oo5′s，12000′E）．

Cent）
The effect of cloud amount on integrated snow

albedois highlightedin Fig2（C）and2（d）also．The

daily variation of average albedo values is plottedagainsttheaveragecloudamount（inocta）forstation−
Iand station−IIrespectively．Each data pointin thefigureis average ofeight，hourly data sets collectedduringtheday．Itisobservedthatthereisanincrease
in albedo values withincreasein cloud amount．The

Sharprisein thefigureindicatesnowfallevents．
3．2．旦酌ヲCfq′／柁eぴαfeγ0乃S乃0ひαJ∂edo

Surfaceincreases，the albedo and reflectance values

decrease．However，forlarge amount of free water

concentrations an interesting phenomenon has beenObserved．After the saturationlimit of the capillaryretentivityis reached，the water trapped between

SnOW grains percolates down suddenly，thereby rc−

ducingthewateramountanddecreasingtheeffective
grain radii．This resultsin suddenincreasein albedo

Values．On further addition of water，the pore space

betweensnowgrainsstartsaccumulatingwateragaln
and reflectance values reduce．

Figure3（a）T3（b）showsthevariationinreflectanceStudyindicates that presence offree waterevenin smallquantities reduces the snow albedo slg−

values with amount of free water over snow surface．

tive addition of impurities makes the snow surface
dirty and reduces the reflectance values in visible

down the snow when saturation increases to about

A reductionin reflectance values withincreasein freenificantly．Themeltingsnowcrystalsreleasesthecon−WaterCOntentisclearlyobserved．Duringthestudies
densation nuclei（pollen／dust）from theice grain，Ondielectricpropertiesofwetsnow，SweenyandCol−
Which gets deposited on snow surface．The cumula−beck（1974）showed that the water starts draining
7％ofporevolume．Watersaturationlevelsupto14％
SpeCtrum．Thusevenafterrefreezingofthewater，the Can be obtained when drainageisimpeded by analbedo does not return toits originalvalue．Anotherimportant factor that contributes towards reduction
inalbedovaluesisincreaseintheeffectivesnowgrain
Size due to refreezingof water．Theincreasein snow
grainsize reducesreflectance valuesinnearIRspec−
tralrange（WiscombeandWarren，1980）．

Experimentalinvestigationsinlaboratory andfieldindicate that as the amount of water over snow

Obstacle．Thesevaluesofwaterdrainagematchestheresultsobtainedinre日ectancemeasurements，aSWhen

thequantityofwateroversnowsurfaceincreasesby

4to7％of the sample by weight，（0．04kg to O．06kgWater）freewaterpercolatesdowntherebyincreasing
the reflectance values．This phenomenon repeatsagainwhentheamountofwaterreachesbetween12−13％（0．08kgtoO．1kgwater）ofsamplebyweight．Itis

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Fig．2．（a）and（b）：Diffused and direct reflectance ofsnow for grain diameters O．5andl．Omm with radiationincident on

SnOWSamplesat300，（c）and（d）：Variationofaverageintegratedalbedowithcloudamountforstation−Iandstation−ⅠⅠ．

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Fig．3・（a）and（b）：Variationin reflectance of snow with free water content；（c）reductioninintegrated albedo withincreaseinfreewateramountfornaturalmeltprocessoverAntarcticiceshelf；（d）variationinintegratedalbedowith

freewatercontentforartincialwatersprayingexperimentoverAntarcticiceshelf．

Vashisth∂7

evidentfromthefigurethatthatsmallersnowgrains
Sample（0．5mm）hashigherreflectancevaluesascom− Acknowledgments
pared tolarger snow grain samples（1．20mm）．How−
ever，the reductionin reflectance valuesin both theCaSeSremalnSSame．
Figure3（c）highlights the decreaseinintegrated

The author gratefully acknowledges the co−Operation and help extended by Maj．Gen．S．S．Sha−rma，（Ex−Director）andMr．D．N．Sethi（Jt．Director），Of

albedo withincreasein free water amount over Antr

Snow and Avalanche Study Establishment，Manali，arcticice shelf．Each pointinfigureis an average of（India）duringthecourseofstudy．

four tofive datasets taken during a day．Thefigure
indicatesadecreaseinintegratedalbedovaluesby5−
8％ withincreasein water amount by2−12％．The
SCatterin datais due to variationin various otherparametersinfluencing albedolike snow drift onice

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Choudhury，B．）．andChang，A．T．C．（1981）：Radiativeproper−
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