M96-1 The Optical Properties Of Sea Ice
96-1The Optical Properties of Sea IceMONOGRAPHDonald K. PerovichMay 1996
AbstractSea ice is a translucent material with an intricate structure and complexoptical properties. Understanding the reflection, absorption, and transmission of shortwave radiation by sea ice is important to a diverse array ofscientific problems, including those in ice thermodynamics and polar climatology. Radiative transfer in sea ice is a combination of absorption andscattering. Differences in the magnitude of sea ice optical properties are dueprimarily to differences in scattering. Spectral variations are mainly a result ofabsorption. Changes in such optical properties as the albedo, reflectance,transmittance, and extinction coefficient are directly related to changes in thestate and structure of the ice. Physical changes that enhance scattering, suchas the formation of air bubbles due to brine drainage, result in larger albedosand extinction coefficients. The albedo is quite sensitive to the surface state. Ifthe ice has a snow cover, albedos are large. In contrast, the presence ofliquid water on a bare ice surface causes a decrease in albedo, which ismore pronounced at longer wavelengths. Sea-ice optical properties dependon the volume of brine and air and on how the brine and air are distributed.Cover: Dr. T.C. Grenfell measuring melt pond albedos on first-year icenear Barrow, Alaska. A Kipp radiometer, which measures totalshortwave irradiance, is in the foreground. The cylindrical instrument on the tripod is a scanning spectroradiometer that measuresspectral irradiance from 400 to 2500 nm.For conversion of SI units to non-SI units of measurement consult ASTMStandard E380-93, Standard Practice for Use of the International Systemof Units, published by the American Society for Testing and Materials,1916 Race St., Philadelphia, Pa. 19103.
Monograph 96-1US Army Corpsof EngineersCold Regions Research &Engineering LaboratoryThe Optical Properties of Sea IceDonald K. PerovichPrepared forOFFICE OF NAVAL RESEARCHApproved for public release; distribution is unlimited.May 1996
PREFACEThis monograph was prepared by Dr. Donald K. Perovich, Geophysicist, Snow and IceDivision, Research and Engineering Directorate, U.S. Army Cold Regions Research andEngineering Laboratory, Hanover, New Hampshire. Funding for this work was providedby the generous support of the Office of Naval Research under Contracts N0001495MP30002and N0001495MP30031.The monograph was technically reviewed by J. Richter-Menge and G. Maykut.The author deeply thanks Dr. T.C. Grenfell for two decades of illuminating, insightfuland stimulating discussions on the optical properties of sea ice. The author also appreciates the helpful contributions of G. Cota, A. Gow, B. Light, and G. Maykut. Thanks to K.Jones, R. Maffione, S. Pegau, J. Richter-Menge and W. Tucker for helpful reviews of themanuscript.The contents of this monograph are not to be used for advertising or promotionalpurposes. Citation of brand names does not constitute an official endorsement or approvalof the use of such commercial products.ii
CONTENTSPagePreface .Introduction .Background .Theory .Absorption .Scattering .Observations .Albedos .Reflectance .Transmission .Extinction coefficient .Beam spread .Models .Summary and current areas of interest .Literature cited .Appendix A: List of symbols .Abstract .ii1145688111212141519212527ILLUSTRATIONSFigure1. The optical portion of the electromagnetic spectrum .2. Aerial photograph of typical Arctic summer scene taken from an altitudeof 600 m on 3 August 1994 at 78 N, 177 W .3. Range of observed values of total albedo for sea ice .4. Schematic of radiative transfer in sea ice .5. Absorption coefficients of pure, bubble-free ice .6. Absorption coefficients of biota found in congelation ice and frazil ice .7. Observed and calculated phase functions for sea ice .8. Laboratory observations of the increase in spectral albedo duringinitial ice growth .9. Spectral albedos for a possible evolutionary sequence of multiyear ice .10. A spectral albedo sequence that first-year ice might follow througha melt cycle .11. Observations of total albedo vs. brine volume for young ice .12. Spectral albedos of Antarctic sea ice .13. Bidirectional reflectance distribution function at 450 nm for snow-coveredice and bare blue ice .14. The influence of surface conditions on light transmission .15. Observed spectral transmittances for 1.5-m-thick first-year ice .iii33345678991011111213
FigurePage16. Spectral extinction coefficients for nine distinct cases .17. Theoretical estimates of ultraviolet and visible light transmission throughsea ice in the Weddell Sea .18. Calculated estimates of spectral albedo as a function of ice density andgrowth rate .19. Seasonal changes in underice spectral irradiance calculated using abio-optical model .13171819TABLESTable1. Values of i0 and and κt .2. Summary of sea ice radiative transfer models .iv1416
The Optical Properties of Sea IceDONALD K. PEROVICHINTRODUCTIONSea ice is a translucent material with an intricate structure and complex optical properties. Understanding the reflection, absorption, and transmission of shortwave radiation by sea ice isimportant to a diverse array of scientific problems. It is of fundamental concern in treating largescale problems in ice thermodynamics and polarclimatology. The summer melt cycle of the Arcticsea ice cover is driven by shortwave radiation,making the interaction of shortwave radiationwith sea ice a critical component of the heat balance of the ice cover (Maykut and Untersteiner1971, Maykut and Perovich 1987, Thorndike 1992,Ebert and Curry 1993). Of particular climatological concern is understanding the sea ice albedofeedback mechanism (Ingram et al. 1989). Duringthe summer the ice cover begins to melt due tothe input of solar radiation. This melting tends todecrease the surface albedo and increase the heatinput, thereby accelerating the melt process. Because of the climatological interest in the heatbalance of sea ice, there is also a need for largescale spatial and temporal information on ice packalbedos. Properly interpreted, the reflected radiance measured by visible and near-infrared satellite sensors can provide such information. In addition, the amount and spectral composition ofshortwave radiation transmitted through sea icestrongly impacts primary productivity and biological activity in and under a sea ice cover (SooHoo et al. 1987, Arrigo et al. 1993). Visible lightbenefits ice biota by contributing to photosynthesis, while ultraviolet light can damage organisms.This monograph focuses on the optical properties of sea ice. The goal is to provide an introductory tutorial to the topic, not to be a completecompendium of work in the field. The physicalprinciples underlying radiative transfer in sea ice,including scattering and absorption, are discussed,along with the importance to optics of the sea icephysical state and structure. Observational resultsare presented, with the emphasis placed on explaining the wide variability in sea ice opticalproperties in terms of ice physical properties andradiative transfer theory. An overview is given ofexisting sea ice radiative transfer models presenting their basic characteristics, solution schemes,strengths, and limitations. Finally, current researchareas and problems of interest in sea ice opticalproperties are discussed. Since the presence of asnow cover can greatly impact light reflection andtransmission through sea ice, some mention ismade of the optical properties of snow. An excellent review of the optical properties of snow isprovided by Warren (1982). The optical properties of ice biota and particulates found in the ice(Arrigo et al. 1991, Roesler and Iturriaga 1994)are also discussed briefly because of their impacton radiative transfer in sea ice.BACKGROUNDBy “optical” we refer to the portion of the electromagnetic spectrum that is coincident with thewavelength range of radiation from the sun, fromroughly 250 nm to 2500 nm (Fig. 1). The solarportion of the electromagnetic spectrum is alsoreferred to as shortwave radiation. The opticalregion can be divided into three segments: ultraviolet light from 250 to 400 nm, visible light from400 to 750 nm, and near-infrared light from 750 to2500 nm. The ultraviolet can be further dividedinto UV-C from 200 to 280 nm, UV-B from 280 to320 nm, and UV-A from 320 to 400 nm. Because ofstrong absorption in the atmosphere, essentiallyno UV-C reaches the Earth’s surface. It is in theUV-B where light levels are substantially enhancedby the depletion of stratospheric ozone (Frederickand Lubin 1988, Lubin et al. 1989, Tsay and
total, albedo αt is often a quantity of interest,since it is a measure of the total solar energyabsorbed by the ice and ocean (Maykut andUntersteiner 1971, Maykut and Perovich 1987). Itcan be expressed in terms of the spectral albedoand the spectral incident irradiance asStamnes 1992, and Smith et al. 1992a) and canhave a deleterious impact on living organisms(Smith 1989, Smith et al. 1992b). The familiar spectrum of visible light is also shown in Figure 1from violet (400 nm) to blue (450 nm) to green(550 nm) to yellow (600 nm) to red (650 nm).“Properties” refers to the parameters that areused to describe the reflection, absorption andtransmission of solar radiation by sea ice. Theterminology of radiative transfer is intricate andvoluminous. It also has the unfortunate attributethat the same physical quantity may have a different name, depending on whether an oceanographer, an astrophysicist or a biologist is speaking. To avoid a Babel of jargon we shall limitourselves to the terms needed for a basic understanding of the optical properties and shall follow the terminology conventions of the sea iceliterature.The spectral radiance I(θ,φ,λ) is the power in aray of light in a particular direction, where θ isthe zenith angle (0 pointing downward, π pointing upward), φ is the azimuth angle and λ is thewavelength. The spectral radiance is defined asthe radiant flux/nanometer per unit area per unitsolid angle in a particular direction and has unitsof W m–2 sr–1 nm–1. The spectral irradiance F(λ) issimply the radiance projected onto a plane surface and integrated over a hemisphere. Becauseof this projection the radiance is scaled by cos θ.The downwelling irradiance Fd(λ) is the radianceintegrated over downward directions (e.g., fromthe sky), and the upwelling irradiance Fu(λ) is theradiance integrated over upward directions (e.g.,from the surface). This can be expressed formallyas:Fd (λ ) Fu (λ ) αt R (θ 0 , θ , φ 0 , φ , λ ) I (θ, φ, λ ) cos θ sin θ d θ dφπ φ 0 θ π/2I (θ, φ, λ ) cos θ sin θ d θ dφ .The most studied, and most used, optical property of sea ice is the albedo (α). The spectral albedo is simply defined as the fraction of the incident irradiance that is reflected:α (λ ) dI (θ, φ , λ )cos (θ 0 ) dF (θ 0 , φ 0 , λ )where θ0 and φ0 are the solar zenith and azimuthangle, F(θ0, φ0, λ) is the incident spectral irradiance, and R has units of steradians –1. Formally Ris a derivative quantity, similar to a probabilitydensity function, defined in terms of infinitesimal angles. In practice, the definition is extendedto finite, measurable angles, so that dI I anddF F.Light transmission through the ice is characterized by the transmittance T(λ), which is similar to the albedo in that it is the fraction of theincident irradiance that is transmitted throughthe ice. Light attenuation in the ice is often represented using an irradiance extinction coefficientφ 0 θ 02π(1)The total albedo depends on the spectral distribution of the incident irradiance as well as on thespectral albedo of the surface. Thus a change incloud conditions, and thereby the incident spectral irradiance, can result in changes in the totalalbedo (Grenfell and Maykut 1977).For some problems a knowledge of the angular distribution of the reflected radiance is needed.For example, in climate studies it would be useful to derive large-scale ice albedos from satellitedata. However, satellite sensors have narrow fieldsof view and measure reflected radiance. The keythen is to relate the radiance reflected at the viewing angle of the instrument to the albedo of theice. In order to do this the angular distribution ofreflected radiance, characterized by the bidirectional reflectance distribution function (BRDF),must be known. The formal definition of the BRDFis (Nicodemus et al. 1977, Warren 1982, Perovich1994)2π π / 2 α (λ ) Fd (0, λ ) dλ . Fd (0, λ ) dλκ ( z, λ ) Fu (0, λ )Fd (0, λ ) 1 dFd ( z , λ )Fd ( z , λ )dzwhere F d(z, λ) is the downwelling spectral irradiance at depth z in the ice.Let us now examine the difficulties in deter-where the 0 designates the surface. In sea ice thermodynamic studies the wavelength-integrated, or2
Melting SnowWindpacked SnowNew Snow0.770.810.87Frozen White Ice0.680.70MeltingWhite IceBare (1st yr)0.560.52Mature Pond0.29Refrozen Melt PondPonded (1st yr)0.210.40Old Melt Pond0.15Melting Blue IceOpen Water0.06mining the optical properties ofsea ice by posing a simply statedquestion: “What is the albedo ofsea ice?” Albedos are straightforward to determine. A radiometer is used to measure theirradiance incident on a surfaceand reflected from the surface.The albedo is constrained to liebetween 0, if none of the incident irradiance is reflected, and1, if all the incident is reflected.At first glance this appears to bean easy question to answer.Figure 1. The optical portion of the electromagnetic spectrum. Visible light isFigure 2 is an aerial photofrom 400 nm (violet) to 750 nm (red).graph of a small, roughly onequarter-square-kilometer, area ofa typical summer Arctic scene. The melt seasonhas begun and there is a tremendous amount ofspatial variability in ice surface conditions: snowcovered ice, bare white ice, blue melt ponds,dirty ice, and areas of open water. This variability is also manifested in the wavelengthintegrated albedo, which ranges from 0.05 foropen water, to 0.2 to 0.4 for ponded ice, to 0.5to 0.7 for bare ice, to 0.75 to 0.85 for snowcovered ice. Observations of wavelength-integrated albedo for a full range of sea ice typesand conditions are summarized in Figure 3.Considerable variability in albedo is apparent.Determining that the albedo falls between 0.05Figure 2. Aerial photograph of typical Arctic summerand 0.9 still does not provide an adequate anscene taken from an altitude of 600 m on 3 Augustswer to our question of “What is the albedo of1994 at 78 N, 177 W. The horizontal extent is approxsea ice?” Indeed, while the question is simpleimately 425 m.to state, it is extremely difficult to answer on alarge scale.of the ice. As Figure 2 indicates, sea ice exhibits aConsidering the complicated and variablegreat degree of horizontal variability with diversephysical structure of sea ice, variability in thesurface conditions, including ponds, bare ice, andoptical properties should not be surprising. Tosnow-covered ice, and thicknesses that range fromunderstand and explain this variability, it is necesopen water to pressure ridges over 10 m thick.sary to examine the physical state and structureThere is also vertical complexity,Bulk Albedowith ice properties such as temperature, salinity, brine volumeand air volume changing significantly from the ice surface to theice/water interface. The details ofsea ice physical properties andstructure are summarized inWeeks and Ackley (1982). What is01.00most germane to optics is that seaFigure 3. Range of observed values of total albedo for sea ice. The albedos ice has an intricate structure conare from Burt (1954), Chernigovskiy (1963), Langleben (1971), Grenfell sisting of an ice matrix with inand Maykut (1977), and Grenfell and Perovich (1984).clusions of air, brine, solid salts3
and contaminants, and that it is a material thatexists at its salinity-determined melting point.Therefore, changes in temperature result inchanges in its physical properties and structure.One of the goals of this tutorial is to illustrate, atleast qualitatively, how changes in the ice physical properties are related to changes in opticalproperties. To accomplish this, we must first examine the theoretical underpinnings of radiativetransfer in sea ice.as the equation of radiative transfer for a planeparallel medium (Chandrasekhar 1960): µdI ( τ , µ , φ , λ )dτwhere I the radianceµ the cosine of the zenith angle θφ the azimuth angle.Scattering is included in the S term, which is referred to as the source function. τ is the nondimensional optical depth and is defined asTHEORY[where k is the absorption coefficient, σ is the scattering coefficient, and z is the physical depth. Thesingle scattering albed
plaining the wide variability in sea ice optical properties in terms of ice physical properties and radiative transfer theory. An overview is given of existing sea ice radiative transfer models present-ing their basic characteristics, solution schemes, strengths, and limitations. Finally, current research areas and problems of interest in sea .
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