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Laboratory Studies of the Optical Properties of Young Sea Ice *

  • Donald K. Perovich (a1) and Thomas C. Grenfell (a1)

Abstract

Laboratory experiments were performed to determine the optical properties of young salt ice and to examine correlations between the optical properties and the state of the ice. Ice was grown at different temperatures (–10, –20, –30, and –37°C) from water of different salinities (0, 16, and 31‰). The experiments were conducted in a cylindrical tank 1 m in diameter designed to approximate natural ice growth and to permit in situ optical measurements. Observed incident, reflected, and transmitted irradiances were used in conjunction with a modified Dunkle and Bevans photometric model to determine spectral albedos and extinction coefficients. Cold ice only 0.25 m thick had albedos which were comparable to the values for 2 to 3 m multi-year ice examined by previous researchers during the summer melt season; extinction coefficients were 1.5 to 15 times greater. As the ice temperature and hence brine volume decreased, both albedo and extinction coefficient increased; when the ice temperature dropped below the eutectic point, they increased sharply. In addition, ice grown at lower air temperatures had greater albedos and extinction coefficients even when ice temperatures were the same. Variations in the optical properties of the ice are determined by changes in the amount of brine and its distribution; thus the optical properties of salt ice depend not only on ice temperature but on initial growth rate. Variations in ice salinity over the range 4‰ to 14‰ produced no detectable changes in the optical properties.

Résumé

On a pour suivi des expériences en laboratoire pour déterminer les propriétés optiques de la jeune glace salée et examiner les corrélations entre les propriétés optiques et l’état de la glace. Les glaces étaient fabriquées à des températures différentes (–10, –20, –30, et –37°C) à partir d’eau de salinités différentes (0, 16, et 31‰). Les expériences ont été conduites dans une enceinte cylindrique de 1 m de diamètre dessinée pour se rapprocher des conditions naturelles de croissance de la glace et pour permettre in situ des mesures optiques. On a utilisé les rayonnements incidents réfléchis et transmis ainsi qu’un modèle photométrique modifié de Dunkle et Bevans pour déterminer les albédos spectraux et les coefficients d’extinction. De la glace froide sous seulement 0,25 m d’épaisseur, à des albédos comparables aux valeurs trouvées par d’autres chercheurs examinant pendant la saison de fusion estivale des glaces pluriannuelles de 2 à 3 m d’épaissuer; les coefficients d’extinction étaient de 1,5 à 15 fois plus grands. Lorsque la température de la glace et par conséquent la teneur en saumure diminuait, l’albédo et le coefficient d’extinction s’accroissait; si la température tombe en-dessous du point eutectique ils augmentaient brusquement. De plus, les glaces formées aux plus basses températures de l’air ont des albédos et des coefficients d’extinction plus forts, même lorsque les températures de la glace sont les mêmes. Les variations dans les propriétés optiques de la glace sont déterminées par les changements dans la teneur en saumure et dans sa distribution; par conséquent, les propriétés optiques de la glace salée dépendent non seulement de la température de la glace mais de son mode de croissance initiale. Des variations dans la salinité de la glace de 4‰ à 14‰ n’ont pas produit de changement décelables des propriétés optiques.

Zusammenfassung

Zur Bestimmung der optischen Eigenschaften jungen, salzhaltigen Eises und zum Studium der Zusammenhänge zwischen den optischen Eigenschaften und dem Zustand des Eises wurden Laborversuche angestellt. Eis wurde bei verschiedenen Temperaturen (–10, –20, –30, und –37°C) aus Wasser mit verschiedenem Salzgehalt (0, 16, und 31‰ ) erzeugt. Die Versuche fanden in einem zylindrischen Tank von 1 m Durchmesser statt, da zur annähernd natürlichen Erzeugung von Eis und zur optischen in sit-Messung eingerichtet war. Zur Bestimmung der spektralen Albedo und des Extinktions-Koeffizienten aus einem modifizierten photometrischen Modell nach Dunkle und Bevans wurden die Beobachtungen einfallender, reflektierter und durchfallender Beleuchtungswerte herangezogen. Kaltes Eis von nur 0,25 m Dicke wies Albedowerte auf, die mit denen für 2 bis 3 m dickes, mehrjähriges Eis vergleichbar waren, das von früheren Forschern während der sommerlichen Schmelzperiode untersucht worden war; die Extinktionskoeffizienten lagen 1,5 bis 15-mal höher. Nimmt die Eistemperatur und damit das Solevolumen ab, so wachsen Albedo und Extinktionskoeffizient ; sank die Eistemperatur unter den eutektischen Punkt, so nahmen beide sprunghaft zu. Ausserdem hatte Eis, das bei niedrigen Lufttemperaturen entstanden war, grössere Albedowerte und Extinktionskoeffizienten, auch wenn die Eistemperatur dieselbe war. Änderungen in den optischen Eigenschaften des Eises ergeben sich aus Änderungen des Solegehaltes und ihrer Verteilung; somit hängen die optischen Eigenschaften von salzhaltigem Eis nicht nur von der Eistemperatur sondern auch von der ursprünglichen Wachstumsrate ab. Änderungen im Salzgehalt über den Bereich von 4 bis 14‰ bewirkten keine erkennbaren Änderungen der optischen Eigenschaften.

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Copyright

Footnotes

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Contribution 573, Department of Atmospheric Sciences, University of Washington.

Footnotes

References

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