The Optical Properties of Sea Ice
DONALD K. PEROVICH
along with the importance to optics of the sea ice
INTRODUCTION
physical state and structure. Observational results
Sea ice is a translucent material with an intri-
are presented, with the emphasis placed on ex-
cate structure and complex optical properties. Un-
plaining the wide variability in sea ice optical
derstanding the reflection, absorption, and trans-
properties in terms of ice physical properties and
mission of shortwave radiation by sea ice is
radiative transfer theory. An overview is given of
important to a diverse array of scientific prob-
existing sea ice radiative transfer models present-
lems. It is of fundamental concern in treating large-
ing their basic characteristics, solution schemes,
scale problems in ice thermodynamics and polar
strengths, and limitations. Finally, current research
climatology. The summer melt cycle of the Arctic
areas and problems of interest in sea ice optical
sea ice cover is driven by shortwave radiation,
properties are discussed. Since the presence of a
making the interaction of shortwave radiation
snow cover can greatly impact light reflection and
with sea ice a critical component of the heat bal-
transmission through sea ice, some mention is
ance of the ice cover (Maykut and Untersteiner
made of the optical properties of snow. An excel-
1971, Maykut and Perovich 1987, Thorndike 1992,
lent review of the optical properties of snow is
Ebert and Curry 1993). Of particular climatologi-
provided by Warren (1982). The optical proper-
cal concern is understanding the sea ice albedo
ties of ice biota and particulates found in the ice
feedback mechanism (Ingram et al. 1989). During
(Arrigo et al. 1991, Roesler and Iturriaga 1994)
the summer the ice cover begins to melt due to
are also discussed briefly because of their impact
the input of solar radiation. This melting tends to
on radiative transfer in sea ice.
decrease the surface albedo and increase the heat
input, thereby accelerating the melt process. Be-
cause of the climatological interest in the heat
BACKGROUND
balance of sea ice, there is also a need for large-
scale spatial and temporal information on ice pack
By "optical" we refer to the portion of the elec-
albedos. Properly interpreted, the reflected radi-
tromagnetic spectrum that is coincident with the
ance measured by visible and near-infrared satel-
wavelength range of radiation from the sun, from
lite sensors can provide such information. In ad-
roughly 250 nm to 2500 nm (Fig. 1). The solar
dition, the amount and spectral composition of
portion of the electromagnetic spectrum is also
shortwave radiation transmitted through sea ice
referred to as shortwave radiation. The optical
strongly impacts primary productivity and bio-
region can be divided into three segments: ultra-
logical activity in and under a sea ice cover (Soo
violet light from 250 to 400 nm, visible light from
Hoo et al. 1987, Arrigo et al. 1993). Visible light
400 to 750 nm, and near-infrared light from 750 to
benefits ice biota by contributing to photosynthe-
2500 nm. The ultraviolet can be further divided
sis, while ultraviolet light can damage organisms.
into UV-C from 200 to 280 nm, UV-B from 280 to
This monograph focuses on the optical proper-
320 nm, and UV-A from 320 to 400 nm. Because of
ties of sea ice. The goal is to provide an introduc-
strong absorption in the atmosphere, essentially
tory tutorial to the topic, not to be a complete
no UV-C reaches the Earth's surface. It is in the
compendium of work in the field. The physical
UV-B where light levels are substantially enhanced
principles underlying radiative transfer in sea ice,
by the depletion of stratospheric ozone (Frederick
including scattering and absorption, are discussed,
and Lubin 1988, Lubin et al. 1989, Tsay and
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