retical, needs to be done investigating scattering
transmitted light. This deficiency has impeded
in sea ice. The albedo is quite sensitive to the
radiative transfer modeling efforts, ice heat bal-
surface state. If the ice has an appreciable snow
ance studies, and bio-optical investigations. This
cover, visible wavelength albedos are above 90%
is beginning to change as advancing technology
and little light is transmitted to the ocean. In very
leads to improved instrumentation and innova-
cold ice (T<24C), hydrohalite precipitates, caus-
tive new approaches to measuring light in and
ing a sharp increase in albedo and extinction co-
under the ice are developed. New sensors make
efficient to values comparable to snow. There is a
it possible to measure detailed spectral transmit-
less pronounced, but still potentially significant,
tances even under thick snow-covered ice. Fiber
effect at temperatures below 8C where mirabilite
optic probes can be frozen in the ice to measure
precipitates. The presence of liquid water on the
the radiance distribution within the ice. Powerful
surface causes a decrease in albedo, which is more
techniques are being applied to measure in-situ
pronounced at longer wavelengths. If the surface
profiles of transmitted irradiance, beam spread,
drains, the brine pockets become air bubbles, re-
and diffuse attenuation coefficient.*
sulting in more scattering and an increase in al-
Many pressing issues concerning sea ice opti-
bedo and extinction coefficient. Ice that is grown
cal properties can only be addressed through in-
faster has more platelets and more brine inclu-
terdisciplinary studies. A combined effort is
sions, and consequently, large albedos and ex-
needed to examine such issues as assessing ice
tinction coefficients. The optical properties de-
albedo feedback, ascertaining the impact of en-
pend not only on the volume of brine or air, but
hanced incident levels of ultraviolet irradiance
on how that brine or air is distributed.
on biota living in or under the ice, and using
Sea ice optical properties is currently a research
satellite-measured microwave signatures as a
area of considerable interest and activity. Even
proxy for large-scale ice albedo. Recent experi-
though much has been learned about the optical
mental programs have recognized this and have
properties of sea ice, there are still numerous im-
emphasized acquiring a comprehensive data set,
portant and intriguing problems extant. A major
including information on the ice state and struc-
goal is quantifying relationships between the
ture, biota, particulates and microwave signatures,
physical and the optical properties. Achieving this
as well as complete optical measurements.
goal entails not only a better understanding of
Another approach to these problems is through
the optical properties, but a better understanding
modeling, in particular through the integration
of the physical properties. Because of the poten-
of models. As the previous section demonstrated,
tial climatological impact of icealbedo feedback,
there are several good radiative transfer models
one area of particular concern is determining how
for sea ice that include information on the physi-
the changes in the physical state of the ice during
cal properties of the ice (Grenfell 1983, 1991, Jin et
the summer melt season affect the albedo of the
al. 1994). There are also models that treat the
ice cover.
physical properties of sea ice during the first year
An improved understanding of scattering in
of growth (Cox and Weeks 1988, Wade and Weeks
sea ice is needed. This can be addressed through
in press). Thermodynamic sea ice models include,
laboratory studies of the scattering properties of
typically in a parameterized fashion, the reflec-
small sea ice samples (Miller et al. 1994) and
tion, absorption and transmission of solar radia-
through field studies investigating the spread of
tion. The effects of biogenic material on transmit-
a collimated beam of light in ice (Longacre and
ted irradiance can be considered (Arrigo et al.
Landry 1994). Another approach to estimating the
1991), and there has been progress towards de-
scattering properties of sea ice is to use Mie theory
veloping a true bio-optical model where the intri-
(Bohren and Huffman 1983). A statistical descrip-
cate interplay between the light levels in and un-
tion of the ice microstructure is needed for this
der the ice and the amount of biological activity
approach, including detailed information on the
can be fully explored (Arrigo et al. 1993).
inclusion size distributions for the air bubbles
General, comprehensive, interdisciplinary
and brine pockets (Perovich and Gow 1991). Little
models are needed models that couple the ice
is known regarding these size distributions and
how they vary with ice physical properties such
as brine volume, density and growth rate.
* Personal communication with S. Pegau, College of
In the past there has been an abundance of
Oceanic and Atmospheric Sciences, Oregon State Uni-
albedo measurements, but few observations of
versity.
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