Figure 17. Theoretical estimates of ultraviolet and visible light transmission hrough sea ice in the Weddell Sea

Table 2. Summary of sea ice radiative transfer models

Figure 18. Calculated estimates of spectral albedo as a function of ice density and growth rate

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17

Figure 17. Theoretical estimates of ultra-

Integrated

violet and visible light transmission

through sea ice in the Weddell Sea. Ice

thickness, snow depth and physical prop-

0.05

erties data are from Lange and Eicken

(1991). Transmitted UV-B and visible

irradiance were computed using a two-

100

stream model (Perovich 1990, 1993)

Position (m)

cal properties data from Lange and Eicken (1991)

the roots of the Legendre polynomial. In this for-

were used to define the type and thickness of the

mulation, it is no longer necessary to assume that

ice and snow cover (Fig. 17a) along a 100-m

the radiance field is diffuse and that the phase

transect in the Weddell Sea. Ice thicknesses in this

function is isotropic. However, these models, par-

area varied from 0.2 to 1.2 m, while the snow

ticularly for larger numbers of streams, are sig-

depth ranged from 0.0 to 0.2 m. With these input

nificantly more complex computationally.

parameters, the model calculated estimates of

Perovich and Grenfell (1982) developed a two-

transmittance for the biologically harmful UV-B

layer, four-stream model (radiances at two up-

irradiance (280 to 320 nm) and the beneficial pho-

ward and two downward angles) and applied it

tosynthetically active radiation (400 to 700 nm)

to investigate the effects of ice thickness, and the

(Fig. 17b). There is tremendous spatial variability

influence of direct vs. diffuse incident solar ra-

in the UV-B transmittance over the 100-m transect,

diation, on spectral albedo and transmittance.

with values ranging over nearly two orders of

Using experimentally determined phase functions

they found that single scattering albedos (ϖ0) for

magnitude from 0.0015 to 0.09. The primary in-

fluence on transmittance is the snow depth, fol-

young ice were high: from 0.95 for warm melting

lowed by the ice thickness. Maximum transmit-

young ice to 0.9997 for young ice below the eu-

tances are associated with minimum snow depths.

tectic point.

It is evident that the presence of an ice cover

Grenfell (1983, 1991) developed a single-layer,

causes a marked reduction in transmitted light

16-stream model and a multilayer, four-stream

levels. This reduction is greater for the harmful

model to explore relationships between ice physi-

UV-B than for the beneficial visible, implying that

cal properties and ice optical properties. The four-

sea ice may moderate the biological impact of

stream (Grenfell 1991) model significantly ex-

enhanced incident ultraviolet irradiance on biota

tended the work of Perovich and Grenfell (1982)

living in and under the ice.

by including vertically varying ice properties. The

Models based on the discrete ordinates method

single-layer, 16-stream model (Grenfell 1983) gen-

(DOM) of Chandrasekhar (1960) have been used

erated a more detailed angular description of ra-

to treat scattering in more detail and examine the

diance, better represented the phase function, and

angular distribution of radiance. In the DOM, the

improved the treatment of refraction at the air-ice

phase function is approximated by a series of

interface for a homogeneous ice cover. This model

Legendre polynomials (Liou 1973, 1974, Mobley

was used to directly link the physical properties

of the ice, such as the inclusion size distributions

1994). The discrete ordinates refer to particular

angles at which the radiance is computed. These

of air bubbles and brine pockets, to radiative trans-

angles are not arbitrary, but are determined from

fer in the ice. The absorption and scattering coef-

Index

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