3.1 Early stage

With the onset of melt conditions, Arctic sea ice is typically covered by an old snow cover deposited the previous autumn. Spatially, the depth of the volume can vary over metres depending on the surface topography and aeolian processes. In the early stage, snow covers have low liquid-water contents as air temperatures are well below 0°C. FYI snow cover during SIMMS’93 was shallower than in previous years (1990–92)—10 cm maximum in the crystal-pit area (Fig. 3c)—and was predominantly composed of small, rounded grains (radius 0.3–0.5 μm) (Fig. 3b). Compared to FYI, the MYI surface is considerably more spatially heterogeneous. The floe’s annual melt cycle encourages the development of hummocky surface features separated by depressed interstitial areas referred to here as melt-pond areas. In SIMMS’93, hummock tops consisted of a white frazil-like layer overlying darker blue ice, giving the hummocks a grey tone. Large melt-pond areas covered by a thick snow layer (Fig. 3c) were interspersed among these hummocks.

Fig. 3.

Seasanol transition of snow properties at crystal pits: a. snow wetness (near-surface 3cm layer); b. snow-grain radius (near-surface layer); (c) snare depth: ○, first-year-ice site; □, multi-year-ice site.

Figure 2a and b show albedo spectra collected during the early stage (triangle symbols) over FYI and MYI, respectively. Since both surfaces were covered by snow, the albedo at this stage was primarily controlled by the optical characteristics of the overlying snowpack. The albedo at both sites was high in the visible part of the spectrum (FYI α _vis = 0.86; MYI α _vis = 0.89) as most of the incident solar radiation is scattered out of the snowpack. In the near-infrared region, the albedo begins to decrease incrementally with wavelength. The visible and infrared characteristics of snow spectral albedo are controlled through the wavelength-dependent absorption characteristics of pure ice.Figure 4 shows the absorption coefficients for pure ice over the instrument’s spectral sensitivity. It is evident that, over the spectral range sampled, ice crystals are relatively transparent to incident radiation at visible wavelengths, but become moderately absorptive to infrared radiation (Reference Wiscombe and Warren.Wiscombe and Warren, 1980). In the early stage, visible light extinction in the overlying snowpack was dominated by multiple scattering by ice grains, resulting in a high surface albedo as the scattered light eventually left the volume. The incident near-infrared radiation was absorbed by ice crystals in the near-surface layers of the snowpack. thus reducing the albedo at these wavelengths. This absorption increased with wavelength following the absorption coefficient for pure ice.

Fig. 4.

Absorption coefficient of pure ice from 0.4 to 1.0 μm (source: Reference Grenfell and Perovich.Grenfell and Perovich, 1981).

Figure 2a and b also include modelled visible albedo for optically thick, pure snowpacks for days 135 (FYI) and 134 (MYI), respectively. Illumination and geophysical data of each sampling day were used as inputs to Wiscombe and Reference Wiscombe and Warren.Wiscombe Warren’s (1980) snow-albedo model. The modelled albedo averaged over the visible spectrum is 0.93 for the FYI site and 0.96 for the MYI site. It is evident that the magnitude of the measured visible albedo over both sites is lower than would be expected over an impurity-free, optically thick snow volume. One potential reason for the low visible albedo is the possibility that the snowpack was not optically thick, thus allowing the underlying, lower-albedo ice surface to reduce the visible albedo. Another possible reason would be the presence of impurities in the snow volume that were capable of absorbing visible radiation.

Reference Dozier, Davis and Nolin.Dozier and others (1989) have shown that in order for the albedo of a snowpack composed of snow with radii of 1 mm to be independent of the albedo of the underlying surface (semi-infinite), a snow cover of at least 145 mm w.e. is required when incoming solar radiation at a wavelength of 0.45 μm is incident at a solar zenith angle of 60°. Given the FYI snow volume’s average grain-size of 0.96 mm and density of 307 kg m⁻³ a snow depth of 47 cm would be required before the effects of the underlying ice surface are negated and the volume could be considered optically thick for an incident wavelength at 0.45 μm. Since the snow cover on day 135 was far less (10cm), we conclude that the snow cover was not optically thick and the underlying lower-albedo ice surface is the principal reason for the FYI surface’s lower than expected visible albedo.

Since the snow cover on the MYI site was considerably deeper (48 cm) and composed of finer crystals, the underlying ice did not contribute to the visible albedo. The discrepancy between the modelled and sampled results for day 134 is probably due to the presence of absorbing particulates within the snowpack. Similarly, the lower albedo over the FYI site may also be partially due to impurities. Reference Warren and Wiscombe.Warren and Wiscombe (1980) have shown that grey absorbers within a snow volume, such as soot particles, can reduce the visible albedo of snow. Unfortunately, we are unable to verily this since snow-impurity measurements were not taken. If present, these particulates may have originated as wind-blown crustal material from nearby islands and/or soot from the nearby camp.

3.2 Transitional Stage

The transitional stage was marked by the arrival of positive air temperatures which subsequently transformed FYI and MYI snow covers from a relatively low-water (funicular) state to a saturated, freely draining (pendular) volume. In this stage, the FYI site was covered by a thin, rapidly decomposing snow volume that allowed the underlying ice to appear as scattered grey patches. The MYI surface area was dominated by melt ponds filled with wet snow. In the transitional stage, snow covers over both ice types experienced significant increases in snow wetness and grain-size and decreases in snow depth (Fig. 3; Table 1). The dramatic increase in grain-size was due to the formation of large, polycrystalline aggregate grains by snow grains clustering to minimize surface free energy (Reference ColbeckColbeck, 1979). Increases in liquid water can further increase the optically effective grain-size by filling pore spaces and coating grains (Reference Dozier, Schneider and McGinnis.Dozier and others, 1981).

Spectra with square symbols in Figure 2a and b represent spectral albedo measured during the transitional stage. Compared to the early stage, the FYI visible albedo (day 162) decreased 6% as the snow depth decreased to 6 cm allowing the underlying ice surface to further reduce the surface albedo. By comparison, there was little change (−2%) in the visible albedo of the MYI site (day 161a). It is evident that in terms of a deep, optically thick snow cover, changes in the snow’s grain structure had little effect on the amount of visible radiation reflected out of the volume at this stage. The most dramatic changes in the spectral albedo of both ice types occurred at the longer, near-infrared wavelengths. The appearance of the large aggregate snow grains in the snow’s surface layer increased the amount of absorbed infrared radiation, consequently lowering the near-infrared albedo by 17–18% at both ice sites. In comparison to the MYI site, larger surface snow grains at the FYI site resulted in a lower near-infrared albedo (FYI α _nir = 0.64, MYI α _nir = 0.67). The inflection at 0.95 μm in spectrum day 162 ofFigure 2a is the result of instrument noise.

Warmer conditions led to increased exposure of ice hummocks on the floe. Spectrum day 161b inFigure 2b was collected over a hummock top adjacent to the crystal-pit area. The hummock surface consisted of a thin, crusty, white ice layer covering milky-blue ice. The surface layer had apparently undergone preferential solar melting, making the darker-toned ice layer visible beneath it. The surface’s spectral albedo appears to decrease linearly with wavelength from 0.5 to 0.8 μm. The upper ice layers of the hummock top scattered the incident light less efficiently than the thick melt-pond snow volumes, resulting in lower average visible and near-infrared albedos (α _vis = 0.73; α _nir = 0.57) (Table 1 ).

Two days later, melt-pond snow covers exhibited a dull colour as they approached saturation levels. Within the melt ponds, freely draining meltwater accumulated within lower snow layers, creating a darker sub-surface slush layer. Spectrum day 163 inFigure 2b was collected in a melt-pond depression that consisted of a 15 cm blue-slush layer covered by 10 cm of very wet snow. The thin snow cover allowed incident visible radiation to interact with the lower-albedo slush layer, decreasing visible albedo by 8% from day 161. Large polycrystalline aggregate grains at the surface (grain radius = 4 mm) decreased the near-infrared albedo by 13% to 0.58 through increased absorption in near-surface zones. Overall, the appearance of a saturated snow surface in the transitional stage caused albedo to decrease across the instrument range, the largest decrease being at near-infrared wavelengths.

3.3 Late Stage

In the late stage, both FYI and MYI surfaces evolved into a heterogeneous mix of standing meltwater and bare snow/ice surfaces as air temperatures slaved consistently above O°C The unique topography of MYI floes, having encouraged the accumulation of deep snow covers, led to the formation of water-filled melt ponds interspersed among the floe’s hummocks. In comparison, increased meltwater drainage and flatter topography at the FYI site prevented the formation of deep water-filled melt ponds.

In Figure 2a and b, spectra with circle symbols represent albedo measurements made during the late stage. Unlike the spectra from the previous stages, these data were collected on clear days. The melt-pond spectra appear to be correct, but the near-infrared part of the day 171a spectrum inFigure 2a is probably overestimated due to the response of the collector and thus truncated at 0.77 μm.

Spectrum day 171a inFigure 2a was taken over a white-ice area at the FYI site. The white patches were not drained ice areas, but rather thin, fragile remnants of the decomposed snow cover. This layer was very fragile and easily broke away to reveal a saturated blue-ice cover. The spectrum is slightly higher at the short, blue wavelengths indicating scattering by the dark, underlying ice layer. Spectra were collected over both FYI and MYI melt-pond areas. Spectrum day 171b (Fig. 2a) was taken at the FYI site over a light-blue or aquamarine-coloured melt area 11 cm deep. A thin, frozen ice laver covered the surface. Most of the FYI surface was covered with melt-water. Spectrum day 173 (Fig. 2b) was measured at the MYI site over a milky-blue melt pond filled with 15 cm of meltwater. A thin (<1 cm), translucent, slush layer lay on the pond surface. At both ponds, solar energy incident at shorter wavelengths (< 0.5 μm) penetrated deeply into the underlying ice, while visible light at wavelengths > 0.5 μm interacted more with the submerged surface ice layer (Reference Perovich and Grenfell.Perovich and Grenfell, 1981). This layer appears to scatter less efficiently, resulting in a decrease in albedo. At near-infrared wavelengths, absorption by meltwater clearly dominates as the albedo levels at an average value of 0.12. The small amount of reflected radiation at these wavelengths is due to volume scattering in the pond-surface layers that were present at both sites.