Albedo evolution

Rain on May 29 marked the beginning of the melt season and a transition from dry snow to wet, melting snow. Albedos from June 3 show a drop to wet snow values of 0.7 to 0.75. In some places, the snow was no longer optically thick, resulting in a modest amount of spatial variability in albedo (~0.1). Melting of the snowpack continued, and by June 15 the albedo line consisted of melting snow, bare melting ice, and a few melt ponds. Spatial variability along the line increased greatly, with albedos ranging from 0.3 for the ponds to 0.7 for the melting snow. 

As the melt season progressed, the snow cover disappeared, the bare ice developed a melting surface granular layer, and the ponds grew deeper and wider. By July 23, albedos had decreased everywhere along the line and the spatial variability of albedo had increased. Albedos varied from a minimum of 0.1 for the deep, dark melt ponds to a maximum of 0.65 for bare ice.

Spatial and temporal variability on the albedo line during melt.

Light/dark melt pond on 23 June.

Light dark melt pond on 8 August.

The evolutionary sequences of bare ice and melt ponds is presented in the figure below. A time series of wavelength-integrated albedos from white ice, a light pond, and a dark pond are presented. Albedos at all three sites are the same until the start of melt season  in early June. However, after just a few days of melting, albedos at these sites diverged significantly. The white ice site remained snow covered with a melting snow albedo of 0.7 to 0.8, while the snow melted completely at the pond sites. At this stage the light pond was bare melting ice, with an albedo of 0.65 and the dark pond was already covered by a few cm of water, with an albedo of 0.5. Albedos continued to decrease at the pond sites as the ponds got deeper. At the bare ice site, all the snow melted, reducing the albedo to 0.6-0.7. On 26 June, there was an increase in albedo at all three sites due to a brief cold spell when there was 1 cm of new snow and an ice skim formed on some of the ponds. The increase was greatest at the light pond site, where the albedo jumped from 0.3 to 0.7 as the surface conditions changed from a melt pond to ice with a thin snow cover. After this, the pond albedos decreased throughout the rest of the summer, reaching a minimum value of 0.1 on 13 August. The albedo of bare ice was fairly constant throughout the summer (0.6-0.7), while there was a progressive decrease in the albedo of ponded ice.

Time series of albedo at selected sites along the albedo line.

Observed changes in albedo were a combination of a gradual evolution due to seasonal transitions and abrupt shifts resulting from synoptic weather events. There were five distinct phases in the evolution of albedo: dry snow, melting snow, pond formation, pond evolution, and fall freezeup. The surface albedo was high (0.8-0.9) and spatially uniform in April. In May, there was a gradual decrease in albedo as the snow cover warmed and the snow grain size increased. Rain in late May caused rapid coarsening of the snow and a sharp drop in albedo from 0.8 to 0.7. Although this event marked the onset of melt, there were periods of cooler temperatures and light snow during the next two weeks, with attendant increases in albedo. After this period of “flickering,” there was a steady decrease in average albedo for the remainder of June and July. By the end of July, the average albedo along the line was 0.4. The spatial variability in albedo was greatest at this time, with values ranging from 0.1 for deep, dark ponds to 0.65 for bare white ice. Starting in late July and early August, there were intermittent periods with air temperatures below freezing. During these periods, ice skims formed on the surface of ponds, and there were occasional snow flurries resulting in an increase of 0.1 in average albedo. Again there was a few weeks of “flickering” during freezeup, with albedos increasing and decreasing depending on the synoptic weather. By the end of August, surface temperatures were consistently below freezing, and the albedo increased as the snowcover got deeper. By October, average albedos returned to their springtime maxima of 0.8 to 0.9 and were spatially uniform.

Albedo    Spectral albedos   Transmittance    UV incident

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