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Evolution of melting processes of sea ice
Melting is an important process in the evolution of a sea-ice floe. It is quite relevant to the role of sea ice in climate. As the data base of sea ice ablation is fairly limited (Fetterer, 1998), more research (i.g. SHEBA) focused on this topic during recent years. Before going into the details of the melting processes, the summary chart provides an overview from the onset of melt to disappearance of the ice. Now let's see in detail each process.
Melting processes of the seasonal ice in Arctic:
Melting is dominated by the contrasts in the albedos of snow and ice (albedo is from 0.1 to 0.9) and open water (albedo is below 0.1) or melt ponds (albedo is from 0.2 to 0.4) (Fetterer, 1998; Perovich, 1996). Albedo is in fact a controlling factor in sea ice melting in summer time. Snow and ice are good reflectors of solar radiant energy, while water is a quite effective absorber of solar radiant energy.
Onset of snow melt: All large scale sea ice melting is triggered by the solar radiation. For sea ice with snow cover, the first to melt is the surface snow flakes due to the absorption of solar energy and the subsequent increase of temperature over freezing point. Meanwhile, solar radiation also penetrates into the snow and causes snow melting, settling and packing, which increase the heat conductivity of the snow, and thus the heat transfer from the snow to the ice is enhanced. Once the snow starts to melt, the accumulating water content will further decrease the surface albedo, thus more solar energy will be absorbed. Owing to the penetration of solar shortwave radiation into the mass of ice, the ice surface temperature is lower than it would be without this effect (Dononin,1977). Penetration of radiation into the ice raises its temperature by the order of a fraction of one degree. With further increase in air temperature and solar radiation the surface layer of the snow is saturated with water and the albedo decreases further.
Impurity particles embedded in the ice or snow decrease the albedo of the surface and become melting centers. Micro-organisms may increase melting of ice in the same way as inorganic sedimental particles. Warm winds from nearby land masses can increase the turbulent heat flux and thus accelerate melting.
Shallow meltwater puddles begin to cover the ice:
With melting, dark patches, which consist of snow saturated with water, are the starting points for the formation of melt ponds. Since the melt rate of ice beneath ponds can be up to 2-3 times more rapid than that of bare ice (Hanson, 1965), the ponds deepen and shrink in diameter, first rapidly and then more slowly.
However, on some cases, especially on large and level first year ice floes, melt snow will form puddles. These puddles usually continue to increase steadily in size during summer after vertical depth reaches some definite value and thus vertical melting proceeds very slowly or even stops. Individual puddles gradually increase in size and join one another to form connected puddle systems.
Vertical melt holes develop and a network of surface drainage canals forms: Accumulation of impurities and organic mater in the deepest parts of the puddles intensifies the heat absorption there, resulting in formation of thaw holes or sometimes called cryoconite holes (Podgorny et al., 1996; Eicken et al., 1994), until reaching the bottom of the ice, creating a drainage system. Ablation holes are formed due to dirt deposition at the low parts of ice.
Internal melting:
Besides the absorption of solar radiation at the snow and sea ice surface, absorption of solar short wave radiation can also occur inside the ice due to penetration of the shortwave radiation. Increase of inner temperature can result in internal melting, increasing the ice porosity and enhancing the desalination of the ice.
Disintegration of sea ice cover: The surface of the ice is extremely irregular from the run-off of over-ice water. The central parts of the ice in the water puddles break up and float up due to their structural weakness. As the more saline the ice the greater its ability to absorb solar heat, and as a general rule the ice near the thermal and dynamic cracks formed in winter has the greatest salinity, it is natural that the ice in these cracks is weakest and the first to melt. Thus they are the natural lines of cleavage of the ice in the spring. Typical small cracks will develop during this stage.
Breaking-up into section along the lines of least resistance: The warm melt water will run off along the cracks of the weakened ice, and melts further the lateral sides of the cracks, enlarging the separations. Ice can be broken apart by the wind, the currents and the waves after the ice is weakened by thawing. The outer contours of these broken parts are accidental and have sharp angles. The collisions among these parts due to their movement caused by winds, current, waves, result in the breaking away of outward projecting parts of the floes, leaving water separation in between.
Leads produced by movement of broken ice: After the sea ice breaks into parts, various forces will drive the floes further apart, leaving leads between.
Bottom ablation and lateral melting on the floe edges: As the waterís albedo is no more than 0.1, i.e. much less than that of snow and ice, solar radiation is absorbed quickly in the leads. Transfer of the heat to the lateral and bottom surfaces of ice results in bottom ablation and lateral melting, eventually leaving only small disintegrated floes and pieces of ice, or we call them mushroom ice. All ice floes will disappear soon.
Melting of multi-year ice:
The physical mechanism for melting of multi-year (MY) ice is the same as the first year (FY) ice. However, due to the different thickness and morphology, ablation of MY ice shows different features.
Onset of melting of snow: Compared with FY ice melting, melt water on top of MY ice usually accumulates in the snow, drains and then re-freezes at the snow-ice interface. And thus, a rough superimposed ice layer (Onstott, 1992) or frozen melt pool (Gogineni, et al., 1992) is formed in the interface between snow and ice.
Subsurface melt pools: In the early summer to midsummer, increase in air temperature results in more melting of snow. Snow thickness decreases, wet snow and slush appear. Melting water accumulates in the low parts, subsurface melt pools appear.
Open puddles: In the midsummer to late summer, as the snow melts completely, open puddles appear. Small melt ponds separated by hummocks at first, can be connected to form large open water. Unlike the FY ice, water in the surface melt pools does not usually run off and is frozen in the following fall and winter to form the frozen melt pools.
Features in melting of multi-year sea ice:
References
Doronin, Yu. P., et al. (1977), Sea Ice, trnslated from Russia, Amerind Publishing Co. Pvt. Ltd., New Delhi.
Eicken, H., et al. (1994) Distribution, structure and hydrography of surface melt puddles, Ber. Polarforsch., 149: 73-76.
Gogineni, S.P., et al., The effects of freeze-up and melt processes on microwave signatures. In: Microwave remote sensing of sea ice, Carsey F. D., editor, Geophysical Monograph 68, American Geophysical Union, Washington, 73-104.
Fetterer, F. et al. (1998) Observations of melt ponds on Arctic sea ice. J. Geophys. Rev., 103: 24821-24835.
Hanson, A.M. (1965) Studies of the mass budget of arctic pack ice floes. J. Glaciol., 5: 701-709.
Onstott, R.G. (1992) SAR and scatterometer signatures of sea ice. In: Microwave remote sensing of sea ice, Carsey F. D., editor, Geophysical Monograph 68, American Geophysical Union, Washington, 73-104.
Perovich, D.K. (1996) The optical properties of sea ice. CRREL Monogr., 96-1, 25pp.
Podgorny, I. A., et al. (1996) Absorption of solar energy in a cryoconite hole, Geophys. Rev. Lett., 23: 2465-2468.
