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Remote Sensing of Sea Ice Hazards
Published in George P. Petropoulos, Tanvir Islam, Remote Sensing of Hydrometeorological Hazards, 2017
Melt ponds are puddles of meltwater from the surface of first-year or multiyear sea ice, usually occurring during the melt onset and lasting till they refreeze or drain into the ocean (Fetterer and Untersteiner 1998). A significant change in the surface roughness, ice morphology, and topography is observed after the melt onset in mid-June in the Arctic (Scharien and Yackel 2005). The meltwater forms a network of ponds over the surface. The depth and area of initially small and shallow melt ponds increase as the summer progresses (Maykut 1986). Eventually, the meltwater drains into the sea through holes called thaw holes. The melt ponds that do not drain through thaw holes refreeze in the fall (Taylor and Feltham 2004). It is the thaw holes that are hazardous and can lead to loss of instruments installed usually during summer on sea ice or accidents during hunting (Laidler et al. 2009). In addition to thaw holes being hazardous, melt ponds also affect the variability of surface albedo of the sea ice cover, thus impacting the heat budget of the atmospheric boundary layer (Morassutti and LeDrew 1996). It should also be mentioned that thaw holes are the sources of sunlight for the flora and fauna underneath sea ice, and Arctic seals usually come out of these holes for sunlight (Digby 1984). Thaw holes serve as one of the favorite spots for polar bears to look for food. The next paragraph discusses how optical and microwave remote sensing techniques can be used to detect and estimate melt pond coverage.
The Ice Environment
Published in Rita A. Horner, Sea Ice Biota, 1985
As mentioned earlier, the summer melt cycle over perennial ice is driven almost entirely by absorbed shortwave radiation. Although Fr reaches a maximum in June, the largest values of net shortwave radiation occur in July because of the decrease in a following removal of the snow cover. The presence of melt ponds significantly increases the net shortwave radiation and hastens the rate of decay and retreat of seasonal ice. In areas of perennial ice, ponds increase shortwave input to the ice by 20 to 40% (Table 1), but have only a small effect on the annual average ice thickness.41 Ice concentration is also an important parameter as leads admit large quantities of energy to the ocean. Once the snow cover has vanished, the amount of energy available for primary production in the underlying water is largely determined by the relative area occupied by leads and melt ponds. Table 2 compares how 1 m thick layers of melt pond/blue ice and white ice distribute Fr for different ice concentrations. When Ai is high, transmission through blue ice accounts for most of the energy absorbed beneath the ice. As Ai decreases, leads quickly begin to dominate solar input to the water beneath the ice.
2014 summer Arctic sea ice thickness and concentration from shipborne observations
Published in International Journal of Digital Earth, 2019
Qingkai Wang, Zhijun Li, Peng Lu, Ruibo Lei, Bin Cheng
The process of deriving SIC from oblique-oriented pictures included two steps: image partition and correction of geometric distortion. During the first step, the sea surface components were partitioned into three categories: sea ice, melt pond, and open water, by manually selecting red, green, and blue (RGB) thresholds based on the color distribution histograms within each image (Perovich, Tucker , and Ligett 2002; Inoue, Curry, and Maslanik 2008; Lu et al. 2010; Huang et al. 2016; Li et al. 2017). The threshold levels for each surface category in each image were independently determined. The RGB thresholds for each category were assumed to be disparate and that there would be no overlap between threshold levels. Melt ponds can be distinguished from surrounding ice because they look bluish and pond reflectance is greater in the blue portion of the spectrum than in the red (Grenfell and Maykut 1977). Similarly, open water can be identified by its darker appearance because of lower reflectance. Based on these criteria, melt ponds, open water, and sea ice can be separated from each other.