Remote Sensing Monitoring Of Spatial And Temporal Variations Of Surface Meltwater On The Greenland Ice Sheet

The Greenland Ice Sheet(Gr IS)is the second largest ice mass on Earth and would rise global sea level of 7.4 meters if it melts completely.In recent years,due to global warming and the Arctic amplification effect,the Gr IS experienced accelerated mass loss,which has become the largest contributor to global sea level rise and created challenges for humanity to answer global climate change.Surface runoff is the main contribution to the Gr IS surface mass loss.During the melt season,a large amount of meltwater is generated and forms the supraglacial hydrological system including supraglacial rivers,lakes,moulins,and crevasses,affecting the surface mass loss and ice sheet movement.Therefore,surface meltwater on the Gr IS has recently gained the attention of the scientific community.In recent years,the Gr IS experienced increasing surface melt,with the expansion of the ablation zone and extension of melt season.There is significant spatial and temporal variation of surface meltwater on the Gr IS.However,at present,most studies of surface meltwater focusing on the southwest Gr IS and only a handful of studies focusing on other parts of the ice sheet,limiting our understanding of the process of meltwater generation,transportation,storage and release,as well as the surface mass balance of the total Gr IS.Therefore,it is an important scientific problem for the Gr IS hydrological research to investigate the spatial and temporal distribution of the pan-Gr IS surface meltwater.In this paper,we used 10 m Sentinel-2 images acquired in 2016 summer to map pan-Gr IS surface meltwater,reveal the meltwater area,elevation,spatial location and hydromorphological characteristics,and explore their response to surface melt,ice surface topography and ice sheet movement.In addition,the meltwater experienced seasonal evolution in summer,which could not be reflected by a snapshot of surface meltwater.In this study,we selected the northeast Gr IS with fast ice flow and substantial surface runoff as typical study area,used Sentinel-2 images acquired on 36 days of 2016to 2020 summer to extract long times serial of surface meltwater,revealing the season evolution of surface meltwater and its response to ice surface melt.(1)pan-Gr IS surface meltwater extraction.In this study,we used 162 Sentinel-2images of the Gr IS acquired in 2016 summer to extract the meltwater,produced 10 m high spatial resolution pan-Gr IS surface meltwater dataset.The results show that during the 2016 melt season,large volume of meltwater formed large and complex supraglacial hydrological system on both the northern and southern Gr IS.The total area of meltwater was about 1.2×10~4 km~2,and 89.7%area of total surface meltwater located below the equilibrium-line altitude.Supraglacial rivers dominate the meltwater area,accounting for 89.5%of the total meltwater area.(2)Spatial distribution characteristics of pan-Gr IS surface meltwater.Based on the pan-Gr IS meltwater mapped in 2016 summer and multi-source data including regional climate model,digital elevation model and ice flow velocity,this study explored the response of surface meltwater to surface runoff,surface topography and ice sheet movement.The results show that there is a positive linear correlation between surface meltwater area and surface melt intensity,and the quantitative relationship is different in different regions.The meltwater hydromorphology is controlled by surface topography,but surface topography cannot reflect the distribution of slush flow zone at high elevation and the water-injection crevasses or moulins at the margin of the ice sheet.The supraglacial drainage patterns vary with different ice flow regimes.There are long rivers and few lakes in areas in fast ice flow zone while there are short rivers and large,narrow lakes in slow ice flow zone.Meanwhile,long,sinuous rivers and small lakes form on the floating ice(3)Seasonal evolution of surface meltwater on northeast Gr IS.In this study,we selected a typical basin of ice sheet ablation zone to extract surface meltwater from2016 to 2020 in order to reveal the intra-and inter-seasonal evolution of surface meltwater.The results show that there are significant differences in surface runoff and meltwater area between 2016 and 2020,and the maximum annual coverage of meltwater increases with the increase of average surface runoff.The morphological characteristics of meltwater remains stable,but its spatial location moves along the ice flow direction.During the melt season,the meltwater extended to the higher elevation as the melt processes.The seasonal evolution of meltwater area was influenced by the interaction of several factors like surface runoff and ice sheet movement.