Arctic Sea Ice Thickness Retrieval From Envisat Altimetry Data

The Arctic has a great impact on the global climate and environment change as the cold source of the world,and is the sensitive and key area of the global climate change research..As one of the most sensitive environmental factors reflecting the climate change in the Arctic,sea ice plays an important role in regulating the regional heat balance.Global warming has led to the continuous melting of sea ice.Especially since the beginning of the 21 st century,the coverage of Arctic sea ice has been significantly reduced,which has directly or indirectly affected the global ocean circulation and atmospheric circulation.Therefore,the study of Arctic sea ice is very urgent and important.As one of the important parameters describing the physical properties of sea ice,the freeboard and thickness of sea ice are important indicators to describe the sea ice variation.Accurate acquisition of the change information of the freeboard and thickness of Arctic sea ice in long time series is not only helpful to the study of global climate change and ecological change,but also of great significance to the Arctic navigation and the development and utilization of Arctic resources.Due to the special geographical and climatic conditions in the Arctic,it is very difficult to obtain the data of sea ice freeboard and thickness in a large range and a long time series.Satellite altimetry is one of the main methods to obtain sea ice freeboard and thickness at present.Envisat began its 10-year continuous altitude observation of the polar region in 2002.Together with other satellite altimeter missions,Envisat has formed a continuous monitoring of the polar region over a long period of time.As a radar altimeter,Envisat's elevation data are susceptible to clutter signals from complex surfaces.Therefore,how to effectively use ENVISAT altimetry data to obtain high-precision freeboard and thickness of Arctic sea ice deserves further study.In this paper,we use ENVISAT altimetry data to invert Arctic sea ice thickness.Main contents of this thesis include:(1)We retrieved the Arctic sea ice freeboard and its variations for the period of2002 to 2012 with Envisat satellite altimetry data.To remove geoid undulations,we reprocessed the Envisat data using a newly developed mean sea surface(MSS)model,named DTU18.We compared the influence of different parameters on the error elimination,and adopted the mean value ±1 times standard deviation for gross error elimination.We validated our freeboard estimates using two radar freeboard products from the European Space Agency(ESA)Climate Change Initiative(CCI)and the Alfred Wegener Institute(AWI),as well as the Operation Ice Bridge(OIB)sea ice freeboard product.Our estimates featured the best performance versus OIB with a mean bias of 0.01 m and RMSE of 0.06 m(2)Based on the results of sea ice freeboard extraction,the seasonal,interannual and spatial variations of the freeboard distribution of Arctic sea ice were analyzed.The results showed that in each growth season,the freeboard of sea ice began to increase in October of each year and reached its maximum in April of the following year.The freeboard of Arctic sea ice showed an obvious trend of decline from 2002 to 2012.The monthly mean freeboard showed two sharp decreases in October 2007 and October 2009,and the lowest annual mean freeboard value was 0.188 m in 2008.Thick sea ice is mainly distributed in the northern of the Canadian Arctic Archipelago and near Greenland,while less sea ice is distributed in the Norwegian Sea and Barron Sea.(3)The appropriate snow thickness and density parameters were selected to calculate the sea ice thickness.The results show that the difference between the sea ice thickness calculated in this paper and OIB is 0.02±0.77 m.The seasonal variation,interannual variation and spatial distribution of sea ice thickness in the Arctic are analyzed.The variation trend of sea ice thickness was similar to that of freeboard,showing a downward trend from 2002 to 2012.The lowest monthly average freeboard value was 1.645 m in October 2007,followed by 1.667 m in October 2009.