| Sea ice is an important part of marine hydrological elements,and it has a great influence on the vertical distribution of marine hydrological elements,ocean dynamic phenomena,local climate change,human production activities and so on.In recent years,satellite remote sensing technology has become an efficient sea ice observation method.Visible satellite sea ice monitoring mainly consists of low and medium-resolution optical sensors,this kind of sensor has the advantages of large width,short revisit period,and visual image which is intuitive and effective for monitoring the processes of sea ice freezing and melting.However,visible light sensors are easily affected by the weather,unable to achieve all-weather observation,and can hardly obtain clear and effective remote sensing images under harsh weather conditions.Radar satellite sea altimeter has all-time and all-weather observation capability.In the past 30 years,radar altimeter has made remarkable achievements in sea surface altitude observation.Radar altimeter also has the ability to monitor sea ice,which plays an important role in monitoring extremely low glaciers.Microwave backscattering occurs on the surface of the sea water,when the microwave scattering on the sea surface.The echo waveform received by the altimeter is Brown waveform.While when the microwave mirror reflection occurs on the sea ice surface,the echo waveform received by the altimeter is peak shape.Compared with the Brown waveform,the amplitude of the sea ice echo waveform is larger,and the trailing edge slope decreases rapidly,which descent speed is much higher than that caused by the pointing characteristic of the altimeter antenna.Based on the difference of microwave echo waveforms on sea water and sea ice surface,an ice-water separation method based on altimeter echo waveform data is proposed in this paper.Liaodong Bay,located in the northern Bohai Sea,is the sea area with the most severe sea ice disaster in the Bohai Sea.At present,there is no similar study on the separation of ice-water using altimeter echo waveform in the Bohai Sea.In this paper,the Jason-2 altimeter in the Liaodong Bay sets of 20 Hz Senior Geographical Data Record data is adopted.By comparing a large number of sea ice and sea water echo waveforms,a two-parameter threshold method for ice-water separation is proposed in this paper,which uses the maximum waveform value and the energy drop rate along the trailing edge.In this paper,the sea ice of Liaodong Bay from the end of 2013 to the beginning of 2014 is estimated by the two-parameter threshold method.The extension range along the orbit of the outer edge of sea ice is calculated and compared with the visible light images observed by the GF-1 satellite.The conclusions indicate that: the sea ice extension range data determined by the two-parameter threshold method based on the Jason-2 altimeter are in good agreement with the results of GF-1 visible light images,the mean square deviation is 4.73 km,and the correlation coefficient is 0.979.Combined with the temperature observation data from the end of 2013 to the beginning of 2014,it is found that the deviation of the extension range of the sea ice outer line calculated by the two methods depends largely on the time difference between the the two groups of observed data.After validating the feasibility of the method,the outer line of sea ice in Liaodong Bay from 2008 to 2016 is estimated by using the two-parameter threshold method,the variation of sea ice in Liaodong Bay during the past eight years from 2008 to 2016 is studied.In order to make a more accurate judgment on the severity of the Bohai Sea ice,according to the maximum extension distance of the sea ice,the ice condition is divided into three levels.Grade I,the sea ice extension distance is less than 60km;Grade II,the extension distance is greater than 60 km and less than 120 km;Grade III,the sea ice extension distance is more than 120 km.The conclusion shows that the Grade I ice situation occurs once and the Grade II ice situation occurs four times and the Grade III ice situation occurs three times.In eight years,the largest extension distance of the sea ice outer edge is about 164km(late 2009-early 2010),and the minimum extension distance is about 52km(Late 2014-early 2015).Based on the correlation analysis of the air temperature,precipitation data during the same period,it is found that there is a negative correlation between air temperature and offshore distance of sea ice outer line,which is based on the two-parameter threshold method,the correlation coefficient is-0.56;and there is no significant relationship between annual precipitation and the increase rate and variation of sea ice outer line. |
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