Mass flux measurements across the margin of the Antarctic ice sheet

Metadata record for data from ASAC Project 2318 See the link below for public details on this project. ---- Public Summary from Project ---- This project is a cooperative venture between Australian and Italian institutes in a study of the mass budget of a large sector of the East Antarctic ice sheet. The mass budget represents the difference between the net input of moisture from the ocean as snow accumulating on the surface and the discharge of ice from the grounded part of the ice sheet. Any mismatch between these input and out put terms produces a direct contribution to change in sea level. Measurements of ice thickness along the coastal margin of the grounded ice sheet, combined with velocity data derived from satellite remote sensing methods, will provide the first direct measurements of the total mass output for the part of the ice sheet draining through this sector of the coastline. The observations will also a baseline against which future change in the mass flux can be assessed.From the abstract of the referenced paper:An air-borne radio echo sounding survey was conducted over the Amery Ice Shelf, East Antarctica, in December 2003. Nine transverse profiles where obtained approximately normal to the flow of direction, together with one longitude profile for a total length of of flight lines of about 1000km. We determined the spatial distribution of the characteristics of the RES signal returned from the ice-water interface at the base of the shelf. The key characteristics are described by the amplitude of the echo and the slope of the leading edge of the returned pulse.For the mass flux we ice thickness values extracted from our RES data, and velocities derived form InSAR maximum coherence tracking in pairs of Radarsat SAR images, with control provided by surface measurements. We calculate the mass budget for a set of discrete areas bounded by our RES survey lines across the shelf, and flow-lines derived from the velocity field (modified in the across shelf components of the velocity) and from linear flow-features identified in the satellite images. using available estimates of the snow accumulation rate at the upper surface, and assuming the ice thickness and velocities are stationary in time, we attribute the mass imbalance within a sub-area to be the result of melt or freeze at the base of the shelf.Both, the distribution of values of melt-freeze rates, and of the characteristics basal echoes form distinct patterns that appear to be influenced by variations in the ice draft and conditions in the sub-shelf ocean cavity. We find a net melt regime associated with the deeper draft to the South and under the central part of the shelf, and net freeze conditions in the North and sides, particularly towards the western margin. the strength of the electromagnetic signals reflected from the ice water-interface shows a similar pattern of variation along all nine transverse profiles. We find a strong association with basal mass balanceSummary of progress 2004/2005The field phase of the project was completed in 03/04 season. Current work involves analysis and interpretation of the Radio Echo Sounding measurements of ice thickness. An initial appraisal was completed of all RES data. A coarsely-spaced set of preliminary estimates of ice thickness were extracted and merged with GPS position data for generation of a first-cut distribution of ice thickness round a large sector of the ice sheet margin. New analysis systems have been developed and to-date applied to 25% of the data for extraction of detail ice thickness information and characteristics of the radar echoes.An initial assessment of all the RES data collected during the field work has been completed. Preliminary ice thickness estimates have been extracted at coarse spacing along the flight lines to generate a first-cut assessment of the ice thickness distribution for planning next phases of the project and for assessment of the need for additional field work. Those data have been merged with GPS position data collected at a ten-second time-interval along the flight track. This work has been carried out primarily by the Italian investigators.In Australia, software systems have been developed to carry out the full-resolution analysis of the digital RES data. This analysis system is based on developments undertaken in AAS project # 2224. To-date, data analysis using this system has been completed for all flights over the Amery Ice Shelf, and analysis of flights over the Law Dome, Totten Glacier, Moscow University Ice Shelf is nearing completion.As part of the depth extraction procedure, algorithms have been developed and implemented that allow extraction of the echo characteristics for an horizon identified in the digital record. For the echoes forming that horizon, the leading-edge slope, peak and base power, echo-width, and height are extracted. These characteristics vary with the characteristics of the reflector in the ice and can be related to roughness, reflectivity coefficient (dielectric contrast), etc.For the flight lines over the Amery Ice Shelf we have found a spatially coherent pattern in the variation in the echo-height and leading-edge-slope of the basal echo that appears to correspond to variation between individual flow-bands of reflection properties of the ice-water interface. This is a recent preliminary finding and work is continuing with these aspects.This detail analysis phase of the project is being carried out jointly by N Young and A Forieri during a 5-month visit by A Forieri to the ACE CRC funded by PNRA (Italy) and ACE CRC.ASAC项目2318数据集元数据记录 请点击下方链接查看该项目的公开详情。 ---- 项目公开摘要 ---- 本项目为澳大利亚与意大利科研机构合作开展的研究课题，聚焦东南极冰盖大片区域的物质收支（mass budget）。物质收支指的是海洋水汽以降雪形式在冰盖表面积累的净输入量，与冰盖接地部分冰体输出量之间的差值。上述输入与输出项间的任何失衡，都会直接导致海平面变化。对冰盖沿海岸接地区域的冰厚测量结果，结合卫星遥感手段获取的流速数据，将首次实现对该海岸扇区排水型冰盖区域总冰体输出量的直接测量。本次观测还将作为基准基线，用于评估未来物质通量（mass flux）的变化情况。 引自相关论文摘要： 2003年12月，研究团队在东南极埃默里冰架（Amery Ice Shelf）开展了航空无线电回声测深（Radio Echo Sounding, RES）调查。本次调查共获取9条近似垂直于冰流方向的横向测线，以及1条经度测线，飞行总里程约1000千米。研究团队测定了冰架底部冰水界面反射的无线电回声测深信号特征的空间分布，核心特征由回波振幅与返回脉冲的前沿斜率表征。 针对物质通量计算，研究团队采用了从无线电回声测深数据中提取的冰厚值，以及基于Radarsat合成孔径雷达（Synthetic Aperture Radar, SAR）影像对的干涉合成孔径雷达（Interferometric Synthetic Aperture Radar, InSAR）最大相干性追踪得到的流速数据，并以地表测量结果作为控制基准。研究团队以跨冰架的无线电回声测深测线为边界，结合从流速场（修正了跨冰架方向的流速分量）及卫星影像中识别的线性流场特征提取的流线，对一系列离散区域的物质收支进行计算。结合已公开的地表降雪积累率估算值，并假设冰厚与流速随时间保持稳定，研究团队将子区域内的物质失衡归因于冰架底部的融化或冻结过程。 融冻速率的分布与基底回波特征均呈现出独特的空间模式，且该模式似乎受冰架吃水深度变化以及冰架下洋腔环境的影响。研究发现，冰架南部及中部下方的吃水深度较大区域呈现净融化状态，而北部与两侧（尤其靠近西部海岸的区域）则为净冻结状态。冰水界面反射的电磁信号强度在全部9条横向测线上均呈现出相似的变化模式，且该模式与基底物质平衡存在显著相关性。 ---- 2004/2005年度项目进展总结 ---- 本项目的野外阶段已于2003/2004季度完成。当前工作聚焦于冰厚无线电回声测深数据的分析与解译。研究团队已完成全部无线电回声测深数据的初步评估，提取了一套粗间距的冰厚初步估算值，并与GPS定位数据融合，生成了冰盖边缘大片区域的首版冰厚分布草图。 研究团队已开发全新的分析系统，目前已将其应用于25%的数据集，以提取精细化的冰厚信息与雷达回波特征。针对野外采集的全部无线电回声测深数据的初步评估已完成。研究团队沿飞行测线以粗间距提取了冰厚初步估算值，生成了首版冰厚分布评估结果，用于规划项目下一阶段工作以及评估是否需要补充野外作业。上述数据已与沿飞行轨迹以10秒间隔采集的GPS定位数据完成融合，该项工作主要由意大利科研人员完成。 在澳大利亚，研究团队已开发出可对数字化无线电回声测深数据开展全分辨率分析的软件系统，该系统基于AAS项目#2224的研发成果。目前，利用该系统已完成埃默里冰架所有飞行测线的数据分析，而针对劳穹（Law Dome）、托滕冰川（Totten Glacier）以及莫斯科大学冰架（Moscow University Ice Shelf）的飞行测线数据分析已接近尾声。 作为冰厚提取流程的一部分，研究团队开发并实现了可从数字化记录中识别层位并提取对应回波特征的算法。针对该层位的回波，研究团队提取了前沿斜率、峰值与基底功率、回波宽度以及回波高度等特征。上述特征随冰体内反射体的特性变化，可用于表征冰面粗糙度、反射系数（介电对比度）等参数。 针对埃默里冰架的飞行测线数据，研究团队发现基底回波的回波高度与前沿斜率的变化存在空间相干模式，该模式似乎与冰水界面反射特性的各流动带之间的差异相对应。这是近期的初步发现，相关研究仍在推进中。 本项目的精细化分析阶段由N Young与A Forieri共同开展，A Forieri受意大利PNRA与ACE CRC资助，前往ACE CRC开展了为期5个月的合作研究。