Investigation of sea ice physical processes in East Antarctica during early Spring - Measuring snow thickness over Antarctic sea ice with a helicopter-borne 2-8 GHz FMCW radar

Public Summary for project 2901
This research will contribute to a large multi-disciplinary study of the physics and biology of the Antarctic sea ice zone in early Spring 2007. The physical characteristics of the sea ice will be directly measured using satellite-tracked drifting buoys, ice core analysis and drilled measurements, with detailed measurements of snow cover thickness and properties. Aircraft-based instrumentation will be used to expand our survey area beyond the ship's track and for remote sampling. The data collected will provide valuable ground-truthing for existing and future satellite missions and improve our understanding of the role of sea ice in the climate system.

Project objectives:
(i) to quantify the spatial variability in sea ice and snow cover properties over scales of metres to hundreds of kilometres in the region of 110 - 130 degrees E, in order to improve the accuracy of sea ice thickness estimates from satellite altimetry and polarimetric synthetic aperture radar (SAR) data.

(ii) To determine the drift characteristics, and internal stress, of sea ice in the region 110 - 130 degrees E.

(iii) To investigate the relationships between the physical sea ice environment and the structure of Southern Ocean ecosystems (joint with AAS Proposal 2767).

Taken from the abstract of the PhD thesis accompanying the dataset:
Antarctic sea ice and its snow cover are integral components of the global climate system, yet many aspects of their vertical dimensions are poorly understood, making their representation in global climate models poor. Remote sensing is the key to monitoring the dynamic nature of sea ice and its snow cover. Reliable and accurate snow thickness data from an airborne platform is currently a highly sought after data product. Remotely sensed snow thickness measurements can provide an indication of precipitation levels. These are predicted to increase with effects of climate change, and are difficult to measure as snow fall is frequently lost to wind-blown redistribution, sublimation and snow-ice formation. Additionally, accurate regional scale snow thickness data will increase the accuracy of sea ice thickness retrieval from satellite altimeter freeboard estimates.

Airborne snow-depth investigation techniques are one method for providing regional estimation of these parameters. The airborne datasets are better suited to validating satellite algorithms, and are themselves easier to validate with in-situ measurement. The development and practicality of measuring snow thickness over sea ice in Antarctica using a helicopter-borne radar forms the subject of this thesis. The radar design, a 2-8 GHz Frequency Modulated Continuous Wave Radar, is a product of collaboration and the expertise at the Centre for Remote Sensing of Ice Sheets, Kansas University.

This thesis presents a review of the theoretical basis of the interactions of electromagnetic waves with the snow and sea ice. The dominant general physical parameters pertinent to electromagnetic sensing are presented, and the necessary conditions for unambiguous identification of the air/snow and snow/ice interfaces by the radar are derived. It is found that the roughness's of the snow and ice surfaces are dominant determinants in the effectiveness of layer identification in this radar. Motivated by these results, the minimum sensitivity requirements for the radar are presented.

Experiments with the radar mounted on a sled confirm that the radar is capable of unambiguously detecting snow thickness. Helicopter-borne experiments conducted during two voyages into the East Antarctic sea-ice zone show however, that the airborne data are highly affected by sweep frequency non-linearities, making identification of snow thickness difficult. A model for the source of these non-linearities in the radar is developed and verified, motivating the derivation of an error correcting algorithm. Application of the algorithm to the airborne data demonstrates that the radar is indeed receiving reflections from the air/snow and snow/ice interfaces.

Consequently, this thesis presents the first in-situ validated snow thickness estimates over sea ice in Antarctica derived from a Frequency Modulated Continuous Wave radar on a helicopter-borne platform. Additionally, the ability of the radar to independently identify the air/snow and snow/ice interfaces allows for a relative estimate of roughness of the sea ice to be derived. This parameter is a critical component necessary for assessing the integrity of satellite snow-depth retrieval algorithms such as those using the data product provided by the Advanced Microwave Scanning Radiometer - Earth Observing System sensor on board NASA's Aqua satellite.

This thesis provides a description, solution or mitigation of the many difficulties of operating a radar from a helicopter-borne platform, as well as tackling the difficulties presented in the study of heterogeneous media such as sea ice and its snow cover. In the future the accuracy of the snow-depth retrieval results can be increased as technical difficulties are overcome, and at the same time the radar architecture simplified. However, further validation studies are suggested to better understand the effect of heterogeneous nature of sea ice and its snow cover on the radar signature.

RAASTI = Radar For Antarctic Snow Thickness Investigation

项目2901公开摘要
本研究将助力2007年早春南极海冰区物理与生物学特性的大型多学科研究。海冰的物理特性将通过卫星跟踪漂流浮标、冰芯分析与钻孔测量直接获取，并同步开展积雪厚度与属性的精细化测量。机载观测仪器将用于拓展调查范围，覆盖船舶航线以外的区域并开展远程采样。所采集的数据将为现有及未来的卫星任务提供宝贵的地面验证数据，并加深我们对海冰在气候系统中作用的认知。

项目目标：
(i) 量化东经110°-130°区域内，海冰与积雪属性在米级至百公里级尺度上的空间变异性，以提升基于卫星测高数据与极化合成孔径雷达（polarimetric synthetic aperture radar，SAR）数据的海冰厚度反演精度。
(ii) 探明东经110°-130°区域内海冰的漂移特性与内部应力。
(iii) 探究南极海冰物理环境与南大洋生态系统结构之间的关联（与AAS项目2767联合开展）。

本数据集配套的博士学位论文摘要如下：
南极海冰及其积雪覆盖是全球气候系统的重要组成部分，但目前对其垂直结构的诸多方面仍缺乏充分认知，导致其在全球气候模型中的模拟精度欠佳。遥感技术是监测海冰及其积雪动态特性的核心手段。当前，机载平台获取的可靠且精准的积雪厚度数据是极具应用价值的科研产品。遥感积雪厚度测量可反映降水水平，据预测气候变化将导致降水增加，但降雪常因风蚀再分布、升华及雪冰形成而流失，因此降水测量难度极大。此外，精准的区域尺度积雪厚度数据可提升基于卫星测高干舷估算的海冰厚度反演精度。

机载积雪深度探测技术是获取上述参数区域估算值的可行方案之一。机载数据集更适用于验证卫星反演算法，且自身也更易通过原位测量（in-situ measurement）完成校准。本论文的研究主题为：采用机载直升机雷达探测南极海冰积雪厚度的技术开发与实用性验证。该雷达为2-8 GHz调频连续波雷达（Frequency Modulated Continuous Wave Radar），由美国堪萨斯大学冰盖遥感中心的合作团队与技术专家共同研发。

本论文综述了电磁波与积雪、海冰相互作用的理论基础，梳理了与电磁遥感相关的核心物理参数，并推导了雷达精准识别气-雪界面与雪-冰界面的必要条件。研究发现，积雪与冰面的粗糙度是影响该雷达层位识别效果的核心决定因素。基于上述结论，本论文给出了该雷达的最低灵敏度要求。

将雷达搭载于雪橇上开展的试验证实，该雷达可精准探测积雪厚度。但在两次南极东近海冰区科考中开展的机载直升机试验显示，机载数据受扫频非线性效应影响显著，导致积雪厚度识别难度加大。本论文建立并验证了该雷达扫频非线性的成因模型，并据此推导了误差校正算法。将该算法应用于机载数据后证实，雷达确实接收到了气-雪界面与雪-冰界面的反射回波。

综上，本论文首次提出了基于直升机搭载调频连续波雷达获取、并经原位测量验证的南极海冰积雪厚度反演结果。此外，该雷达可独立识别气-雪与雪-冰界面，据此可反演得到海冰表面粗糙度的相对估算值。该参数是评估卫星积雪深度反演算法可靠性的核心指标，例如美国国家航空航天局（NASA）Aqua卫星搭载的高级微波扫描辐射计-地球观测系统（Advanced Microwave Scanning Radiometer - Earth Observing System）传感器所对应的反演算法。

本论文针对直升机搭载雷达作业面临的诸多难题给出了详细描述、解决方案与缓解措施，同时也攻克了海冰及其积雪等非均质介质研究中的诸多挑战。未来，随着技术难题的攻克与雷达架构的简化，积雪深度反演结果的精度将进一步提升。不过，仍需开展进一步的验证研究，以更好地理解海冰及其积雪的非均质性对雷达回波信号的影响。

RAASTI = 南极积雪厚度探测雷达（Radar For Antarctic Snow Thickness Investigation）