Measuring snow thickness over Antarctic sea ice with a helicopter-borne 2-8 GHz FMCW radar

摘要

Antarctic sea ice and its snow cover are integral components of the global climate system, yetudmany aspects of their vertical dimensions are poorly understood, making their representation inudglobal climate models poor. Remote sensing is the key to monitoring the dynamic nature of seaudice and its snow cover. Reliable and accurate snow thickness data from an airborne platform isudcurrently a highly sought after data product. Remotely sensed snow thickness measurements canudprovide an indication of precipitation levels. These are predicted to increase with effects of climateudchange, and are difcult to measure as snow fall is frequently lost to wind-blown redistribution,udsublimation and snow-ice formation. Additionally, accurate regional scale snow thickness dataudwill increase the accuracy of sea ice thickness retrieval from satellite altimeter freeboard estimates.udAirborne snow-depth investigation techniques are one method for providing regional estimationudof these parameters. The airborne datasets are better suited to validating satellite algorithms, andudare themselves easier to validate with in-situ measurement. The development and practicalityudof measuring snow thickness over sea ice in Antarctica using a helicopter-borne radar forms theudsubject of this thesis. The radar design, a 2 - 8 GHz Frequency Modulated Continuous WaveudRadar, is a product of collaboration and the expertise at the Centre for Remote Sensing of IceudSheets, Kansas University.udThis thesis presents a review of the theoretical basis of the interactions of electromagnetic wavesudwith the snow and sea ice. The dominant general physical parameters pertinent to electromagneticudsensing are presented, and the necessary conditions for unambiguous identication of the air/snowudand snow/ice interfaces by the radar are derived. It is found that the roughnesses of the snow andudice surfaces are dominant determinants in the effectiveness of layer identication in this radar.udMotivated by these results, the minimum sensitivity requirements for the radar are presented.udExperiments with the radar mounted on a sled conrm that the radar is capable of unambiguouslyuddetecting snow thickness. Helicopter-borne experiments conducted during two voyages into theudEast Antarctic sea-ice zone show however, that the airborne data are highly affected by sweepudfrequency non-linearities, making identication of snow thickness difcult. A model for the sourceudof these non-linearities in the radar is developed and veried, motivating the derivation of an errorudcorrecting algorithm. Application of the algorithm to the airborne data demonstrates that the radarudis indeed receiving reections from the air/snow and snow/ice interfaces.udConsequently, this thesis presents the rst in-situ validated snow thickness estimates over seaudice in Antarctica derived from a Frequency Modulated Continuous Wave radar on a helicopterborneudplatform. Additionally, the ability of the radar to independently identify the air/snow andudsnow/ice interfaces allows for a relative estimate of roughness of the sea ice to be derived. Thisudparameter is a critical component necessary for assessing the integrity of satellite snow-depthudretrieval algorithms such as those using the data product provided by the Advanced MicrowaveudScanning Radiometer - Earth Observing System sensor on board NASA�’s Aqua satellite.udThis thesis provides a description, solution or mitigation of the many difculties of operating audradar from a helicopter-borne platform, as well as tackling the difculties presented in the studyudof heterogeneous media such as sea ice and its snow cover. In the future the accuracy of theudsnow-depth retrieval results can be increased as technical difculties are overcome, and at theudsame time the radar architecture simplied. However, further validation studies are suggested toudbetter understand the effect of the heterogeneous nature of sea ice and its snow cover on the radarudsignature.