A New Apparatus for Monitoring Sea Ice Thickness Based on the Magnetostrictive-Delay-Line Principle

818 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26 A New Apparatus for Monitoring Sea Ice Thickness Based on the Magnetostrictive-Delay-Line Principle RUIBO LEI AND ZHIJUN LI State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, China YANFENG CHENG Polar Research Institute of China, Shanghai, China XIN WANG State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, China YAO CHEN Wu Xi Fengrun Science and Technology Ltd., Wuxi, China (Manuscript received 10 December 2007, in final form 11 October 2008) ABSTRACT High-precision ice thickness observations are required to gain a better understanding of ocean–ice–atmosphere interactions and to validate numerical sea ice models. A new apparatus for monitoring sea ice and snow thickness has been developed, based on the magnetostrictive-delay-line (MDL) principle for positioning sensors. This system is suited for monitoring fixed measurement sites on undeformed ice. The apparatus presented herein has been tested on landfast ice near Zhongshan Station, East Antarctica, for about 6 months during the austral autumn and winter of 2006; valid data records from the deployment are available for more than 90% of the deployment’s duration. The apparatus’s precision has been estimated to be 60.002 m for the deployment. Therefore, it is possible that this apparatus may become a standard for sea ice/snow thickness monitoring. 1. Introduction ice energy equilibrium, and investigating flooding on ice surfaces (Kawamura et al. 1997; Saloranta 2000; Eicken Sea ice plays an important role in the global climate et al. 2004). Although for an area as large as the Arctic system (Vavrus and Harrison 2003) and also is the most and Antarctica, and even a global scale, the final ex- sensitive indicator of local and global climate change pression of the ocean–ice–atmosphere interactions is (Vinnikov et al. 1999; Heil 2006). Sea ice thickness is the achieved through numerical modeling (Me´lia 2002; Liu most fundamentally integrative and crucially important et al. 2003). Measuring sea ice and snow thickness at a parameter for describing ice conditions. The uncertainty fixed site can be utilized, in conjunction with satellite of ice thickness measurements is the major difficulty in remote sensing and numerical models, to estimate re- setting the ocean heat flux from ice mass and tempera- gional ice growth and melt (Perovich et al. 2003; Perovich ture measurements (Heil et al. 1996; Perovich et al. 1997; and Richter-Menge 2006; Richter-Menge et al. 2006). Perovich and Elder 2002). The snow cover depth is cru- Thus, field programs focus ice thickness measurement in cial for parameterizing surface albedo, evaluating snow– general, for example, during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment from 1997 to 1998 (Perovich et al. 2003). Meanwhile, since 2000, Corresponding author address: Ruibo Lei, State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Tech- more and more ice mass–balance buoys (IMBs) have been nology, Dalian 116024, China. deployed in the Arctic to measure the ice cover mass E-mail: [email protected] during an annual cycle (Richter-Menge et al. 2006). DOI: 10.1175/2008JTECHO613.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 09/29/21 08:32 PM UTC APRIL 2009 L E I E T A L . 819 At present, although the numerical model has been velocity in ocean water (specifically, the physical prop- developed well (Smedsrud et al. 2006), the measurement erties such as density, salinity, and composition). In such precision of the ice/snow thickness has not improved cases, the practical accuracy of this acoustic sensor would significantly, and so it cannot provide the evidence nec- behave somewhat poorly, possibly ascending to more essary to validate the modeling results. Drill-hole mea- than 0.01 m, although it is deployed slightly below the ice surements and thermal-wire measurements are recog- bottom, ideally around 1–2 m. nized as reliable methods for measuring ice thickness With these methods we have found that it is difficult (Perovich et al. 2003; Heil 2006), but their low efficiency to measure the ice thickness to a millimeter level of cannot satisfy the need for monitoring ice thickness precision. If longer time intervals are considered, the with high temporal resolution. Automatic monitoring accuracy of some traditional methods would be en- techniques including satellite remote sensing, submarine hanced (Perovich et al. 1997; Perovich and Elder 2002), sonar profiling, the combination of laser and electro- but we would lose some short-term information. Thus, magnetic sounding devices, and radar penetration can the error might be significant, when based on these provide large-scale ice thickness distribution, as well as traditional methods, in attempting to measure sea information on the onset of melt and freeze-up (Haas ice thickness with a growth rate of only a few milli- 1998; Bergeron et al. 1999; Rothrock et al. 1999; Laxon meters per day—for instance, landfast sea ice around et al. 2003; Sun et al. 2003; Tamura et al. 2007). Nev- the Antarctic continent in winter (Purdie et al. 2006) ertheless, the response of these methods to the sea ice and sea ice in the Arctic in autumn or spring (Perovich physical properties and ambient conditions, such as the et al. 2003). Therefore, it has been necessary to develop topographical property of ice surface, the volume frac- a new automatic apparatus for fixed-site monitoring of tion of brine within the ice matrix, and ocean-water the sea ice thickness with a high accuracy and a high properties, result in lower resolution for resolving the temporal resolution. This objective has now been ach- air–ice or ice–water interfaces. For much of the year, the ieved with the development of a new apparatus. This temperature profiles derived from the thermistor string apparatus operates most effectively for undeformed ice were used to infer the air–ice and ice–water interfaces and is based on the magnetostrictive-delay-line (MDL) (Perovich et al. 1997). Purdie et al. (2006) came to the principle. The application of the apparatus is very good conclusion that since the thermodynamic properties for thickness monitoring (with several sites) and for near the ice bottom are considerably unstable, the ice conducting basic research into the physics of sea ice temperature profile is insufficient to infer the ice–water growth and melting. This apparatus was utilized to interfaces. In particular, this method is severely limited monitor landfast sea ice thicknesses around Zhongshan during the summer melt season when there is no sig- Station, East Antarctica, for about 6 months during the nificant thermal contrast between the lower layer of austral autumn and winter of 2006. ice and the upper part of the water column (Perovich This paper discusses the principles of the design, the et al. 1997). Moored upward-looking sonar and acoustic configuration of this apparatus, and the results from its sensors equipped in the IMBs are recognized as the first field applications in Antarctica. The apparatus’s most effective techniques for recording the variations in precision in the field is also described. sea ice thickness automatically (Hudson 1990; Strass 1998; Perovich and Elder 2001; Perovich et al. 2004; 2. Design principles Richter-Menge et al. 2006). When compared with drill- hole measurements, the accuracy of the moored up- According to the MDL technique, a positioning sen- ward-looking sonars is typically 0.20 m, and the relative sor that is able to detect the positions of a variety of error is 11.5%, owing to the uncertainty involved in the moving permanent magnets set at various distances from conversion process from sonar records to the reliable the sensing elements has been developed; its operating measurements of the ice thickness (Strass 1998). In principles were reported in detail by Hristoforou et al. addition, moored upward-looking sonars cannot record (1997) and Karagiannis et al. (2003). The sensor is il- the time series of the snow depth and cannot be utilized lustrated in Fig. 1. Responding to the stimulation of the to monitor the sea ice thickness in shallow alongshore pulse current, the circular magnetic field is shaped by areas. The accuracy of the acoustic sensor is 60.005 m the magnetic loop. Subsequently, the annular torsional when it was not utilized for measurements, in other distortion would come into being and be spread to both words, when not concerned with the measurement ends of the MDL in the form of a torsional wave. The conditions (Richter-Menge et al. 2006). However, it has echoes of the torsional waves can be detected by a been demonstrated that the accuracy of the below-ice checking machine. The position of the magnetic loop 1, acoustic sensor depends on the resolution of the acoustic L1, can be expressed as L1 5 0.5y t1; here, y is the Unauthenticated | Downloaded 09/29/21 08:32 PM UTC 820 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26 pipe and a cable. The air pipe is used to connect the cyl- inder, which is fixed in the apparatus box, with the buoy, which is equipped in the lower magnetic loop; the cable is used to connect a battery pack with a miniwinch and to transmit data records to the datalogger, which is fixed in the apparatus box. The air pipe leads to one stainless steel tube in the measuring pole, and the MDL is set in one stainless steel tube in the other measuring pole. Two moving magnetic loops are fixed in the movement machines along the measuring poles, and one settled magnetic loop is fixed at the top of the measuring pole. The measuring operation is completed by controlling the movement of the magnetic loops.

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