818 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26

A New Apparatus for Monitoring 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 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

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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), 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 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

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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. When the mea- suring operation is performed, the upper magnetic loop, FIG. 1. The MDL positioning sensor. which is jointed with a heavy hammer with a total propagation velocity of the torsional wave and t1 is the weight of 0.15 kg, moves down under the control of the period from when the stimulating pulse appeared up to miniwinch at a speed of 0.02 m s21. When the underpan the time the echo reached the checking machine. Since of the upper magnetic loop, with a size of 0.005 m2, this propagation velocity of the torsional wave is steady is lowered to the surface of the ice/snow surface, the in different circumstances, this sensor has the charac- miniwinch stops immediately, in order to avoid de- teristic of providing high-precision position detection. pressing the snow surface or causing any change in the In addition, multiple positions can be detected syn- snow structure. At the same time the buoy will be filled chronously when the moving magnetic loops are set in with air supplied by the cylinder through the air pipe. different positions. This positioning sensor has been Afterward, the lower magnetic loop will move up under used widely for measurements of, for example, the the resultant force between gravitation and buoyancy, positions of pneumatic pistons, liquid levels, and abso- and then its overpan, also with a size of 0.005 m2,is lute ground velocity (Hristoforou and Chiriac 2002; finally set under the bottom of the ice. The underpan of Hristoforou et al. 2006). Based on the principles of the the upper magnetic loop and the overpan of the lower MDL positioning sensor and the special requirements magnetic loop could ensure that the magnetic loops are of environmental suitability, automatic measurement, parallel to the ice–snow surface or the ice bottom and and low-power dissipation for field applications, a new minimize the effects of the unevenness of ice–snow apparatus for monitoring sea ice thickness and its cor- surface or ice bottom on the resolution of the mea- responding software has been designed. surement records. The distance from both the upper and This monitoring apparatus is composed of an appa- lower magnetic loop to the settled magnetic loop is ratus box and two measuring poles, as shown in Fig. 2. detected by the MDL positioning sensor. The distance The outsides of the poles are constructed of white between the upper and lower magnetic loop is the total polyethylene hollow rods, which also have been used as thickness, including sea ice and snow cover. The varia- the deployment housing of the thermistor strings for tions of the ice/snow surface and the ice bottom are deployment in sea ice or glacier ice (Preunkert and calculated by comparing the positions of the upper and Wagenbach 1998; Pringle et al. 2007). This device avoids lower magnetic loop with their original values, respec- preferential freezing or melting, as its thermal conduc- tively. The measurement data then will be transmitted tance is very small. The diameter (0.02 m) of the poles is to the datalogger. so small that the disturbances of the poles to the heat When the detection operation is completed, the upper flux exchange at the snow surface and the ice bottom, magnetic loop will move up under the draft of the and the wind shielding at the snow surface, are at the miniwinch, which is assembled at the top of the mea- same levels as the housed rod of the acoustic sensor or suring poles, through a steel-wire string; meanwhile, the the thermistor string. There were two stainless steel lower magnetic loop will move down after the air in the tubes fixed in the polyethylene hollow rods, which buoy has been sent back to the cylinder through the air strengthen the measuring poles consumedly and pre- pipe. This process, after detection, not only prevents the vent the measuring poles from bending under external upper magnetic loop from being covered by snow and forcing, especially during strong winds. The horizontal the lower magnetic loop from being frosted under ice, space between these two poles is 0.15 m. The apparatus but also minimizes the possibility of the disturbance to box and the measuring poles are connected by an air the heat flux exchange at the snow surface and the ice

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FIG. 2. The monitoring apparatus for sea ice thickness. bottom. The miniwinch, and the air pump that is as- of 2006. The apparatus was disposed at the site (69822990S, sembled in the cylinder, are driven by the power of the 768219450E) with a water depth of 9 m on 27 March 2006 battery packs. The PC software used includes functions [yearday (YD) 85]. The deployment work is similar to for restoring and cleaning up the data in the datalogger, that involved in deploying the array of thermistors in displaying and adjusting a real-time clock, displaying sea ice (Pringle et al. 2007). When the apparatus was the temperature of the battery packs, controlling the installed, there were 0.01 m of snow, 0.25 m of ice, and charge, and updating the program. 0.025 m of freeboard. After a vertical ice hole of 0.25 m The low-temperature operating environment has been diameter had been drilled through the thickness of the taken into account adequately. Thus, the apparatus box ice cover, and the ice brash in the hole had been cleaned is characterized by heat insulation and corrosion pre- out, the measuring poles were fixed in the hole. The vention, and it is waterproof. A special latex material measurement ranges for detecting the ice/snow surface was adopted for the buoy, cylinder seal, and air pipe to and the ice bottom were 0.75 and 2.25 m, respectively. prevent problems arising from hardening and other The apparatus box was deposited on sea ice, with a 2-m changes brought about by low temperatures. The bat- distance from the measuring pole, perpendicular to the tery packs can withstand a temperature of 2308C for dominant wind direction, to avoid wind shielding. The more than 10 days when the measurement interval is set deployment work, including ice-hole drilling, apparatus for 10 min, in one power-supply cycle. fixing, and apparatus testing, was supported by three Based on capability testing in the laboratory, the winter-over crew members from Zhongshan Station, principal technical criteria for this apparatus are as fol- and was finished in 1 h. Figure 3 presents a field pho- lows: 1) the resolution is 0.0001 m, 2) the design precision tograph of the apparatus, which is denoted by MDL. of the MLD positioning sensor assembled in the appa- Three days later the offset between the record obtained ratus is 60.001 m, 3) the temperature range is from 2558 by the apparatus and the record derived from the drill to 508C, and 4) the measuring interval is 10–180 min. hole at a distance of 1 m from the apparatus was less than 0.03 m. It was proven that the ice hole was natu- rally refreezing. Subsequently, the portion of the hole 3. In situ experiments above the water level was artificially frozen in by infilling water. The disturbance of the deployment a. Instrument disposition hole in the measure records can be limited in the first The monitoring apparatus was utilized to measure week after deployment. From April onward, the in situ landfast sea ice thickness around Zhongshan Station, measurements were carried until 21 September 2006 East Antarctica, during the austral autumn and winter (YD 263).

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quent snowstorms, periodic variations in water levels and tide currents, the unintentional disturbance of ani- mal life under the ice, etc. The field conditions during the experiment are summarized in Table 1. With a diameter of 0.02 m, the measuring poles sound rather flimsy, but the degree of its flexure during the strongest wind with a speed reach to 25.9 m s21 was less than 58, and no perpetual distortion occurred after any gale event for the measuring poles. By the end of our field experiment, however, the measuring poles re- mained vertical to the ice surface. Thus, we can consider the intensity of the measuring poles to be sufficient to resist the external forcings in the field. Some mechanical malfunctions of the apparatus oc- curred during the experiment; for example, the lower magnetic loop was obstructed by an unidentified object under the sea ice, and the upper magnetic loop was obstructed by snow adhering along the measuring poles when measurements were taken. These malfunctions would eventually cease by themselves, but a few records were lost. The steel-wire string, which is used to connect the lower magnetic loop with the miniwinch, was de- FIG. 3. The monitoring apparatus (denoted by MDL) and the stroyed during the snowstorm event. This malfunction thermal-wire thickness gauges (denoted by TWTG). was dealt with by changing the steel string when the field support crew visited the deployment site. Fortunately, In addition, for comparison, 16 thermal-wire thick- this malfunction occurred only 2 times during the ex- ness gauges consisting of a stainless steel wire with a periment. These mechanical malfunctions can be solved steel rod attached as a crossbar on the bottom end and a without any questions in the future by augmenting the wooden handle on the top end, spaced every 15 m along space between the moving magnetic loops and the a section, were installed near our apparatus in late measuring poles; choosing a new material for the string; March 2006. The principles of this method for moni- and optimizing the connection among the miniwinch, toring the sea ice mass balance have been described in the steel-wire string, and the lower magnetic loop. detail by Untersteiner (1961) and Perovich et al. (2003). b. Experimental results To make a measurement, first, the stainless steel wire has to be loaded a 36-V dc and then its electrical resis- During the experiment the measurement interval was tance is melted free. Finally, it can be pulled up and read set for 3 h except for a few days when it was set for 2 h. against a scale on a gauge anchored into the ice, when In all, 1460 records for the ice/snow surface and 1460 the crossbar on its bottom end moves up against the ice records for the ice bottom were obtained. Based on bottom. The gauges also were used as snow-depth stakes. field observations, the growth rate was not more than The uncertainty levels of the stake and gauge readings 0.05 m day21, and the intraday variations of the ice/ were typically less than 60.005 m. These thermal-wire snow surface were not more than 0.2 m during the ex- thickness gauges are denoted by TWTG in Fig. 3. periment; thus, the record for the ice bottom is rejected The deployment site was periodically visited to re- where the offset is more than 0.05 m and the record for trieve data, change the battery pack, and check the the ice/snow surface is rejected where the offset is more operational status of the apparatus for ensuring the ac- than 0.2 m when compared with their daily average quisition of uninterrupted data, every 7–10 days according values, respectively. After these rejections, 1352 records to meteorological conditions, although the battery pack for the ice/snow surface and 1368 effective records for could usually function for more than 20 days. Sea ice at the ice bottom remained. The effective fractions of the the deployment site was undeformed, no snow ice for- data records for detecting the ice/snow surface and the mation occurred, and the freeboard was positive during ice bottom were 92.6% and 93.7%, respectively. All the experiment. The operation of the apparatus was the obviously erroneous records were caused by me- hampered by the extremely harsh field conditions, chanical malfunctions of the instrument as described in which were characterized by low air temperature, fre- the above paragraphs. When the upper magnetic loop was

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TABLE 1. Field conditions during the experiment. thermal-wire gauges in May and August are shown in Figs. 4c and 4d. Although the monthly average ice Conditions Value growth rate derived from the thermal-wire gauges is Lowest air temperature 230.28C close to that derived from the apparatus, there still is Lowest monthly average air temperature 218.68C Maximum extreme wind speed 25.9 m s21 some artificial information in the records derived from Gale event 93 days the thermal-wire gauges when taking into account the Range of specific humidity 19%–92% daily ice growth rate. For instance, the data records Lowest sea level pressure 923.7 hPa from the thermal-wire gauges show that an ice-melting Snowfall event 52 days event occurred on 29 August 2006 (YD 240), and that Snowstorm event 11 days Polar night duration 58 days the stability and continuity of the ice growth rate was Maximum daily gap of water level 1.82 m artificially weakened remarkably. Thus, the natural properties of ice growth would be distorted by the tra- ditional measurement methods owing to their low pre- obstructed and could not reach the snow surface, the cision and temporal resolution. However, considering snow depth was recorded as being so much greater than its high precision and temporal resolution, our appara- could be believed. Likewise, when the lower magnetic tus can help us to gain better accuracy on an hourly basis loop was obstructed and could not reach the ice bottom, when estimating the equivalent latent heat flux from ice the data were also beyond belief. Thus, this small thickness measurements and also when estimating the amount of obviously erroneous data was rejected easily ocean heat flux under sea ice. and did not interfere with the process of identifying and Solid precipitation was observed rarely during the experiment. There was a gale event, with wind speeds analyzing the ice growth and snow accumulation pro- 21 cesses. Therefore, this apparatus can be considered to higher than 17 m s following a snowfall event. Thus, be highly efficient and reliable when operating under the depth of the snow cover that accumulated on the extreme polar field conditions. Sea ice growth rates sea ice surface around the apparatus was very small were calculated by computing the first derivatives of the throughout the experiment. Nevertheless, there was a curves of the time series of ice thickness using short but strong snowfall event that lasted from 30 to 31 July. The time series of the wind speed, from 29 July Hi(tj1 ) Hi(tj) (YD 209) to 3 August (YD 214) in 2006, is plotted in f 5 1 , (1) tj11 tj Fig. 5a. The time series of the snow depth derived from the apparatus, and the thermal-wire gauge that was in- where Hi is the ice thickness, tj is the time of one mea- stalled nearest to the apparatus at a distance of 5 m, surement, and tj11 is the time of the next measurement during the same period, is shown in Fig. 5b. As the plot (Perovich et al. 2003). The sea ice growth rate was in the in Fig. 5a shows, the wind speed increased distinctly range of 0–0.0173 m day21 during the experiment. The after 1700 local time (LT) on 31 July (YD 211). Al- monthly average sea ice growth rate is greatest in May though the snow depth increased before 1700 LT on 31 (from YD 120 to 150) with a value of 0.0106 m day21; July, owing to the solid precipitation, and decreased the monthly average sea ice growth rate is smallest in subsequently at a good pace owing to wind drift, the August (from YD 212 to 242) with a value of 0.0058 m rapid change in snow depth was captured accurately by day21, as shown in Figs. 4a and 4b. From Fig. 4, it can be our apparatus. However, the rapid change in snow seen that the ice growth rate was in the range of 0.0019– depth would be missed by the thermal-wire gauge 0.0173 m day21 in May and in the range of 0.0017– measurements because of its lower temporal resolution. 0.0093 m day21 in August. These results indicate that c. Precision estimation the ice growth rate was only a few millimeters both in the autumn months and in the winter months; further- As the snow depth was small throughout the experi- more, the ice growth rate was markedly unstable be- ment, only the precision estimation for records of the ice cause of the fluctuation of the air temperature and bottom is discussed here. Because there was no ice oceanic heat flux, as well as the seasonal variations of surface ablation or snow ice formation during the ex- solar radiation, etc. During the experiment from March periment, this precision estimation of the ice bottom can to September, the mass balance was manually measured be recognized as that of ice thickness. every 1–2 days using the thermal-wire gauges, and 132 In an attempt to investigate the precision of the ice sets of records including ice thickness and snow depth thickness records, it must be made certain that the data were obtained. The variations of the average ice thick- are valid primarily. For this investigation, a total of ness and average ice growth rate according to these 16 24 drill-hole records were collected intermittently over a

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FIG. 4. Variations in sea ice thickness and growth rate in May and August 2006: (a),(b) data records derived from the apparatus and (c),(d) average data records derived from the thermal-wire gauges. The arrow in (a) denotes the record on 20 May (YD 139).

2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 distance of 2 m around our apparatus. The ice thickness 2 6åjThj Thjj å(Thj Thj) 7 holes were drilled by a slim drill with a stainless steel Erj 5 4 1 5, and auger 0.05 m in diameter. The drill-hole records, with a m m precision of 60.005 m, ranged from 0.335 to 1.570 m. (3) These drill-hole records and the apparatus records, obtained at the same time, are shown in Fig. 6. The Er 5 max (Eri,Erjj), (4) overall absolute dispersion between the drill-hole rec- ords and the apparatus records is 0.0015 6 0.001 m. It is where Eri,Erj, and Er are the positive error, the nega- proven that the apparatus records are valid, based on tive error, and the general error, respectively; Thi, Thi, the precision of drill-hole records and the natural spatial variations in both the ice surface and ice bottom. As the precision and the time resolution of both the drill-hole records and the thermal-wire thickness gauges records were not very high, and there were some spatial variations in both the ice surface and ice bottom, the precision of the apparatuses in the field cannot be esti- mated by comparison with the drill-hole records or thermal-wire thickness gauges records. To estimate the precision of our apparatuses subtlety, an approach whereby the growth rate of the sea ice on a certain day is constant has been assumed. Based on this approach, a linear regression according to the least squares method is first applied for data records derived from a particular day, then the slope of the linear-regression trend is recognized as the growth rate of the sea ice on that day, and finally the precision of the monitoring apparatuses in the field on that day can be estimated as follows:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 åjThi Thij å(Thi Thi) Eri 5 1 , (2) FIG. 5. Time series of (a) wind speed and (b) snow depth, derived n n from 29 Jul to 3 Aug 2006 (YDs 209–214).

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FIG. 7. Positive error, negative error, and the linear-regression datum line of records obtained from the monitoring apparatus during the experiment (arrows denotes an error on 20 May, YD 139).

The estimated precision of the apparatus is slightly FIG. 6. Sea ice thickness records obtained from drill holes and the monitoring apparatus during the experiment. worse than the designed precision of 60.001 m for the MDL positioning sensor assembled in our apparatus, owing to the extremely harsh field conditions in Ant- and n are the ice thickness records, which are larger arctica. However, there is no doubt that this apparatus than their estimated linear-regression values, the esti- has the capacity to detect variations in ice/snow thick- mated linear-regression values according to Thi, and the ness with high precision at the millimeter level and high number of Thi, respectively; and Thj, Thj, and m are the temporal resolution on an hourly basis. ice thickness records, which are smaller than their esti- mated linear-regression values, the estimated linear- 4. Concluding remarks regression values according to Thj, and the number of Thj, respectively. Our new apparatus for monitoring ice/snow thickness For example, the positive error, the negative error, based on MDL principles, which can be used under and the general error on 20 May 2006 (YD 139, denoted harsh environmental conditions in polar regions and in by the arrow in Fig. 7) are 0.0004, 20.0005, and 0.0005 m, shallow seas, addresses the low efficiency of the tradi- respectively. The maximal value among the general tional manual methods effectively. The effects of the sea errors on all field days can be recognized as the preci- ice physical properties and circumstantial conditions on sion of the apparatuses during the experiment. the measure resolution, which are difficult to avoid when The errors from all field days have been estimated, as using the techniques of satellite remote sensing, sub- shown in Fig. 7. The solid lines denote the positive error marine sonar profiling, moored upward-looking sonars, and the negative error, and the dashed line denotes the electromagnetic sounding, radar penetration, and acous- linear-regression datum line, which expresses a link among tic sensors, can be minimized by using this apparatus. the linear-regression lines on all field days, and has been Practice in the field proves that the precision of this transferred from the original values to the zeros. The apparatus is 60.002 m, which is more accurate than any maximal absolute value of both the positive error and records derived from other techniques; the effective the negative error is 0.002 m. This estimation approach fractions of the records for detecting the ice/snow surface leads to a slightly conservative estimation of the preci- and the ice bottom are 92.6% and 93.7%, respectively. sion of the apparatuses, because some slight variations Based on current techniques, this apparatus cannot in the rate of ice growth are evident even in the intraday replace the acoustic sensor equipped in the IMB be- period, and the estimating strategy according to Eqs. cause the equipping battery pack can only operate for (2)–(4) seems to be slightly rigorous. In other words, the 10–20 days, but this technical problem can be resolved actual precision of the apparatuses may be somewhat by improving the driving mechanism of the moving better than the estimated values, and this estimated magnetic loops and strengthening the capability of the value can be considered to be the upper limit of the battery pack, which then could be installed in the IMBs, actual precision of the apparatuses in the field. When enhancing the quality of future data records. applying the moving average value of every eight rec- This apparatus also has some limits for field applica- ords in every day instead of the estimated linear- tions: 1) it can be utilized only for fixed-site undeformed regression values in Eqs. (2) and (3), the positive error ice, but cannot measure ice thickness along a line or and the negative error are 0.0018 and 0.0017 m, re- in a large area, or be used to estimate profiles of ice spectively. ridges; and 2) although this apparatus has the ability

Unauthenticated | Downloaded 09/29/21 08:32 PM UTC 826 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 26 to detecting ice/snow surface melting, it cannot be uti- ——, P. D. Dimitropoulos, and J. Petrou, 2006: A new position lized to detect how snow contributes to the sea ice mass sensor based on the MDL technique. Sens. Actuators, 132A, balance. 112–121. Hudson, R., 1990: Annual measurement of sea ice thickness using In conclusion, this monitoring apparatus could have a an upward-looking sonar. Nature, 344, 135–137. significant role to play in the family of ice/snow thick- Karagiannis, V., C. Manassis, and D. Bargiotas, 2003: Position ness monitoring techniques. The measured data have sensors based on the delay line principle. Sens. Actuators, the potential to express, at the basic-research level, the 106A, 183–186. ice growth and decay rate; to estimate the ocean heat Kawamura, T., K. I. Ohshima, T. Takizawa, and S. Ushio, 1997: Physical, structural, and isotopic characteristics and growth flux coupled with sea ice temperature measurements processes of fast sea ice in Lu¨ tzow-Holm Bay, Antarctica. J. obtained from other instruments; and to supply evi- Geophys. Res., 102 (C2), 3345–3355. dence for the need to modify certain sea ice numerical Laxon, S., N. Peacock, and D. Smith, 2003: High interannual models. variability of sea ice thickness in the Arctic region. Nature, 425, 947–950. Liu, J., G. A. Schmidt, D. G. Martinson, D. Rind, G. Russell, and Acknowledgments. This study was supported by the X. Yuan, 2003: Sensitivity of sea ice to physical parameteri- National Natural Science Foundation of China under zations in the GISS global climate model. J. Geophys. Res., Contracts 40676001 and 40233032, and by the National 108, 3053, doi:10.1029/2001JC001167. Key Technology Research and Development Program Me´lia, D. S., 2002: A global coupled sea ice–ocean model. Ocean under Contract 2006BAB18B03. We are grateful to the Modell., 4, 137–172. Perovich, D. K., and B. C. Elder, 2001: Temporal evolution of Chinese Arctic and Antarctic Administration for its Arctic sea-ice temperature. Ann. Glaciol., 33, 207–211. logistic support during our experiment; to Mr. Li Dong ——, and ——, 2002: Estimates of ocean heat flux at SHEBA. and Mr. Xiufeng Liu, who were part of the 2006 winter- Geophys. Res. Lett., 29, 1344, doi:10.1029/2001GL014171. over crew at Zhongshan Station in the 22nd CHINARE, ——, and J. A. Richter-Menge, 2006: From points to poles: Ex- for their field-work support; to the Meteorological Of- trapolating point measurements of sea-ice mass balance. Ann. fice of Zhongshan Station for providing meteorological Glaciol., 44, 188–192. ——, B. C. Elder, and J. A. Richter-Menge, 1997: Observation of data; and to the National Tidal Centre, Australian Bu- the annual cycle of sea ice temperature and mass balance. reau of Meteorology, for providing tide-prediction data. Geophys. Res. Lett., 24, 555–558. Three anonymous reviewers are thanked for their ——, T. C. Grenfell, J. A. Richter-Menge, B. Light, W. B. Tucker III, comments, which considerably improved this work. and H. Eicken, 2003: Thin and thinner: Sea ice mass balance measurements during SHEBA. J. Geophys. Res., 108, 8050, doi:10.1029/2001JC001079. ——, B. C. Elder, K. J. Claffey, S. Stammerjohn, R. Smith, S. F. REFERENCES Ackley, H. R. Krouse, and A. J. Gow, 2004: Winter sea-ice properties in Marguerite Bay, Antarctica. Deep-Sea Res. II, Bergeron, C. J., Jr., J. R. Brusstar, N. Yi, Y. Wu, and J. W. Ioup, 51, 2023–2039. 1999: A new vertical continuation procedure for airborne Preunkert, S., and D. Wagenbach, 1998: An automatic recorder for electromagnetic field data from the modified image method. air/firn transfer studies of chemical aerosol species at remote Geophysics, 64, 1364–1368. glacier sites. Atmos. Environ., 32, 4021–4030. Eicken, H., T. C. Grenfell, D. K. Perovich, J. A. Richter-Menge, Pringle, D. J., H. Eicken, H. J. Trodahl, and L. G. E. Backstrom, and K. Frey, 2004: Hydraulic controls of summer Arctic pack 2007: Thermal conductivity of landfast Antarctic and Arctic ice albedo. J. Geophys. Res., 109, C08007, doi:10.1029/ sea ice. J. Geophys. Res., 112, C04017, doi:10.1029/2006JC003641. 2003JC001989. Purdie, C. R., P. J. Langhorne, G. H. Leonard, and T. G. Haskell, Haas, C., 1998: Evaluation of -based electromagnetic-induc- 2006: Growth of first-year landfast Antarctic sea ice deter- tive thickness measurements of summer sea-ice in the Bel- mined from winter temperature measurements. Ann. Glaciol., lingshausen and Amundsen Seas, Antarctica. Cold Regions 44, 170–176. Sci. Technol., 27, 1–16. Richter-Menge, J. A., D. K. Perovich, B. C. Elder, K. Claffey, I. Heil, P., 2006: Atmospheric conditions and fast ice at Davis, East Rigor, and M. Ortmeyer, 2006: Ice mass-balance buoys: A tool Antarctica: A case study. J. Geophys. Res., 111, C05009, for measuring and attributing changes in the thickness of the doi:10.1029/2005JC002904. Arctic sea-ice cover. Ann. Glaciol., 44, 205–210. ——, I. Allison, and V. I. Lytle, 1996: Seasonal and interannual Rothrock, D. A., Y. Yu, and G. A. Maykut, 1999: Thinning of the variations of the oceanic heat flux under a landfast Ant- Arctic sea-ice cover. Geophys. Res. Lett., 26, 3469–3472. arctic sea ice cover. J. Geophys. Res., 101 (C11), 25 741– Saloranta, T. M., 2000: Modeling the evolution of snow, snow ice 25 752. and ice in the Baltic Sea. Tellus, 52A, 93–108. Hristoforou, E., and H. Chiriac, 2002: Position measuring system Smedsrud, L. H., W. P. Budgell, A. D. Jenkins, and B. A˚ dlandsvik, for applications in field sports. J. Magn. Magn. Mater., 249, 2006: Fine-scale sea-ice modelling of the Storfjorden polynya, 407–410. Svalbard. Ann. Glaciol., 44, 1–7. ——, ——, M. Neagu, and V. Karayannis, 1997: On the calibration Strass, V. H., 1998: Measuring sea ice draft and coverage with of position sensors based on magnetic delay lines. Sens. Ac- moored upward looking sonars. Deep-Sea Res. I, 45, 795– tuators, 59A, 89–93. 818.

Unauthenticated | Downloaded 09/29/21 08:32 PM UTC APRIL 2009 L E I E T A L . 827

Sun, B., J. Wen, M. He, J. Kand, Y. Luo, and Y. Li, 2003: Sea ice Untersteiner, N., 1961: On the mass and heat budget of Arctic thickness measurement and its underside morphology analy- sea ice. Arch. Meteor. Geophys. Bioklimatol., 12A, 151– sis using radar penetration in the Arctic Ocean. Sci. China, 182. 46D, 1151–1160. Vavrus, S., and S. P. Harrison, 2003: The impact of sea-ice dy- Tamura, T., K. I. Ohshima, T. Markus, D. J. Cavalieri, S. Nihashi, namics on the Arctic climate system. Climate Dyn., 20, 741– and N. Hirasawa, 2007: Estimation of thin ice thickness and 757. detection of fast ice from SSM/I data in the Antarctic Ocean. Vinnikov, K. Y., and Coauthors, 1999: Global warming and J. Atmos. Oceanic Technol., 24, 1757–1772. Northern Hemisphere sea ice extent. Science, 286, 1934–1937.

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