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The infiltration of melted snow water and rainwater into snow can drastically change the form of snow layers. This process is an important factor affecting wet snow avalanches. Accordingly, numerous field surveys and cold room experiments have been conducted to investigate the distribution of water in snow. The common methods of water content measurement (calorimetric and dielectric methods) are implemented by disturbing snow samples to measure them. However, the resolutions obtained are of the order of several centimeters, which hinders the continuous measurement of the water content of a particular sample. Magnetic resonance imaging (MRI), which is typically used in the medical field, can be used to generate a high-resolution three-dimensional (3D) image of the water distribution in samples without destructing them. The luminance of images produced by MRI depends on the volumetric water content of the sample, with luminance increasing with volumetric liquid water content. Therefore, the volumetric liquid water content of the sample can be estimated from its luminance value. Considering this concept, we developed a method to measure the volumetric liquid water content of wet snow samples using MR images. To evaluate the developed method, we prepared several wet snow samples and measured their various volumetric liquid water contents using MRI (θMRI) and the calorimetric method (θcal). θMRI, and θcal showed good correlation when compared, with values in the range 0.02–0.46. Therefore, our system can accurately and non-destructively measure water content. The developed method using MRI can measure 3D volumetric liquid water contents with a high resolution (2 mm). Using the developed method, we investigated the hysteresis of the water retention curve of snow based on the measurements of a wetting process (boundary wetting curve) and a drying process (boundary drying curve) of the water retention curve for each sample. Our results indicate the existence of hysteresis in the snow water retention curves and the possibility of modeling it by adopting contexts of soil physics.

The Tenth Symposium on Polar Science/Ordinary sessions: [OM] Polar Meteorology and Glaciology, Wed. 4 Dec. / Entrance Hall (1st floor) , National Institute of Polar Research

The Ninth Symposium on Polar Science/Ordinary sessions: [OM] Polar meteorology and glaciology, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

Under the Japanese Antarctic Research Expedition (JARE) program, sea-ice thickness has been routinely monitored off Lutzow–Holm Bay (East Antarctica) during the summer (mid-December to early January) since 2000/01, using an electromagnetic induction (EM) instrument onboard the icebreaker Shirase. Analysis of these data over a 10-year period, combined with visual observations using a simplified form of the ASPeCt (Antarctic Sea ice Processes and Climate) protocol, suggests a strong interannual variability in sea-ice thickness in this region. For the repeat pack-ice observation area, where the sea-ice thickness averaged over the nine seasons is ∼1.9 m, mean thicknesses of observed sea-ice in 2010/11 and 2011/12 are exceptionally large, at ∼3.3 and ∼5.8 m, respectively. This result is strongly related to regional patterns of sea ice dynamics. Ice convergence caused by anomalous northerly winds was particularly high in 2011/12, suggesting that the extremely thick ice observed in that season resulted largely from sea-ice deformation processes (including pressure ridging). Longer-term analysis of data from the past 34 years confirms that sea-ice conditions and thickness off Lutzow–Holm Bay in summer are determined mainly by the large-scale pattern of atmospheric pressure in December.

Flow fingers develop due to the infiltration of snow meltwater or rain water into a dry snowpack. The width, spacing, and areal coverage of the flow finger path and occurrence of capillary barriers have been demonstrated using dye tracer experiments that were conducted in the field and in cold laboratories. Because dye tracer experiments require the destructive analysis of the snow, the initial shape, and the temporal development of fingering and lateral flows are unclear. To examine this, we carried out nondestructive observations of flow finger development, and the formation of capillary barriers using two different dry snow samples in a cold laboratory using magnetic resonance imaging (MRI) with a three-dimensional rapid imaging method. X-ray micro-computed tomography (μCT) was also used to determine the relationship between the water flow phenomena and the snow microstructure. The experimental results showed that flow fingers developed in dry snow samples. The number of flow finger paths increased, and a lateral flow developed through the continuing water supply. The small discontinuity of the pore size and snow density in the vertical direction acted as a capillary barrier and induced lateral flow. These results showed that both the persistence of the flow finger path and the increase in the number of flow paths are important for understanding the infiltration of water into dry snow and for developing a snowpack model that includes water-moving processes. Nondestructive observation using MRI is a promising technique to follow the development of preferential flow path and lateral flow. Detailed information regarding the heterogeneity of the pore distribution and snow microstructures in a layered snowpack is required to understand the water flow phenomena in a natural snowpack.

The surface hoar layer is known to be one of the most typical types of weak layers in dry slab avalanches. To investigate the features of snow cover, including the surface hoar layer, we developed apparatuses that make surface hoar artificially. Two growth systems were developed using circuit wind tunnels. The first apparatus supplied wind using a circuit wind tunnel. A few millimeters thick of snow were placed on a cooling panel (0.12 m2) to form the fundamental snow layer. Surface hoar was then grown on the snow surface using the difference between the temperature and relatively high water vapor pressure of the air; and the cooled snow surface. The shapes of the artificially grown surface hoar crystals were simple plate, sector plate, dendrite, needle, column and cup. It was suggested that the crystal shape depended on snow surface temperature and wind condition. Both dendrite and needle shapes required low wind conditions. The trend of the temperature dependence of the crystal shape was often in agreement with the habit of artificial snow crystals and depth hoar crystals. In the second apparatus, we set nine cooling panels and made a surface hoar layer on the fundamental snow cover. It was possible to make surface hoar with an area of 1.08 m2 with one experiment using this system. Three surface temperatures could be selected in a single experiment because the panels were cooled by three different thermostatic baths. These systems are useful for investigating the features of snow and the surface hoar layer efficiently.

Heavy sea-water spray icing is a major problem in the maintenance of lighthouses situated in northern harbors facing the Sea of Japan. Moreover, low ice adhesion materials or anti-icing coatings are ineffective over long periods. In this study, we have investigated the characteristics of saline ice adhesion on hydrophilic and hydrophobic pliable polymer sheets that can be used to envelop and thereby protect small lighthouses. We conducted experiments in cold room laboratories, and further, we verified the deicing conditions. Ice adhesion strength tests using fresh water indicated that a highly hydrophobic surface exhibited lower adhesion strength than other test materials. The results of saline ice adhesion tests at −20°C indicated that the adhesion strength decreases very rapidly as the salinity increases for all materials; this trend was particularly remarkable in case of superhydrophilic materials. The water spray icing tests were verified by experiments conducted using an ice model basin. Wet...

高分子膜材を活用することにより,着氷体表面の性質とその変形による着氷の自然剥落をはかるという複合的な除氷効果が期待される.本研究は,高分子膜材を用いた着氷防除において,適当な膜材の選定指針を得ることを目的として,低温室内における高分子膜材の着氷力測定試験を行った.本研究では,難着氷性や可動性などの観点から膜材を選択し,試験試料とした.純水を用いた実験では,撥水性素材の着氷力が最も小さく,超親水性フィルムは比較試料の値よりも大きいという結果が得られた.また,-5℃から-25℃までの実験結果から,ほとんどの試料で温度が低下するにつれ着氷力は大きくなる傾向が見られたが,超親水性フィルムは-10℃で着氷力がピークを示した.塩水を用いた場合,着氷力は純水の氷に比べ大幅に小さかった.これは着氷界面に溜まったブラインの影響が大きいと考えられ,このことは超親水性フィルムにおいて特に顕著であった.

2004年2月22日夜から23日朝にかけて北海道空知,日高および根室地方で着氷性の雨が降り,雨氷が発達した.気温の鉛直分布から,札幌では2月22日21時に,根室では23日9時には上空で気温がプラス,地上付近で気温が再び氷点下となる温度逆転層が存在し,着氷性の雨が降る条件であった.2月24日に空知の広域観察を行った結果,岩見沢から砂川にかけて雨氷を確認することができた.また,後日の調査で日高,根室において雨氷が成長したことが分かった.激しい着氷雪は気象測器に可動不良を起こすことから,アメダスおよび国道の気象テレメータの風向風速データの欠測期間から着氷発生期間の推定を試みた.雨氷の発生域をさらに把握するために,メソ客観解析データおよび数値地図標高データを用いて,降雪粒子の融解条件と雨滴の凍結条件を雨氷発生モデルにより検討し,北海道全域における着氷性の雨の発生域の推定を行なった.その結果,空知中・南部,日高の標高の低い地域および根室で広範囲に雨氷が発生したことが明らかになった.地上観察と測器の動作異常から推定した雨氷の発生域と,メソ客観解析データを用いた本研究の解析手法による結果を比較したところ,良い一致が見られた.

MR microscopy was designed to visualize and quantify the three-dimensional structure of snowpack and tested on snow and ice samples. We studied the structure of four types of packed ice particles: ice spheres, large rounded polycrystals, small rounded monocrystals, and depth hoar. Because the nuclear magnetic resonance (NMR) signal from the ice was very weak, the air space of snow was filled with a fluid that had a strong NMR signal. By imaging the fluid, we inferred the ice shapes and positions. Both dodecane and aniline could be used, provided that they were doped with iron acetylacetonate. Test imaging of dodecane showed that 0.5–2 h were needed to obtain one 3D image; thus, we developed a specimen-cooling system to maintain the sample at a constant temperature. The chamber had a double pipe cylinder through which cold air flowed, and the temperature of the sample holder was controlled by adjusting the volume of cold airflow. Experiments using the above ice particles and the system allowed us to obtain 3D microscopic images. For an image matrix of 2563, the voxel size was 120 μm on a side, whereas image matrices of 1283 and 643 had voxel sizes of 200 and 400 μm, respectively. The imaging sequence used 3D gradient echoes. We also compared the 3D images with 2D data that was obtained using the conventional section plane method. MR microscopy is thus a very useful method to visualize the microstructure of snowpack.

MR microscopy technique was introduced to visualize and quantify the three-dimensional structure of snowpack. Since the NMR signal from the ice was week, we looked at the air space instead filling with dodecane or aniline doped with iron acetylacetonate. Four types of snow were tested: ice spheres, large rounded poly crystals, small rounded mono-crystals and depth hoar crystals. A specific specimen-cooling system was developed to keep the temperature below 0 degrees C. In the experiments 0.5 to 2 h were necessary to accumulate the signals enough to obtain a 3D micro-image; the image matrix 128(3), voxel size (200 microm)3 or 256(3) (120 microm)3. Comparison with the 2D data using the conventional section plane method was also carried out and MR microscopy is proved to be a very useful method to visualize the microstructure of snowpack.

The mechanism of sun-crust formation was investigated through laboratory experiments in a cold wind tunnel. The experiments were carried out by controlling the energy balance and the sun crust formed was consistent with that observed in Nature, i.e. a thin ice layer composed of ice particles and cavities due to internal melting beneath the sun-crust layer. The surface-cooling rate was between −50 and −100 W m−2, and the absorption of shortwave radiation exceeded 200 W m−2. The energy balance during the formation of the sun crust is consistent with the data observed in Nature.The sources of H2O for the sun-crust formation were investigated through changes in δ18O. Taking into account that the δ value of sun crust was larger than that of the snow beneath it and the calculations of the latent-heat emission with the sun-crust formation, it was concluded that the retention of meltwater by capillary force and refreezing in a thin layer was the dominant mechanism of this metamorphism.

サン・クラストは短時間で氷板が形成される特徴的な雪面形態であるが, 滑らかな氷板が広く雪面を覆い斜面の安定度に影響を及ぼすと考えられる.著者らはサン・クラストの形成をくわしく観測しサン・クラストの構造および形成される気象条件, 雪質条件を明らかにした.さらに熱収支条件を再現して風洞実験を行いサン・クラストを形成することができた.サン・クラストは雪面が放射冷却によって冷やされ, 積雪内部は透過した日射によって融解が生じるような気象条件で密度の大きな積雪上に形成された. その結果サン・クラストの下には内部融解による空洞がしばしば形成された.積雪表層で起きる変態と水の輸送を推定するために, サン・クラスト形成に伴う酸素安定同位体組成の変化を観測した.サン・クラストのδ18O値は下層よりも大きくなり, 雪面直下で作られた融解水が積雪表層で再凍結することがサン・クラストの形成要因であると考えられた.

An MR imaging system was developed to visualize brine drainage channels in sea spray icing. Brine pockets trapped in spray-ice matrices during ice growth are structural features of sea spray icing. Brine in the spray ice sample had drained out; therefore, using a suction pump, we filled the air gaps in the drainage channels with dodecane. In the experiments, 0.5-1.0 h was necessary to accumulate signals sufficient to obtain a 3-D micro-image; the image matrix comprised 128 3 voxels (each voxel was 200 μm per side). The MIP view showed that sea spray icing has a developed drainage-channel network structure.

A compact magnetic resonance imaging (MRI) system for a cold (-5 degrees C) room has been developed to acquire MR images below the freezing point of water. The MRI system consists of a 1.0 T permanent magnet, a higher-order shim coil set, and a gradient coil probe, installed in the cold room, and a compact MRI console installed in a room at normal temperature (20-25 degrees C). The most difficult problem for the installation of the MRI system in the cold room was the degradation of the field homogeneity of the permanent magnet shimmed at 25 degrees C. To overcome this problem, higher-order shim coils were developed and the temperature variation of the magnetic field distribution was measured using a standard phantom with and without shim coil currents. As a result, it was confirmed that the homogeneity (the difference between the minimum and maximum values) of the magnetic field in the 17x17x19 mm(3) rectangular parallelepiped region was improved from 117 to 59 ppm using an appropriate combination of shim coil currents. A snowpack immersed in dodecane (C(12)H(26)) was imaged using a driven-equilibrium three-dimensional (3D) spin-echo sequence at -5 degrees C. The visualized 3D structure of the snowpack demonstrated the effectiveness of our approach.

The mechanism of sun crust formation was investigated through field observations for five winters from 1991 to 1995. During the observation periods, the sun crust formed 15 times. The sun crust observed in this study was a thin ice layer, which is 1 to 2 mm thickness and composed of ice particles, with cavities formed under it. In the all cases, the snow type before the sun crust formation was wet rounded grains. The heat balance during the formation of sun crust was calculated revealing the following process. Evaporation made the surface smooth and flat. Longwave radiative flux and latent heat flux cooled the snow surface, while beneath the surface, shortwave radiation was absorbed and internal melting occurred giving rise to cavities. The longwave radiation refroze some meltwater which remained within the layer near the surface, and a thin ice layer was formed.