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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 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.

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.

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.