Kenji Kosugi

Snow ripples and their contribution to the mass transport in drifting snow

Morphological characteristics of snow ripples formed by drifting snow were investigated as functions of wind velocity in a cold wind tunnel at -15 °C. Wave-length, wave height and migration rate of snow ripples increased from 5 to 20 cm, 3 to 5 mm and 1 to 8 cm/min, respectively, with increasing wind velocity from 5 to 7 m/s. Measured size distributions of snow particles in snow ripples showed sorting of large particles in ridges, suggesting that the snow ripple migration is caused by creeping of large particles. The snow drift rate caused by creep, that is, by the ripple migration, was estimated to amount, at least, to 6% of the total snow drift rate.

Experiments on ice-sphere flows along an inclined chute

Abstract Inclined chute experiments with ice spheres were carried out in a cold laboratory to investigate the snow avalanche dynamics. The chute was 5.4 m long and 0.08 m wide. The chute inclination angles were varied from 30° to 40° and the temperature were varied from 0 to −30 °. The motion of ice spheres were recorded with the high speed video system and the velocity and density profiles were obtained under various conditions. Two types of bed conditions were selected to investigate how flowing particles interact with the bed. One was intended to simulate a rough boundary condition by placing ice spheres on the bed surface. Another was to simulate a smoother boundary by placing sand papers. A velocity gradient with a blunt shape and a significant amount of slip were found in all conditions. The slip velocity increased with chute inclinations and temperatures. Low density regions were observed near the bed, typically in the order of four to five particle diameter thick, where there existed large velocity fluctuations. Boundary conditions have a significant influence on the flow behavior; flows on the rough bed condition seem to be more agitated near the bed and travel slower in general.

Development of saltation layer of drifting snow

Abstract The saltation length of aeolian snow particles and a new parameter, the ejection factor, which expresses the degree of erosion due to drifting snow, were obtained as functions of friction velocity by means of wind-tunnel experiments for semi-hard snow cover. The saturated-snowdrift transport rate was also obtained experimentally as a function of friction velocity. Based on these characteristics and the parameter, the development of the saltation layer of drifting snow along the fetch was simulated under various conditions such as snow hardness, wind speed and snowfall intensity. The main results are as follows. The developing distance denoting the distance required for the saltation layer to attain saturation, X sat, is determined by saltation length, ejection factor and saturated-snowdrift transport rate, all of which depend on wind speed. It is also affected by the magnitude of snowdrift transport rate at the starting point and by the intensity of snowfall if it exists. The dependence of Xsat on wind speed is not simple in the case of semi-hard snow cover: Xsat increases with wind speed under weak to moderate wind conditions and then decreases under moderate to strong wind conditions. It is sensitive to snow hardness: it is about one order longer on hard snow cover than on semi-hard snow cover. Snowfall reduces not only the value of Xsat but also its dependence on snow hardness.

Grain size dependence of eolian saltation lengths during snow drifting

Eolian saltation, a primary process in the transport of fine granular material by wind, produces a variety of geophysical effects on Earth and other planetary surfaces. Wind-tunnel experiments were carried out to investigate the dependence of saltation on grain size. The saltation length of snow particles was estimated at size intervals of 0.05 mm in diameter by measuring local vertical mass fluxes in 17 snow collectors arrayed at the lee end of the snow surface. The measured mean saltation length of snow particles of 0.01–1 mm in diameter ranged from 0.1 to 1.0 m at wind velocities of 5–10 m/s. Mean saltation length decreased with increasing diameter and decreasing wind speed. We suggest that the probability of the saltation length of a particle at each diameter is described by a monotonically decreasing distribution function, that is, the shorter the saltation length, the higher the frequency of its occurrence. One ramification of this distribution is that the mean saltation length does not imply the dominance of saltating particles of this length.

Estimation of the electrostatic charge of individual blowing-snow particles by wind tunnel experiment

Abstarct There are some reports on the measurement of the charge-to-mass ratio of blowing-snow particles, but there are few studies concerned with individual snow-particle charge. We measured the charge-to-mass ratios using snow particles selected according to size, and discussed individual charges. Experiments were conducted in a cryogenic wind tunnel. Charge-to-mass ratios measured in our experiment were all negative and their absolute values tended to increase with a decrease in particle diameter. Individual snow-particle charges were calculated from the average of particle diameter distributions. The charges were all approximated by the power function of diameter at each temperature. Assuming that the coefficient of these approximations is a function of air temperature, we could easily predict the individual snow-particle charge.

Visibility reduction based performance evaluation of vision-based safety sensors

This paper describes the indoor snowfall simulation chamber and its application for evaluating visibility performance of vision-based sensors affected by environmental conditions. First, vision-based safety sensors are introduced and outdoor environmental requirements for these sensors are discussed. And also, the relationship between the visibility and the performance of vision based sensors is discussed and the current problems for evaluating visibility performance are described. Next, the indoor snowfall simulation method using expanded polystyrene beads (EPB) is explained and the spectral transmission measurement equipment for evaluating the visibility reduction is presented. Finally, the visibility reduction caused by simulated and artificial snowfall are measured for verifying the proposed indoor snowfall simulation chamber. Preliminary experiments are also carried out in both snowfall systems in order to evaluate various kinds of vision-based safety sensors for personal care robots.

Effects of Snowfall on Drifting Snow and Wind Structure Near a Surface

Wind-tunnel and numerical experiments were performed to investigate the effects of snowfall on the wind profile and the development of drifting snow. Wind profiles and mass-flux profiles of drifting snow were measured with and without artificial snowfall over a snow surface within the tunnel. Wind and shear-stress profiles and the impact speeds of the snowflakes during snowfall were also investigated numerically. During snowfall, snowflakes transfer part of their horizontal momentum to the air, which increases the stress close to the snow surface; however, the resultant modifications of the wind profiles are small. Because snowflakes have large momentum, the decomposed snow crystals that result from their collision with a surface can form a saltation layer, even over a hard snow surface where entrainment of the grains from the surface does not occur. Additionally, during snowfall, the threshold friction velocity can be lower than the impact threshold because snowflake fragmentation facilitates snow drifting. The broken crystals contribute to the increase in the number of drifting snow grains, even below the impact threshold.