Norikazu Maeno

A continuous 770-year record of volcanic activity from east Antarctica

A 100-m ice core from site G 15 (accumulation rate 0.1 m water yr−1, mean annual temperature −38°C) on the Mizuho plateau, Dronning Maud Land, East Antarctica, has been analysed using the dielectric profiling (DEP) technique. The capacitance and conductance of the core were measured at ac frequencies (20 Hz-300 kHz). The high-frequency conductivity profile shows variations that are primarily related to the strong acids derived from volcanic activity. The Tambora (1815) eruption can be identified with the aid of an approximate chronology based on the firn densification rate, other historic eruptions can then be recognised. Beyond about 300-years historical observations are very few, however if a constant overall accumulation rate is assumed, a well-known eruption of 1259 A.D. can be found near the bottom of the core. Other peaks in the conductivity profile can then be assigned dates accurate to within a few years. Using the conductivity profile it is possible to estimate the relative acid deposition fluxes produced by the main eruptions with reasonable accuracy. The estimated acid deposisition fluxes realtive to the Tambora (1815) eruption, of Agung (1963) is 27%, Krakatoa (1883), 25%, the signal of 1601, 28%, and that of 1259, 53%.

Adhesion shear theory of ice friction at low sliding velocities, combined with ice sintering

Adhesion and shear deformation of ice have been traditionally considered to be responsible for ice friction at sliding velocities lower than about 10−2 m/s, but the simple mechanism cannot explain the recent finding that the ice–ice friction coefficient increases with decreasing sliding velocity. This article proposes an improved adhesion shear theory, which takes account of junction growth of asperities at the sliding ice interface due to sintering. At lower sliding velocities and higher homologous temperatures, contacts of ice asperities develop resulting in the increase of friction force.

Mechanical strength of polycrystalline ice under uniaxial compression

Abstract Mechanical strength of polycrystalline ice Ih was investigated by uniaxial compression tests at wide ranges of temperature, −10 to −173°C, and of strain rate, 4×10−4 to 4×10−6 s−1. A systematic change of the deformation type from brittle fracture to ductile deformation was observed to take place at a critical strain rate and temperature. A systematic increase of the strength was also found with decreasing temperature and increasing strain rate. In both the ductile and brittle regions, a similar relation was found to hold: e =Aσ n exp (−E/RT) where e is the strain rate, σ is the peak stress for ductile deformation and failure stress for brittle fracture, R is the gas constant and T is the absolute temperature. The apparent activation energy E and exponent n are 48 kJ/mol and 6.5 in the brittle region and 64 kJ/mol and 3.4 in the ductile region, respectively. The maximum stress (σmax: peak and failure stress) has a good correlation with the strain (emax) at that stress irrespective of the temperature and strain rate. An empirical equation, emax=0.41+0.015σmax, where the unit of σmax is MPa, can be applied to both the ductile and brittle regions.

Shear cell experiments of snow and ice friction

Snow and ice friction was investigated with a shear cell in which two surfaces of annular snow and/or ice samples were in contact and sheared by rotation. The temperature ranged from 0 to −25 °C, normal stress from 205 to 1292 Pa, and velocity from 0.9 to 25.3 m/s. The total friction coefficients measured by friction of two hard sintered snow plates ranged roughly from 0.2 to 0.8, which were separated into velocity‐independent dry friction and linearly dependent viscous friction; only at velocities larger than 15 m/s did friction increase parabolically, evidence of turbulent friction. The dry friction coefficient (μD) was found to consist of Coulomb friction and adhesion, both of which were strongly dependent on temperature and hardness of snow; μD decreased linearly with lowering temperature from 0.47 at 0 °C to 0.22 at −25 °C. Viscosity was found to be dependent on snow type (grain characteristics) but independent of temperature; the average kinematic viscosity was roughly 5×10−5 m2/s for snow of densit...

Measurements of snow mass flux and transport rate at different particle diameters in drifting snow

Abstract Wind tunnel experiments were carried out to investigate the snow mass flux and the snow transport rate as functions of friction velocity and particle diameter. Friction velocities obtained with an ultra sonic anemometer was from u*=0.15 to 0.39 m/s. The number flux and the particle size of drifting snow were measured with a new snow particle counter at heights of 16 to 61 mm above the snow surface. The horizontal snow mass flux at each particle diameter d, qd, decreased exponentially with height qd∝exp(−λdz/(u*2/g)), where z is the height, g is the acceleration due to gravity and λd is a dimensionless parameter. The constant λd was independent of friction velocity, and at larger particle diameters, approached to 0.3. The total snow transport rate increased with the power (3.96) of friction velocity, but the total snow transport rate at each particle diameter showed that the power decreased with increasing particle diameter, and approached to 3.

MEASUREMENTS OF RESTITUTION COEFFICIENTS OF ICE AT LOW TEMPERATURES

Abstract Measurements of the restitution coefficient (e) of a smooth water ice sphere (radius = 1.5 cm) are made in a wide range of impact velocities (1≤υi≤700cms−1) and temperatures (113≤T≤269K). The impact velocity dependence of e is different in the quasi-elastic and inelastic regimes separated by a critical velocity (υc) at which fracture deformation occurs at the impact point of ice samples. In the quasi-elastic regime (υi≤υc), the value of e is almost constant (0.88) and ice samples show no fracture deformations. In the inelastic regime (υi>υc), e decreases with increasing υi and ice samples have fracture patterns. The velocity dependence of e is fitted as e(υ i ) = ( υ i υ c ) − log ( υ i υ c ). vc is shown to increase with decreasing temperature from 25cms−1 (269K) to 180cms−1 (113–215K).

Rapid Growth of Asteroids Owing to Very Sticky Interstellar Organic Grains

We experimentally found interstellar grains covered with organic matter in an asteroid belt, and more importantly, the organic matter played an essential role in the formation of the asteroids. The sticking threshold velocityof 5 m s-1 of the millimeter-sized organic grains was several orders of magnitude higher than those of the coexisting silicate and ice grains. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. In contrast, there was no coagulation of the silicate and ice grains in the terrestrial and Jovian regions, respectively.

Wind-Tunnel Measurements Of Restitution Coefficients And Ejection Number Of Snow Particles In Drifting Snow: Determination Of Splash Functions

Wind-tunnel experiments of drifting snow were carried out andsplash functions were formulated to describe probability distributions of vertical restitution coefficient, horizontal restitution coefficient and ejection number when a natural snow particle collided at a natural snow surface. The following results were obtained:(1) The vertical restitution coefficient was usually larger than unity and decreased sharply with impact angle. At smaller impact angles around 5 degrees the vertical restitution coefficient exceeded a magnitude of ten.(2) The horizontal restitution coefficient, ranging from -1 to 1.5, decreased with impact velocity, but was not clearly dependent on impact angle.(3) The ejection number amounted to five per impact and increasedwith impact velocity.(4) Three splash functions to express the probability distributions of the vertical restitution coefficient, horizontal restitution coefficient and ejection number were formulated, which will be used in future computer simulations of the snow drifting process.

Wind-Tunnel Experiments on Blowing Snow

Blowing snow was produced artificially in a cold wind-tunnel, and various measurements were conducted including particle diameters, concentrations, saltation lengths, heat transport and electric charge. The mean diameter of blowing snow particles decreased only slightly with increasing height; in the saltation layer, standard deviation was large and velocities were scattered in a wide range, suggesting the complex dynamic process on taking-off. The mean saltation length ranged from a few cm to 40 cm, increasing with wind velocity.

Observations of the dynamic structure of snow avalanches

During two winters, 1990-92, the dynamic structures of snow avalanches were studied in western Norway. Artificially released wet-snow avalanches ran down the avalanche chute and stopped in front of the retaining dam. Running velocity distributions were obtained not only by video tape recorder, but also by various other recording instruments. Internal velocity was derived for the last avalanche by frequency analysis of impact pressure and optical sensor data. The vertical velocity shear of the avalanche flow has been estimated to be in the range 1lOS-