Abstract. In blizzards and sandstorms, wind transport of particles is associated with separation of electrostatic charge. Moving particles develop charge of sign opposite the electrostatic charge on stationary surface particles. This electrification produces forces in addition to the gravitational and fluid friction forces that determine trajectories for particles being transported in saltation. Evaluating electrostatic forces requires the electric field strength very near the saltation surface and charge-to-mass ratios for the moving particles. In a low-level blowing sand event we measured an average charge-to-mass ratio of +60/xC kg -• on the saltating particles at 5-cm height and a maximum electric field of + 166 kV m -• at 1.7-cm height, in wind gusts near 12 m s -• at 1.5-m height. The electrostatic force estimated from these measurements was equal in magnitude to the gravitational force on the saltating particles. Including electrostatic forces in the equations of motion for saltating particles may help explain discrepancies between measurements and models of saltation transport. IntroductionWind is a powerful force capable of transporting vast quantities of particulate matter. By some estimates, wind transports more than 5 x 108 metric tons of dust on Earth each year, with nearly one tenth of our planet being blanketed in a layer of wind-deposited soil ranging in depth from 1 to 100 m. Besides understanding the importance of aeolian processes for life on Earth, we rely increasingly on that understanding to make interpretations about atmospheres that surround neighboring planets. The presence of atmospheric dust in a planet's environment is a factor in predicting its temperature and climatic characteristics. Improving our understanding of the wind transport process benefits many areas of research. The bouncing mode of particle transport very near the surface, called saltation, is a key mechanism in aeolian processes. The objective of our experiment was to simultaneously measure the near-surface electric field and particle charge-to-mass ratio, from which to assess the significance of electrostatic force acting on the saltating sand. The measurements in lowlevel sand transport on August 31, 1995, in southwest Wyoming indicated electrostatic forces of magnitude similar to the gravitational force on the particles.Copyright 1998 by the American Geophysical Union. Paper number 98JD00278.0148-0227/98/98JD-00278509.00 Aeolian Saltation TrajectoriesSaltating particles rebound from elastic impact with surface particles to follow long, low trajectories in response to forces of fluid drag and gravitation. Using a high-speed camera to photograph trajectories of saltating glass spheres in a wind tunnel, White and Schulz [1977] found trajectories were higher and longer than predicted from equations involving only fluid drag and gravitational forces. They improved agreement with the observed trajectories by adding Magnus lift to the theoretical equations. However, matching measured trajectories by the addition of Magnus...

A small avalanche path near the Bridger Bowl ski area in southwestern Montana has been instrumented to measure density, velocity and dynamic friction in a flowing avalanche. These measurements, made by an array of sensors mounted in the avalanche path, have been carried out for several dry-snow avalanches. Measurements of density were made using a capacitance probe that measures the dielectric constant of any material that passes in front of it. Through a calibration procedure, the dielectric constant of a given type of snow can be related to the density of that snow. Optical sensors were used to measure light reflected from the avalanche as it passed by the sensors. Signals from adjacent optical sensors were cross-correlated to determine velocity. Density and velocity measurements were made at several heights in the avalanche, with particular attention directed near the running surface. Results indicate that avalanche deformation is concentrated near the running surface where the snow density is found to be largest. Upward from the surface, the velocity gradient falls off greatly while the density also declines.Finally, the dynamic-friction coefficient at the base of the avalanche was found by measuring shear and normal forces on a roughened 23 cm × 28 cm aluminum plate mounted parallel and flush with the avalanche running surface. The ratio of the shear force to normal force on the plate provides a measure of the dynamic-friction coefficient at the base of the avalanche.

In blizzards and sand storms, wind transport of particles is associated with separation of electrostatic charge. Moving particles develop charge of sign opposite the electrostatic charge on stationary surface particles. This electrification produces forces, in addition to the gravitational and fluid friction forces, that determine trajectories for particles being transported in saltation. Evaluating electrostatic forces requires the electric field strength very near the saltation surface and charge-to-mass ratios for the moving particles. Measurements in a blizzard provide an electric field profile, with measured fields as high as 30 kV m-1 measured at the 4-cm height. Reversal of charge sign on samples of saltation particles collected in a blizzard indicates a mixture of positive and negative particles in transport. This result points out the need for measurements of charge on individual particles, and an apparatus designed to make these measurements is detailed. Using measured charge-to-mass ratio for individual saltation particles (Schmidt et al., 1998) of +72 mC kg-1 and-208 mC kg'1 we estimate electrostatic forces as large as the gravitational force on some saltating particles. Including forces of this magnitude in the equations of motion for saltating particles shows saltation trajectories altered as much as 60% from those for uncharged particles.

A modified Bingham numerical model is developed and tested for the simulation of the motion of snow avalanches. This two-dimensional, incompressible model takes the form of a two-viscosity system in which a large viscosity is employed in the low stress regions of the flow and a smaller viscosity is used in the high stress regions. The model involves three parameters: the two viscosities, and the value of the stress for the transition between the two flow regimes. A simple no-slip boundary condition is used at the interface between the flowing snow and the stationary snow surface. Model parameters are evaluated by simulating the motion of the leading edge of the flowing snow, velocity versus depth information, and debris distribution of small snow test experiments.

A BSTRACT. A sm a ll avala nche path near the Bridger Bowl ski a rea in so uthwes tern Montana has been in strumented to m eas ure density, velocity a nd dyna mi c fri cti on in a Oowing ava la nche. These measurements, m ade by a n a rray of se nsors m ounted in th e avala nche pa th, have been carri ed out for several dry-snow avala nches. l\1easurem ents of density were ma de using a capac ita nce probe th a t meas ures the dielec tric consta nt of a ny materia l th at passes in front of it. Through a calibra ti on procedure, th e dielec tric consta nt of a given type of snow can be rela ted to the density of that snow. Optical se nsors we re used to m easure light reOected fro m th e avalanche as it passed by th e sensors. Signals fro m adj acent optical sensors we re cross-correla ted to determine velocity. D ensity a nd velocity meas urem ents were m ade at several heights in the avalanche, with pa rticul a r attention directed near the running surface. Results indicate that ava lanche deform ation is concentrated near the running surface where the snow density is found to be largest. U pward from the surface, the velocity gradi ent fall s off g reatl y while the density a lso declines.Fina lly, th e dyna mic-fri cti on coeffi cient at th e base of th e avalanche was fo und by meas uring shea r a nd no rm a l fo rces o n a ro ugh ened 23 cm X 28 cm a luminum pl a te mo unted pa ra ll el and flu sh w ith the avala nche running surface. Th e ra tio of the shear fo rce to norm al force on the plate provides a meas ure of the dynamic-friction coeffi cient at the base of the avalanche.