MEASUREMENTS OF THE ELECTRIC FIELD GRADIENT IN A BLIZZARD

SCOTT SCHMIDT AND JIM DENT

Department of Civil Engineering, Montana State UniversityBozeman, Montana 59717

ABSTRACT

In the transport of heavy particulates by wind, as inblizzards and sand storms, much of the particulate flux occursvery near the surface by a process called saltation. A buildupof electrostatic charge usually accompanies this phenomenon. Todate, the force due to this electrification has not been includedin equations predicting particle transport by saltation.Consideration of electrostatic force on particle saltationtrajectories requires knowledge of the electric field strengthvery near the snow surface. Such measurements have been lacking.This paper reports measurements of the electric field gradient ina blowing snow storm. Given this newly measured electric fieldgradient and previously measured charge-to-mass ratios forsaltating snow particles, the modified equations of motionpredict substantial changes in saltation lengths in agreementwith experimental observations.

INTRODUCTION

Saltation is a transport mechanism by which solid particleshop along a surface in a fluid undergoing shear flow. Whether itbe snow or sand particles in air, sediment in a river bed, orsand in a density current on the ocean floor, the equations thatdescribe this motion are of similar form. The bounce of aparticle, rebounding from elastic impact with the surface, isstretched into a long, low trajectory responding to forces offluid drag and gravitation. Equations of motion currently usedto describe the trajectories of these saltating particles areinconsistent with observed trajectories. The verticaldeacceleration of assinding particles and aceleration ofdecending particles are both greater then the gravitationalacceleration which indicate the presence of a negative liftforce. Lift forces develop if the particle spins (White andSchUltz, 1977) but spin rates needed to produce the necessarylift forces are large. Another force that could produce a liftforce on the particles is an electrostatic force.

Many observers have reported electrification of naturalblowing snow dating back to Simpson (1921). All observers note aconsiderable increase in the positive electric field gradientnormal to the_~arth's surface, during blowing snow events. Largecharge-to-mass ratios have also been measured for blowing snowparticles (Whishart, 1968, and Latham and Montagne, 1970). Whenan electrically charged particle moves through an electric field,

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it is sUbject to a force which has a magnitude equil to theparticles charge times the magnitude of the electric field. Inblizzards, snow particles (carrying a substantial charge) movethrough electric fields of large magnitude. To date, theelectrostatic force on these charged snow particles has not beenincluded in the equations of motion describing particletrajectories, nor has the effects this force has on snow and iceformations been considered. Snow drifts and ice on roadways posea major obstacle to winter travel. Cornice developments on ridgetops are a dangerous source of avalanche triggers. Theseavalanches often affect highway travel and ski area operations aswell as posing a threat to domestic structures in mountaincommunities. If electrostatic forces playa part in the

. formation of these hazards, a better understanding of the forceand the physical basis for its generation should lead to betterprediction and possible control of these phenomena.

ELECTRIC FIELDS IN BLOWING SNOW

The problem with including the electrostatic force in theequations of motion for blowing snow has been lack of electricfield data in the saltation region 0-10 cm above the snowsurface. Latham and Montagne (1970) measured the field in theregion 30-400 cm above the surface and found it variedsignificantly with height. Schmidt and Dent (1993) proposed atheoretical model based on Latham and Montagne's (1970) data atdistances far from the surface and the electric field due to abed of charged ice spheres for the near-surface field.

Electric field Model

Based on measurements of surface particle charge, made in awind tunnel (Maeno et al.,1985), Schmidt and Dent (1993) assumedthat ice spheres, making up a bed surface, have a charge-to-massratio equal in magnitude but opposite in sign to that measuredfor blowing snow particles. The electric field above this bed of

. ice spheres, made up of rings of spheres arranged in closestpacking, was computed as a function of height above the bed bytreating each sphere as a point charge. Contributions to thefield decrease rapidly as the radius of these rings increases, sothat little was contributed beyond 10 rings, or the area coveredby 61 bed spheres.This model predicted changes in saltation trajectory lengths asgreat as 24% for charged particles compared to unchargedparticles. These changes in trajectory lengths were consistentwith observed trajectories (Kobayashi, 1972).

Verification of the model proposed by Schmidt and Dent(1993) requires a measuring device that can be placed close tothe snow surface in a blowing snow storm with minimal effect onthe electric field. In the past it was not possible to measurecloser then 30 cm to the surface without significantly disruptingthe field (Hence Latham and Montagne's 1970 measurements).Recent technologic advancements have resulted in an electric

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FIGURE 1: End viewof electric fieldprobe in electricfield.

A cylinder rotates in a twodimensional electric field (E = Ex + Ey)with the axis of the cylinder perpendicularto the field. The cylinder is cut in halflength-wise and the two halves separated bya dielectric. The electric field induces acharge on the surface of the cylinder(Figure 1). This induced charge isproportional to the field. There are twoelectrodes, mounted 180 degrees from eachother, on the cylinder's surface. As the cylinder rotates thecharge on its surface remains fixed with respect to the electricfield thereby inducing a current between the electrodes. Thiscurrent is converted to optical pulses by a light emitting diodeand input into a optical fiber were they are transmitted to areceiver system and converted into an electric field output.

field meter capable of making the needednear-field measurements. This meter wasdeveloped by the Jet Propulsion Laboratory(JPL) at the California Institute ofTechnology. The DC electric field probe isdescribed in detail by Kirkam and Johnston(1988). ,The following ~s a.briefdescript10n of the phys1cs 1nvolved.

ELECTRIC FIELD MEASUREMENTS

Harold Kirkam made a generous loan of the instrument toMontana State univ. for the winter of '93-94 in order that it'soperation could be tested in harsh blizzard environments. Theprobe worked exceptionally well in winter conditions and inJanuary '94 measurements were made of the electric field in theregion .002-4 m above the snow surface.

On 6 January 1994, a mobile laboratory was moved to alocation just south of the Cooper Cove interchange of InterstateHighway 80, 50 kID west of Laramie, Wyoming, at 41°31' N,106°05'W, 2360 m elevation. Consistent west wind duringdrifting, over nearly level terrain with short-grass vegetation,make the site ideal for such measurements. These conditionsexist for approximately 1 kID upwind of the measuring location.10 cm of new snow fall provided light drifting due to moderatewinds. At 2458 hours the wind increased, ranging from 4 mjs to10 mjs on up to 22 mjs.

Instruments

Measurements included average wind speed and air temperatureat aIm height. Anemometers (19 cm diameter, 3-cup plasticrotors) produced signals with frequencies proportional to windspeed. A thermistor shielded with a plastic pipe 'T' connectorproduced voltages proportional to air temperatures. Relativehumidity at 2 m height was measured in an enclosure mounted on

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the side of the mobile lab. The inclosure was screened withpolyester filter fabric. The electric field probe was mounted ona tripod equipped with a ten-turn potentiometer. Oncecalibrated, the potentiometer produced voltages proportional tothe height of the probe above the snow surface. The electricfield probe was connected by 15-m fiber-optic cables to thereceiver unit, housed in the mobile laboratory.

Procedure

computers in the mobile lab recorded data from runs of 1­hour duration for storage on magnetic disks. Measurements ofaverage windspeed, temperature, and humidity, as well as probeheight and horizontal and vertical electric field components wererecorded every minute. The procedure was to set the probe at agiven height for a period of 15 min. Each time the probe heightwas changed, a manual measurement was taken and compared with theheight reading given by the potentiometer to insure accuracy inthe electronic reading. A series of vertical profiles for theelectric field above the snow surface was measured in the period0100-h to 0630-h.

Results

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ELECTRIC FIELD IN BLOWING SNOW

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FIGURE 2: Comparison of theoreticelectric field proposed by Schmidt andDent to measured electric field. Powerlaw regression used to fit data.

DISCUSSION

Measurements ofelectric field wereaveraged at each heightinterval. A comparisonof the measuredelectric field as afunction of height tothe model electricfield theorized bySchmidt and Dent (1993)is shown in Figure 2.

The electricfields measured in thisexperiment were asgreat as 30-kV/m at aheight of 4-cm ascompared to the fairweather electric fieldof O.06-kV/m measuredat the same height ..Electric fieldsmeasured in a blowingsnow storm compare surprisingly well with the theoretical modeldeveloped by Schmidt and Dent (1993). This indicates that theeffects of electric fores can not be neglected when describingthe motion of saltating snow particles. Based on previouslyreported charge-to-mass ratios, electric forces can affect snow

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particle saltation trajectories by as much as 24 % (Schmidt andDent, 1993). Schmidt and S~hmidt (1992~ predict that previouslymeasured charge-to-mass ratl0s for blowlng snow are at least anorder of magnitude too small. If this is the case, theelectrostatic force on a saltating snow particle could be of thesame order of magnitude as the gravitational force.

Electrostatic forces calculated by multiplying the electricfield measured in this experiment with measured particle charge­to-mass ratios may explain many wind-related snow and iceformations. Wind tunnel experiments (Maeno et.al., 1985) showedsaltation surfaces also become charged during blowing snowstorms. The magnitude of the charge-to-mass ratio isapproximately equal to the charge-to-mass ratio of the saltatingparticles but the sign of the surface charge can be positive ornegative. The sign change appears to coincide with localdeposition and erosion areas on the surface. saltating snowparticles carry charge whose sign is predominantly negative butSchmidt and Schmidt (1992) show that this sign fluctuates. Thechange in sign for the saltating particles seems to be related tothe presence of newly fallen snow being transported as opposed tosnow that has been on the ground for several days. This changein sign may explain why freshly fallen blowing snow is more aptto form large drift formations, cornices, and icing on roads. Arelationship between surface charge, saltation particle charge,and electric field may provide information from which a modelpredicting, in particular, roadway icing can be developed.Understanding the electrostatic relation should also result inbetter predictions of drift formation and may lead to anexplanation for cornice formation.

As expected from a review of the charging mechanisms for iceparticles (Hobbes, 1974) the measured electric field appears tobe a strong function of blowing snow particle flux. Temperatureand humidity must also play a role in the charging of the blowingsnow particles. For the measurements made here temperature andhumidity remained constant but if a general model for snowelectrification is to be developed, experiments relating particleflux, temperature, and humidity with electric field must beconducted.

CONCLUSION '

Ground blizzards transport a great quantity of snow. It hasbeen observed that this transported snow is accompanied by anincrease in the electric field normal to the earth's surface.Measurements of electric fields in a blowing snow storm give aquantitative verification indicating that electric fields areenhanced by more then four orders of magnitude. Thesemeasurements also indicate that the electric field in a blowingsnow storm varies significantly as a function of height above thesnow surface as predicted by Schmidt and Dent (1993). Thepresence of such a large electric field may help to explain snowand ice formations such as dunes, cornices, and roadway icing.

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ACKNOWLEDGEMENTS

The Authors would like to extend his gratitude to HaroldKirkam, Jet Propulsion Laboratory, for the generous loan of theD.C. electric field meter. Many thanks to Dr. R. A. Schmidt,u.S. Forest Service, for providing the mobile research lab andhis help in making these measurements.

REFERENCES

Hobbs, P.V. 1974, Ice Physics. Oxford. Clarendon Press.

Johnston, A.R. and Kirkham, H. 1989. A Miniaturized Space­Potential DC Electric Field Meter. IEEE Trans Power Deliv.,4(2), 1253-1261.

Kobayashi, D. 1972. Studies of snow transport in low-leveldrifting snow. contrib. Inst. Low. Temp. Sci. Hokkaido Univ.,Sere A(24), 1-58.

Latham, J. and Montagne, J. 1970. The possible importance ofelectrical forces in the development of cornices. J. Glaciol.,9(57), 375-384.

Maeno, N., Naruse, R., Nishimura, K., Takei, I., Ebinuma, T.,Kobayashi, S., Nishimura, H., Kaneda, Y., and Ishida, T., 1985.Wind-tunnel experiments on blowing snow. Annals of Glaciology 6,63-67.

Schmidt, S. and Dent, J.D. 1993. A Theoretical Prediction ofthe Effects of Electrostatic Forces On Saltating Snow Particles.Ann. Glaciol., 18, 234-238.

Schmidt, S. and Schmidt R.A. 1993. The Sign Of ElectrostaticCharge On Drifting Snow. Proc. International Snow ScienceWorkshop 1992, 351 -360.

Simpson, G. C. 1921. British Antarctic Expedition, 1910-1913.Vol. 1, London, Harrison and Sons.

White, B. and Schultz, J. 1977. Magnus effects in saltation.J. Fluid Mech., 81, part 3, 495-512.

Wishart, E. R. 1968. Electrification of Antarctic driftingsnow. Proc. Int. Symposium Antarctic Glaciological Exploration,Sept. 3-7, 1968, Hanover, NH,USA. lASH Pub. No. 86, 1970, p.316-324.