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A Detailed Assessment of Snow Accumulation in Katabatic Wind Areas on the Ross Ice Shelf, Antarctica

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A Detailed Assessment of Snow Accumulation in Katabatic Wind Areas on the Ross Ice Shelf, Antarctica JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. D25, PAGES 30,047-30,058, DECEMBER 27, 1997 A detailed assessment of snow accumulation in katabatic wind areas on the Ross Ice Shelf, Antarctica David A. Braaten Departmentof Physicsand Astronomy,University of Kansas,Lawrence Abstract. An investigationof time dependentsnow accumulation and erosiondynamics in a wind-sweptenvironment was undertakenat two automaticweather stations sites on the RossIce Shelf betweenJanuary 1994 andNovember 1995 usingnewly developedinstrumentation employinga techniquewhich automatically disperses inert, colored (high albedo) glass microspheresonto the snowsurface at fixed intervalsthroughout the year. The microspheresact as a time markerand tracerto allow the accumulationrate and wind erosionprocesses to be quantifiedwith a high temporalresolution. Snow core and snowpit samplingwas conducted twice duringthe studyperiod to identify microspherehorizons in the annualsnow accumulation profile, allowing the snowaccumulation/erosion events to be reconstructed.The two siteschosen for thisinvestigation have characteristically different mean wind speedsand therefore allow a comparativeexamination on the role of wind on ice sheetgrowth. Mass accumulationrate at the twosites for the 14-dayintegration periods available ranged from 0.0 to >2.0 kg m-2 d -l. The meanmass accumulation rate duringthe studyperiod was greaterat the site with strongerwinds (0.69kg m -2 d-1) than the site with lower mean wind speeds (0.61 kg m-2 d-l); however,the differencebetween the two meansis not statisticallysignificant. Accumulationrates derived from an ultrasonicsnow depth gauge operated at one of the sitesare comparedto the actualtracer-derived accumulationrates and show the limitationsof only having a measureof snow surfaceheight with no instantaneousmeasurements of the snowdensity profile. Snow depthgauge derived accumulationrates were foundto be greatlyoverestimated during high-accumulation periods and were greatlyunderestimated during low-accumulation periods. 1. Introduction complex and has been examinedexperimentally by Budd et al. [ 1966], Kobayashi [ 1978], Wendlet [1989], and Maeno et ai. Polar ice sheets are an important component of the global [1995] using innovative experimental techniquesto examine climate system; however, their remote and harsh environment various characteristicssuch as mass flux and snow grain size confound and limit experimental attempts to understandtheir as a function of height and wind speed. Naturally, these dynamics and interactions with the atmosphere and oceans. processescan only occur when the wind speed exceeds some Polar ice sheetscontain approximately2% of the Earth's water threshold speed for snow grain movement (saltation), which and therefore negative changesin ice sheet mass balance can usually dependson the condition of the snow surface. Age- potentially have serious implications due to a rise in global hardened snow has a much higher threshold saltation speed sea level, although it is still an open question whether polar than recentlydeposited snow. Bromwich [1988] gives a range ice sheets grow or decay under a warming climate scenario of thresholdspeeds between 7 and 13 m s-l; however,during [Jacobs, 1992]. Polar ice sheets are also valuable archives of this study the saltation thresholdwind speedson the Ross Ice direct and indirect evidence of millennial-scale climatic events Shelf were observed to be between 5 and 6 m s-l. Wind and past atmosphericcomposition of trace gasesand aerosols. movement of surface snow results in a variety of different Proper interpretation of this evidence requires a thorough surface features which have been classified as depositional knowledgeof the dynamical processeswhich govern ice sheet and/or erosional patterns by Kobayashi and Ishida [1979]. growth. They concludethat wind speedsless than 15 m s-l (at 1 m Processes which control local ice sheet changes can be above the surface) producetransverse features such as ripples, categorized in terms of source and loss parameters. Source waves, and barchans, whereas wind speedsgreater than 15 m parameters include episodic snowfall events associatedwith s-1 producelongitudinal features such as dunes and sastrugi. synoptic scale features such as cyclones and fronts The microphysicalprocesses which producethese featuresare [Bromwich, 1988] and hoar frost deposition. Loss parameters complex, and essentially nonlinear, in that the turbulent include sublimation [Fujii and Kusunoki, 1982] and snow airflow distorts the snow surface which in turn distorts the erosion caused by drifting or blowing snow; however, snow flow. External characteristicssuch as amplitude,wavelength, grains removed by erosionmay be replacedby snow which has shape, and movement have been examined by Budd et al. eroded from upwind locations. The processof snow erosionis [1966] and Kobayashiand Ishida [1979]; however,important aspects such as snow grain transport and mixing into the Copyright1997 by the AmericanGeophysical Union. accumulation profile remain largely unknown. Since most Papernumber 97JD02337. precipitation events at the coastal margins of Antarctica are 0148-0227/97/97JD-02337509.00 associatedwith wind speedsgreater than the thresholdspeed 30,047 30,048 BRAATEN: SNOW ACCUMULATION IN KATABATIC WIND AREAS I I I I I I 166øE 167"E 168øE 169øE 170øE 171 øE Ross Sea 77.25 ø S- McMurdo Sound Ross Island 77.50"s- Ross Ice Shelf 77.75" S- - McMurdo/ Station Ferrell AWS Willie Field WS 78.00" S- 78.25" Wind Rose Scale Wind Rose Legend l• • , • I .... I --<5m/s 0 10 20 >=5m/s Percentage of Observations i i i i i 18o ø 90 øE 90 øW o Figure 1. Location of automatic weather stations where snow accumulation experiments were conducted. Wind roses summarize the winds during the study period, showing the prevailing directions from which the winds were coming. for blowing snow, sourceand loss parametersare interrelated new automatedsystem to dispersecolored glass microspheres in that layers of accumulatedsnow are generally a mixture of onto the snow surface at preset time intervals to act as a tracer. snow producedby individual stormsand from previousstorms These sites are Willie Field automatic weather station (AWS) which has been eroded and transportedfrom upwind locations (77.85øS, 167.08øE) located approximately 10 km east of [Bromwich, 1988]. McMurdo Station (MCM) and Ferrell AWS (78.02øS, This investigation was conducted to obtain greater 170.80øE)located approximately 100 km eastof MCM (Figure quantitative insight into the time dependentprocesses of ice 1). Evaluation of snow accumulation at each site was sheet snow accumulation in windswept areas. Experiments conductedin two phasesbetween January 1994 and November were carried out at two locations on the Ross Ice Shelf using a 1995. For the Willie Field AWS the two evaluationperiods BRAATEN:SNOW ACCUMULATION IN KATABATIC WIND AREAS 30,049 wereJanuary 18, 1994,to January28, 1995,and February l, corridor from 210 ø with a directional standard deviation of 1995, to November 2, 1995, and for the Ferrell site the 18.5'for wind speeds > 5 m s-l. evaluationperiods were January25, 1994, to January21, At Willie Field only, a CampbellScientific ultrasonic snow 1995, andJanuary 25, 1995,to November17, 1995. depthgauge and CRI0 data loggerwith a datastorage module supplementsthe AWS data,providing hourly measurements of distance to the snow surface with an accuracy of +10 mm. 2. Wind and PrecipitationCharacteristics These data were retrieved once per year. For most days, more 2.1. Automatic Weather Stations than 90% of these hourly measurements are valid. Occasionally, erroneous data were recorded for several The two AWS sitesused in this studyare part of a large array consecutivedays which was most likely due to ice crystal of stationsdeployed on the continent[Holmes et al., 1996]. buildup(hoar frost) on the sensorhousing. In 1994,data were EachAWS provides10 min measurementsof air temperature, missingfrom 23% of the days with the longestconsecutive wind speedand direction,atmospheric pressure, relative period being 30 days, and 22% of the days in 1995 were humidity, and vertical differential temperature(2 m) missingwith the longestconsecutive period being 20 days. transmittedto polar-orbiting satellites, although between Figures 2a and 2b give the daily averagesnow depth gauge 15% and 20% of the measurementsare intermittently lost derived accumulationduring the study period at Willie Field. becausea satelliteis not in positionto receivethe broadcast. These data show both large positive and negative Data are collected through the Argos Data Collection and accumulations on short timescales with a substantial long- LocationSystem (ARGOS) and processed by SpaceScience and term positiveaccumulation. Positivechanges are associated EngineeringCenter, University of Wisconsin-Madison with precipitation, wind blown transportof snow into the [Stearnsand Wentiler, 1988]. Sinceboth AWS sitesare target area, and/or the formationof a surfacefeature on the locatedwhere annual snow accumulationis approximately0.5 target area. Negative changescan resultfrom wind erosion, m yr-l, theheight of thewind speed measurement at both sites sublimation, gravitational forces which compact the snow, decreasedfrom --3 to -2 m during the studyperiod. The two and metamorphicprocesses of the snow crystals[Colbeck, studysites have characteristicallydifferent wind regimes, 1983]. Figure 2a correspondsto the period before the first althoughboth sites experience katabatic winds
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