Ninth International Conference on Permafrost ... - IARC Research

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Long-Term Monitoring of Sensible and Latent Heat Fluxes Using Eddy Covarianceat a High Arctic Permafrost Site in Svalbard, NorwayS. WestermannAlfred-Wegener-Institute for Polar and Marine Research, Potsdam, GermanyJ. BoikeAlfred-Wegener-Institute for Polar and Marine Research, Potsdam, GermanyK. PielAlfred-Wegener-Institute for Polar and Marine Research, Potsdam, GermanyJ. LüersUniv. of Bayreuth, Germany, Dept. of MicrometeorologyIntroductionLand-atmosphere interactions are an important elementin the energy and water budgets in permafrost regions. Theeddy covariance method has proven to be the most reliableway to directly measure sensible and latent heat fluxes(Foken 2006). However, due to the difficult logistics and theextreme environment, very few long-term eddy covariancestudies exist in arctic regions (Grachev et al. 2007). Previousmeasurements on Svalbard, Norway, were limited to thesummer season (Lloyd et al. 2001). Here we present longtermeddy covariance measurements of sensible and latentheat fluxes at a High Arctic continuous permafrost site onSvalbard.MethodsThe eddy covariance measurements were performed nearLeirhaugen hill, located approximately 2 km southwest ofthe village of Ny-Ålesund. The site is situated in hilly tundraat the foot of two major glaciers, and is characterized bysparse vegetation alternating with exposed soil and rockfields.The eddy covariance system consisted of a Campbell CSAT3D sonic anemometer and a LiCor LI-7500 CO 2and H 2Ogas analyzer, which were sampled at 20 Hz using a CR3000Campbell Scientific datalogger. The evaluation of the rawdata was performed with the software “TK2” (Mauder &Foken 2004) from the University of Bayreuth, Germany. Thequality assessment scheme of Foken & Wichura 1996 (seealso Foken 2006), which is based on tests for stationarityand integral turbulence characteristics, was used to assessthe quality of the flux measurements.Measurements were collected from April to September2007, covering a period from late winter until the end of thesummer season. To account for the changing height aboveground of the flux sensors due to accumulation or meltingof snow, the snow depth directly at the EC-site was recordedusing a Campbell Scientific SR50 distance sensor. Themeasured heights above ground ranged from a minimum of2.0 m to 3.2 m at the end of the snow ablation period. Aftera complete snow melt, the EC-instruments were lowered toa height of 2.5 m above ground.The net radiation was recorded at a climate station in thevicinity of the eddy covariance site, so that it is possible tocompare the magnitude of the sensible and latent heat fluxeswith the radiation balance.ResultsDuring the entire snow-covered period, either a stable or aneutral near surface atmospheric stratification was recorded,corresponding to z/L (measurement height over Obukhovlength) significantly greater than zero or approx. zero,respectively. Hereby, a stable stratification was associatedwith low horizontal wind speeds of less than 5 m/s, whilea neutral stratification was found predominantly for higherwind speeds. Particularly at stable conditions, the use ofthe eddy covariance method, which depends upon a fullydeveloped turbulence field, is questionable. This was alsoreflected applying the Foken & Wichura quality assessment:a significant part of the data measured during the snowcoveredperiod was classified as “only for orientationpurposes” or “to be discarded” both for the sensible and latentheat flux. The data, which withstood the quality assessment,typically yielded low fluxes of less than 20 W m − ². Hereby,the sensible heat flux was usually negative, corresponding toa sensible flux directed from the atmosphere to the ground(longwave radiation forcing), while the latent heat flux waspositive, corresponding to weak but still existing sublimationand/or evaporation processes of snow or melt water.The appearance of large snow-free patches around June26 triggered a strong increase of both sensible and latent heatfluxes, with now both fluxes being positive, correspondingto a warming of the tundra surface forced by shortwaveradiation. During this period the latent heat flux, with amaximum of 90 W m − ², was more than twice as large asthe sensible heat flux, likely due to very wet soil conditionsdirectly after snowmelt. This situation reversed during July,when the tundra increasingly dried up throughout most ofthe potential fetch area of the eddy covariance site. Aroundthe middle of July, both heat fluxes were approximatelyequal, the sum of both peaked at values of more than 200W m − ². Towards the end of July, the sensible heat fluxsubsequently became dominant over the latent heat flux byapproximately a factor of two. The immediate surroundingof the measurement site could then be characterized asmoderately damp tundra. From mid of August onwards, both341
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tfluxes decreased steadily. At this time, the latent heat fluxwith peak values around 50 W m − ² was found to dominateonce again over the sensible heat flux.During the polar day season, the sensible and latent heatflux displayed a strong diurnal course with peak fluxesassociated with maxima of solar radiation around midday. Atthe lowest sun angles, around midnight, both fluxes usuallydecreased to close to zero, but remained positive.From the completion of snowmelt through the middle ofAugust, the atmospheric stratification (according to the z/Lratio) was found to be either unstable or neutral, resultingin a good data-quality assessment. Towards and after theend (approximately middle of August to September) of thepolar day season the general pattern could be characterizedas neutral to weak unstable atmospheric stratification duringthe day and stable atmospheric stratification during the night.The quality assessment still indicated a good data qualityduring the day, with an increasingly poor data quality duringthe night.Lloyed, C.R., Harding, R.J., Friborg, T. & Aurela, M. 2001.Surface fluxes of heat and water vapour from sites inthe European Arctic. Theor. Appl. Climatol. 70: 19-33.Mauder, M. & Foken T. 2004. Documentation and instructionmanual of the eddy covariance software packageTK2. Work Report University of Bayreuth, Dept. ofMicrometeorology Vol. 26, ISSN 1614-8916.DiscussionThe highest amount of the net radiation was observedaround beginning of July to be around 300 W m − ²,corresponding with a total of sensible and latent heat fluxesreaching values around 200 W m − ². This clearly shows theimportance of the sensible and latent heat fluxes regardingtheir parts in the whole energy budget of permafrost soilsaround Ny-Ålesund. Thus, eddy covariance measurementsmust be regarded as an essential tool in obtaining a completepicture of the energy budget and the allocation of theavailable energy during the summer period.At snow-covered times, that is, for approximately twothirdsof a year, the situation is yet more difficult to assess.On the one hand, the quality of a significant portion of thedata is questionable according to quality assessment (Foken& Wichura 1996), and only low fluxes were observed. On theother hand, such quality assessment schemes were developedin and for temperate zones, and are therefore not necessarilywell suited for conditions found in the Arctic. Furthermore,low but sustained sensible and latent heat fluxes might haveto be taken into account in the energy budget of the snowcoveredground. It is of great importance to critically reviewand possibly modify the evaluation and quality assessmentfor eddy covariance data for these circumstances.ReferencesGrachev, A.A., Andreas, E.L., Fairall, C.W., Guest, P.S.& Persson, P.O.G. 2007. SHEBA flux–profilerelationships in the stable atmospheric boundarylayer. Boundary Layer Meteorol. 124: 315-333.Foken, T. 2006. Angewandte Meteorologie,Mikrometeorologische Methoden, (2 and extendededition). Heidelberg: Springer, 326 pp.Foken, T. & Wichura, B. 1996. Tools for quality assessmentof surface-based flux measurements. Agric ForestMeteorol. 78: 83-105.342
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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