Ninth International Conference on Permafrost ... - IARC Research

Impacts of Small-Scale Surface Variations on the Energy Balance of PolygonalTundra on Samoylov Island, Lena River Delta, SiberiaM. LangerAlfred 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, GermanyG. StoofAlfred Wegener Institute for Polar and Marine Research, Potsdam, GermanyIntroductionTo quantify the magnitude of energy balance variations ofa typical polygonal tundra system, two typical polygons atdifferent succession stages were chosen for study. At eachpolygon site, surface characteristics were analyzed underthe aspect of energy exchange processes. The focus restsupon the energy transfer processes due to the emission andabsorption of radiation. The study is complemented throughthe analysis of soil heat fluxes and the correspondingthermal ground characteristics, such as soil heat capacityand conductivity.Site DescriptionThe investigated polygons have the followingcharacteristics: The center of the first polygon consists ofwater-supersaturated peat. The groundwater level is at or justabove the soil surface. The first polygon rim is elevated 10cm above the water table. The uppermost soil layers consistof peat with high air content. The soil surface is covered bydifferent moss types associated with dry conditions.The polygonal crack is 10 cm wide and extends downto the ice wedge. The crack is filled with a combination ofwater and organic material of hydrophilic mosses.The second polygonal rim consists of an elevated region anda lower transition zone towards the second polygon center.The soil in this elevated region still holds a considerable airfraction, but has an increased mineral fraction compared tothe first polygonal rim. The transition zone shows increasingsoil moisture toward the second polygon center. The secondpolygon center consists of water-saturated peat with thewater table just underneath the ground surface. The secondpolygonal center is dominated by different hydrophilicmosses.MethodsFor detecting variations of the radiation balance, theinvestigated polygons were instrumented with a radiationscanner system bridging a 10 m transect. The scanner iscapable of measuring reflected and emitted shortwave andlongwave radiation at high spatial and temporal resolutionsof 20 cm and 30 min, respectively.The system consists of a photo pyranometer and an infraredsensor that were fixed on the scanner wagon 70 cm abovethe ground surface. An air temperature and relative humidityprobe, as well as a photosynthetic active radiation sensor,were attached adjacent to the main sensors. Meteorologicalvariables, such as incoming radiation, were measured at aclimate tower located 5 m from the scanner system.Soil heat fluxes were calculated based on temperature andmoisture measurements directly beneath the soil surface ateight different locations along the scanner transect. Two-pointtemperature measurements were used for thermal diffusivitycalculations based on a one-dimensional numerical solutionof the heat transfer equation. Soil heat capacities wereapproximated based upon literature values of different soiltypes and soil moisture measurements.ResultsThe scanner system directly provides surface temperaturesand reflected solar radiation fluxes. By estimating surfaceemissivities, the surface temperatures can be transformedinto emitted longwave radiation fluxes. The consistency ofthe data was controlled by comparing the spatial mean of theemitted and reflected radiation to the radiation measurementsat the climate tower.The scanner system was operated during periods ofdifferent weather conditions. The surface temperaturesalways varied significantly along the profile. At periods ofhigh solar angles the surface temperatures of the dry rims aregenerally increased compared to the wet locations. Duringthese time spans the polygonal rimes are about 5°C warmerthan the wet polygonal centers. Less evident is the reversesituation that occurs during low sun positions, when thewet locations are slightly warmer than the dry polygonalrims. Observed temperature differences are about 3°C. Thehighest spatial temperature differences were observed duringperiods of clear sky conditions when radiative transfer is ata maximum.The observed albedo values also display spatial variationscorresponding to the different surface structures. Thereflectivity at the rims is about 20% and about 15% at thepolygonal centers. These observations are verified throughspectrometer data. Instrumentation failures such as sensor163