Ninth International Conference on Permafrost ... - IARC Research

The Effect of Spatially Distributed Snow Cover on Soil Temperatures:A Field and Modeling StudyIntroductionAnna LiljedahlUniversity of Alaska Fairbanks, USALarry HinzmanUniversity of Alaska Fairbanks, USASergei MarchenkoUniversity of Alaska Fairbanks, USASvetlana BerezovskayaUniversity of Alaska Fairbanks, USADue to its insulative properties, snow cover has beenshown to play a significant role in the 20 th century warmingof permafrost (Osterkamp 2007, Stieglitz et al. 2003) andthermokarst formation (Osterkamp 2007). Although theeffects of snow cover characteristics on soil temperatureshave been studied by others (Ling & Zhang 2003, Osterkamp2007, Overduin et al. 2007, Stieglitz et al. 2003), small-scalegeographical variations have been given limited attention.The combination of snow redistribution, hydraulic gradientsinduced by polygonal features, nutrient limited vegetation,soil mineralization, and respiration impaired by the cold soilscalls for a fine-scale analysis to understand the interactionsamong the Arctic systems and how they would respond toany changes in air temperature and precipitation.We examine the effects of snow depth and soil thermalproperties on micro-scale (>0.6 m horizontal, >0.01 m vertical)soil temperature distribution through field measurementsand computer modeling across a single polygon.Study siteThe study site is located inside the boundaries of theBarrow Environmental Observatory (BEO) close to Barrowon the North Slope of Alaska (Fig. 1). The BEO hosts anintensively monitored area called the Biocomplexity site(71°16′47ʺN, 156°36′2ʺW), which is a 1 km 2 watershedcontaining a drained lake basin and low- and high-centeredpolygons. Sedges and sphagnum moss represent the majorvegetation at the Biocomplexity site.The mean annual air temperature in Barrow during theyears 1973–2006 was -12°C with 32% of mean daily airtemperatures colder than -20°C. The latter ranged from-45°C to 16°C yr 1973–2006. Adjusted precipitation (Yanget al. 1998) averaged 168 mm/yr with 72 mm falling June–August (1973–2006). Summer 2007 was a warm summer(5.4°C compared to 3.3°C yr 1973–2007) with a low amountof precipitation (24 mm), resulting in unusually dry soilsprior to freezing. The Circumpolar Active Layer Monitoringsite (Brown et al. 2000), located at the northern end of theBEO, experienced a mean active layer depth of 35 cm yr1995–2007. The area of focus for this study is a low-centeredpolygon represented by relatively high (0.5 m) and wide (5m) rims.Figure 1. The location of the Biocomplexity site inside theboundaries of the Barrow Environmental Observatory. Originalimage courtesy of K.M. Hinkel.MethodsWe combined distributed modeling and field observationsof snow depth and soil temperatures, focusing on the activelayer thermal regime. Output from a snow distributionmodel was used as input to a soil thermal regime modelin both daily and hourly time steps. SnowTran-3D (Listonet al. 2007) provided wind-driven snow depth evolutionover topographically variable terrain. Required modelinputs include topography, vegetation, and weather data(precipitation, air temperature, humidity, wind speed anddirection). In order to simulate the spatially distributedtemperatures of permafrost and active layer thickness forthe investigated area, both equilibrium and transient modelshave been applied. The equilibrium model is a spatiallydistributed permafrost model based on an approximateanalytical solution of soil freezing and thawing, whichincludes an estimation of thermal offset due to the differenceof frozen and thawed soil thermal properties (Romanovsky &Osterkamp 1995). The numerical (transient) model simulatessoil temperature dynamics and depth of seasonal freezingand thawing by solving nonlinear heat equation with a phasechange. In this model, the process of soil freezing/thawingoccurs in accordance with the unfrozen water content curveand soil thermal properties, which are specific for each soillayer (Marchenko et al. 2008). The model requires input of183