Ninth International Conference on Permafrost ... - IARC Research

The Importance of Snow Cover Evolution in Rock Glacier Temperature ModelingMatteo DallAmicoDepartment of Civil and Environmental Engineering, University of Trento, ItalyStefano EndrizziDepartment of Civil and Environmental Engineering, University of Trento, ItalyRiccardo RigonDepartment of Civil and Environmental Engineering, University of Trento, ItalyStephan GruberDepartment of Physical Geography, University of Zurich, SwitzerlandIntroductionThe snow cover evolution is one of the crucial factorsaffecting the thermal and hydraulic regime of rock glaciers(Mittaz et al. 2000), as snow strongly controls soil energybalance through its high albedo and insulating properties.Therefore, accurate modeling of the snowpack is absolutelynecessary to reliably describe soil temperatures. Theimportance of accurate snow modeling entails the use ofsophisticated models based on the solution of the snow energybalance and, consequently, on a good parameterization ofradiation and turbulent fluxes (e.g., Jordan 1991). An advanceor delay in estimating the time of snow disappearance wouldcause a strong error in the calculation of the energy balanceat the soil surface, altering the ground heating or freezingand, therefore, affecting the soil temperature profile for thewhole summer.The goal of this work is to simulate and discuss the rockglacier snow evolution in order to analyze the influenceof the snow cover and accumulation/melting time on thetemperature regime of the active layer of a rock glacier.Modeling Features and Case StudyThe model used in the simulation is GEOtop (Rigon etal. 2006), a distributed physically-based model which jointlysolves the energy and water balance of soil (Bertoldi et al.2006) and snow (Zanotti et al. 2004), and accounts for thegeotechnical parameters of unsaturated soils affecting slopestability (Simoni et al. 2007). The model has been improvedrecently to include a correct treatment of frozen soil (Endrizziet al. 2008) and to model snow with a multilayer schemecapable of describing snow metamorphism and watercirculation and refreezing in the snowpack (Endrizzi 2007).Commonly, in alpine climates the soil exchanges heatdirectly with the atmosphere only in a short time window,roughly spanning from June to October, whereas duringwinter and early spring, heat transfer between soil and atmosphereis mediated by the snowpack. Consequently, the heatflux reaching the soil surface is strongly reduced due to highsnow albedo, which reduces net energy input, and to snowinsulating properties, which cause heat conduction to be verysmall below the upper snow layers. In fact, the snow energybalance equation can be written as follows (Oke 1990):∆Q S+∆Q M= R n+ P − H − L − G [W/m 2 ] (1)where the terms in the left-hand side (LHS) represent the heatstorage rate in the snowpack due to sensible heat (∆SQ S) andto latent heat (∆Q M, melting/refreezing and rain on snow). Inthe right-hand side (RHS), R nis the net all-wave radiation,P is the sensible heat flux supplied by precipitation, Hand L are, respectively, the sensible and latent heat fluxesexchanged between the surface (be it snow or soil) and theatmosphere, and G is the heat flux reaching the soil surfaceacting as soil energy input. When the ground is snow-free,the LHS in equation (1) is null, and G is equal to the net∆Q energy S+∆Q flux M exchanged = R n+ Pwith − Hthe − Latmosphere. − G [W/m On 2 ] the other (1)hand, for snow covered ground, G is proportional to thetemperature gradient at the snow-soil interface, namely:G sn=−KT sn− T S1(D + D ) [W/m 2 ] (2)2 sn Swhere K is the snow-soil averaged thermal conductivitycalculated as a harmonic mean, T snis snow temperaturein the layer close to the soil surface, T Sis the soil surfacetemperature, and D snand D Sare the depths of the snow andsurface layer, respectively.Investigated siteSimulations have been carried out on the active rockglacier Murtèl (Upper Engadin, Swiss Alps: 46°26′N,9°49.5′E, 2670 m a.s.l., 15° slope with NW aspect) in whichthe oldest temperature time series of Alpine Permafrost hasbeen measured (Vonder Mühll & Haeberli 1990, Hoelzle etal. 1999). Input data include incoming shortwave radiation(both direct and diffuse), incoming longwave radiation, airtemperature, wind speed and direction, air pressure, andprecipitation.Simulations and ResultsThe simulation spans a period of two hydrological yearsbeginning from October 1997. As the first snowfall normallyoccurs in November, this choice allows the avoidance of theproblem of determining the initial condition of snow on thesurface. Most of the parameters used by the snow model ofGEOtop were simply taken from literature, for example,snow reflectance and snow thermal and hydraulic properties.As only total precipitation was available, the calibration wasreduced to the definition of the threshold air temperaturesabove (below), where precipitation is considered to occur asrain (snow).57G sn=−K1T sn− T S[W/m 2 ] (2)