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Features and performance of the NCAR Community Land Model (CLM): Permafrost, snow, and hydrology David Lawrence NCAR / CGD Boulder, CO
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Components of CLM T2,T2, T1,T1, T3,T3, T 10, Photosynthesis model Soil thermodynamic and hydrology model … Snow Model Calculates the surface energy, water, momentum, (and carbon) fluxes River Transport Model
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Community Land Model Subgrid Structure Gridcell GlacierWetlandLake Landunits Columns Plant Functional Types UrbanVegetated Soil Type 1 15 total PFTs
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Snow model 5-layer model Treats processes such as accumulation, melt, compaction, water transfer across layers, snow aging State variables are mass of liquid and ice water, snow temperature, and layer thickness Near-term improvements: –Vertical distribution of solar heating –Improved snow cover fraction and snow burial fraction –Improved snow aging –Aerosol deposition
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Soil hydrology No perpetually frozen layer At least one perpetually frozen soil layer Vertical redistribution - Richard’s equation TOPMODEL-based surface runoff Coexistence of liquid and ice water at T < 0 o C Dynamic water table position controls sub- surface runoff Groundwater model (unconfined aquifer) soil depth (m)
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Soil thermodynamics Solution of heat diffusion equation Thermal properties (conductivity, heat capacity) function of soil texture (sand, silt, clay) and soil liquid and ice water contents Recent improvements –Organic soil –Deeper soil column
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CLM soil carbon density dataset Source data from Global Soil Data Task Kg m -2 Lawrence and Slater, 2007 Calculate thermal and hydraulic soil parameters as a weighted combination of organic and mineral soil values
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) + f sc,i Θ sat,sc (1 - f sc,i ) ( Θ sat,i = 0.489 − 0.00126 %sand i Thermal and hydraulic parameters for organic soil Soil typeλ sat λ dry Θ sat k sat Sand3.120.270.370.023 Clay1.780.200.460.002 Peat0.550.05 a 0.9 a,b 0.100 b a Farouki (1981), b Letts et al. (2000) λ sat sat. thermal conductivity λ dry dry thermal conductivity Θ sat volumetric water at saturation k sat sat. hydraulic conductivity f sc,I = ρ sc,i / ρ peat fraction of layer i that is organic matter
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Impact on soil temperature (SOILCARB – CONTROL)
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Deeper soil column Motivation: account for thermal inertia of deep soil layers Solution: add additional layers, hydrologically inactive 3.5m 10 levs ~50-125m 15-17 levs Ignore geothermal heat flux (up to ~0.1 W m -2 )
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Annual cycle-depth soil temperature plots Siberia SOILCARB + 50m Soil Column
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Near-surface permafrost extent and active layer thickness (1970-1990) 8.5 million km 2 10.7 million km 2 11.2 to13.5 million km 2 observed Continuous + discontinuous SOIL CARBON + DEEP SOIL
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Other features of CLM Other Features –Terrestrial carbon-nitrogen cycle model –Dynamic global vegetation model –Can be run at resolution independent of atmos resolution –(Greenland ice sheet model) –Broad community participation in model development (contributions from CCSM Land and Biogeochemistry Working Groups)
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Current limitations of CLM specific to Arctic processes Limitations –No excess soil ice –No thermokarst, subsidence process –Static wetlands, lakes –Dynamic vegetation does not include shrubs –No methane emission model –Sporadic, isolated permafrost not explicitly represented
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Summary: Towards simulating Arctic terrestrial high-latitude feedbacks in CCSM Microbial activity increases Enhanced [nitrogen] Shrub growth CO 2 efflux Lakes drain, soil dries Global warming CH 4 efflux Expanded wetlands Carbon sequester Permafrost warms and thaws Arcticwarming
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Summary: Towards simulating Arctic terrestrial feedbacks in an Earth System Model Microbial activity increases Enhanced [nitrogen] Shrub growth CO 2 efflux Lakes drain, soil dries Global warming CH 4 efflux Expanded wetlands Carbon sequester Permafrost warms and thaws Arcticwarming TOPMODEL based approach permits dynamic wetlands? Thermokarst? CLM-CN: simulates large Arctic soil carbon store CLM-DGVM: currently no shrub type Organic soil and deep soil column improve permafrost simulation Large-scale degradation of near- surface permafrost likely Wetland-CH 4 emission model Peatland soil type and/or moss vegetation type?
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Near-surface permafrost degradation Lawrence and Slater, 2007b Observed estimate continuous + discontinuous
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Deep permafrost (10-30m) Most deep permafrost still exists at the end of the 21 st century
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Soil carbon in CLM-Carbon/nitrogen (CLM-CN) Soil carbon content: CLM-CNSoil carbon content: Obs
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Thermal and hydraulic parameters for organic soil Soil typeλ s W m -1 K -1 λ sat W m -1 K -1 λ dry W m -1 K -1 c s J m -3 K -1 x10 6 Θ sat k sat m s -1 x10 -3 Ψ sat mm b Sand 8.613.120.272.140.370.023-47.33.4 Clay 4.541.780.202.310.460.002-633.012.1 Peat 0.25 a 0.550.05 a 2.5 a 0.9 a,b 0.100 b -10.3 b 2.7 b a Farouki (1981), b Letts et al. (2000) λ s soil solid thermal conductivity λ sat saturated thermal conductivity λ dry dry thermal conducitivity c s heat capacity Θ sat volumetric water at saturation k sat saturated hydraulic conductivity Ψ sat saturated matric potential b Clapp and Hornberger parameter
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Impacts on climate CAM-CLM CLM
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CCSM3 projections of degradation of near-surface permafrost We define near-surface permafrost as perpetually frozen soil in upper 3.5m of soil Lawrence and Slater, 2005
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