Features and performance of the

NCAR Community Land Model (CLM):

Permafrost, snow, and hydrology

David Lawrence

NCAR / CGD Boulder, CO

Components of CLM

T2, T1,

T3,

T10,

Photosynthesis model

Soil thermodynami

c and hydrology

model…

Snow Model

Calculates the surface energy, water, momentum, (and carbon) fluxes

RiverTransportModel

Community Land Model Subgrid Structure

Gridcell

Glacier Wetland Lake

Landunits

Columns

Plant Functional Types

UrbanVegetated

Soil Type 1

15 total PFTs

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

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 < 0oC

• Dynamic water table position controls sub-surface runoff

• Groundwater model (unconfined aquifer)

so

il d

ep

th (

m)

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

CLM soil carbon density datasetSource 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

) + fsc,i Θsat,sc

(1 - fsc,i) (

Θsat,i = 0.489 − 0.00126 %sandi

Thermal and hydraulic parameters for organic soil

Soil type λsat λdry Θsat ksat

Sand 3.12 0.27 0.37 0.023

Clay 1.78 0.20 0.46 0.002

Peat 0.55 0.05a 0.9a,b 0.100b

a Farouki (1981), b Letts et al. (2000)

λsat sat. thermal conductivity

λdry dry thermal conductivity

Θsat volumetric water at saturation

ksat sat. hydraulic conductivity

fsc,I = ρsc,i / ρpeat fraction of layer i that is organic matter

Impact on soil temperature (SOILCARB – CONTROL)

Deeper soil column

Motivation:

account for thermal inertia of deep soil layers

Solution:

add additional layers, hydrologically inactive

3.5m10 levs

~50-125m15-17 levs

Ignore geothermal heat flux (up to ~0.1 W m-2)

Annual cycle-depth soil temperature plotsSiberia

SOILCARB + 50m Soil Column

Near-surface permafrost extent and active layer thickness (1970-1990)

8.5 million km2 10.7 million km2

11.2 to13.5 million km2 observedContinuous + discontinuous

SOIL CARBON + DEEP SOIL

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)

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

Summary: Towards simulating Arctic terrestrial high-latitude feedbacks in CCSM

Microbial activity

increases

Enhanced[nitrogen]

Shrubgrowth

CO2

efflux

Lakes drain, soil dries

Globalwarming

CH4

efflux

Expandedwetlands

Carbonsequester

Permafrost warms and

thaws

ArcticArcticwarmingwarming

Summary: Towards simulating Arctic terrestrial feedbacks in an Earth System Model

Microbial activity

increases

Enhanced[nitrogen]

Shrubgrowth

CO2

efflux

Lakes drain, soil dries

Globalwarming

CH4

efflux

Expandedwetlands

Carbonsequester

Permafrost warms and

thaws

ArcticArcticwarmingwarming

TOPMODEL basedapproach 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-CH4 emission model

Peatland soil type and/or moss vegetation type?

Near-surface permafrost degradation

Lawrence and Slater, 2007b

Observed estimatecontinuous + discontinuous

Deep permafrost (10-30m)

Most deep

permafrost still

exists at the end of

the 21st century

Soil carbon in CLM-Carbon/nitrogen (CLM-CN)

Soil carbon content: CLM-CNSoil carbon content: Obs

Thermal and hydraulic parameters for organic soil

Soil type λs

W m-1 K-1

λsat

W m-1 K-1

λdry

W m-1 K-1

cs

J m-3 K-1x106

Θsat ksat

m s-1x10-3

Ψsat

mm

b

Sand 8.61 3.12 0.27 2.14 0.37 0.023 -47.3 3.4

Clay 4.54 1.78 0.20 2.31 0.46 0.002 -633.0 12.1

Peat 0.25a 0.55 0.05a 2.5a 0.9a,b 0.100b -10.3b 2.7b

a Farouki (1981), b Letts et al. (2000)

λs soil solid thermal conductivityλsat saturated thermal conductivityλdry dry thermal conducitivitycs heat capacity

Θsat volumetric water at saturationksat saturated hydraulic conductivityΨsat saturated matric potentialb Clapp and Hornberger parameter

Impacts on climate

CAM-CLM

CLM

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