LandSurface Modeling

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Land-Surface Modeling

• Simple Biosphere Model (SiB)
• Community Land Model (CLM)
• Scott Denning, CSU

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LSP for NWP Models

• Exchanges between atmosphere and surface of
• Heat
• Water
• Radiation
• Momentum
• Biophysical consistency
• Scalability (grid independence?)
• Computational efficiency

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LSP and Climate Models

• Biophysics as for NWP, plus
• Biogeochemistry, particularly as it affects
  atmospheric CO2
• Treatment of human land use (e.g., agriculture)
  and land-use change
• Vegetation distribution changes over time with
  climate

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Carbon and Climate Futures?
Given nearly identical human emissions, different
models project dramatically different futures.
Carbon cycle feedbacks are among the largest
sources of uncertainty for future climate.
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First Generation Biophysics (c. 1984)

• Storage of water and heat in simple reservoirs
• Independent specification (maps) of surface
  properties like roughness and albedo

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Leaf Anatomy
Stomate (pl. stomata)
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Second Generation Biophysics (c. 1990)

• e.g., BATS, SiB
• Circuit analogy for fluxes
• Biophysical consistency, with surface properties
  based on veg type

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Stomatal Conductance, c. 1990

• Maximum conductance scaled down by
  empirically-derived factors
• Assumed independence of limitations

BATS, Dickinson et al, 1986
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SiB2 Vegetation and Soil Structure

• Single vegetation canopy layer
• Three soil layers
• Sfc layer evaporates directly into air
• Root layer transpires
• Recharge layer drains and provides capillary
  source to root zone

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SiB2 Heat Fluxes

• Three prognostic temperatures (m, c, g)
• 3 resistors
• Ta is diagnostic (weighted mean of the others)

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SiB2 Water Fluxes

• Only em and W2 are predicted
• 4 resistors
• Temp-dependent saturation e of canopy and ground
  is diagnosed

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Carbon and Water

• Plants eat CO2 for a living
• They open their stomata to let CO2 in
• Water gets out as an (unfortunate?) consequence
• For every CO2 molecule fixed about 400 H2O
  molecules are lost

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SiB2 carbon fluxes

• Assimilation by photosynthesis
• Carbon efflux due to respiration and
  decomposition
• Only Cm is predicted
• Other Cs are diagnosed

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Third generation Ecophysiology(c. 2000)

• Prediction of photosynthesis and respiration
• Linkage of photosynthesis, stomatal function,
  transpiration

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Photosynthesis and Conductance
Stomatal conductance is linearly related to
photosynthesis
(The Ball-Berry-Collatz parameterization)
RH at leaf sfc
photosynthesis
stomatalconductance
CO2 at leaf sfc
Photosynthesis is controlled by three
limitations(The Farquahar-Berry model)
Enzyme kinetics(rubisco)
Light
Starch
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Farquhar Photosynthesis Model
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Farquhar Model (contd)
The Dark Reactions of photosynthesis, aka the
Calvin Cycle

• Reducing CO2 to organic matter in an oxidizing
  atmosphere requires energy (ATP) and reductant
  (NADPH2)

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Leaf Physiology in SiB2

• Heat, water, and carbon fluxes are coupled by
  physiology
• Scaling to canopy and landscape fluxes based on
  resource allocation

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Spatial Extrapolation with Models and Imagery
Applied at multiple spatial scales
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SiB2 Parameters

• Leaf area index is computed from AVHRR imagery
  (red and NIR reflectance)
• Spatial, seasonal, and interannual variability
  are captured
• Now available globally at 8 km since 1981

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SiB2 Parameters

• Soil water potential at saturation
• Derived from textural database
• Available at 1 km in US, 10 km globally

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Community Land Model (CLM2) in CCSM

• SiB-like photosynthesis and transpiration
• 10-layer soil with variable root uptake
• 0-5 layer snowpack accomodates strong temperature
  gradients (insulation)
• Subgrid-scale tiling of plant functional types
• Fractional coverage by evergreen trees, deciduous
  trees, grass, crops, bare ground, etc.

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Simple Biosphere Model, v3

• New Features
• Prognostic T, e, CO2, other tracers in canopy
  air space
• 10-layer soil (T and w), with adjustable water
  extraction profile (roots)
• Snowpack of 0-5 layers
• Mixed canopy physiology (e.g., savanna)
• Stable isotope fractionation of CO2

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Prognostic Canopy Air Space
temp
turbulence
canopy
ground
snow
vapor
CO2
respiration
turbulence
photosynthesis
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Prognostic CAS (contd)
heat
water

• Solution method is fully implicit
• Efficient matrix solver for 4 canopy layers, 0-5
  snow layers, and 10 soil layers

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Stable Isotope Fractionation in SiB3
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Local Testing at Flux Tower
CO2
Water
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Continuous Vegetation Fields(DeFries et al, 1999)

• Subgrid fractional area covered by trees, grass,
  bare soil at 1 km derived from AVHRR timeseries
• Woody fraction subdivided into deciduous/evergreen
  and broadleaf/needleleaf

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LS Parameters from RS Imagery
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Succession Following Disturbance
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CLM3 in CCSM (by late 2003)

• New architecture to accommodate dynamic
  vegetation with competition, disturbance,
  recovery
• CSU to port prognostic SiB features
• Prognostic CAS
• Stable isotopes
• Options to drive from imagery
• Full belowground biogeochemistry
• Nutrient limitations
• Tracking of decomposing biomass and isotopes

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New Subgrid Data Structure CLM3
Each PFT or mix is treated as an instance of the
biophysics
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