Land Model Requirements from the Atmospheric Model

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Land Model Requirements from the Atmospheric Model

• Robert Dickinson
• Georgia Institute of Technology

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Mechanisms of Atmosphere Impacting Land

• Removes moisture from surface (or adds dry air
  from above) and so amplifies ET (e.g. BL
  we acts as a conductance)
• Through clouds, reduces and diffuses solar
  radiation, increases downward LW.
• Through precipitation, delivers water.
• Through its distribution of imbalanced pressure
  forces in the BL, delivers momentum (u) to the
  surface

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Mechanisms of Land Impacting Atmosphere

• Through ET delivers water to fuel precipitation.
• Through its sensible fluxes generates boundary
  layer buoyancy and gradients these drive
  convection systems on various time scales
• Through its LW, heats the atmosphere

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Current Scaling Mismatch

• Land model is necessarily formulated as a single
  point column model thus its inputs are needed
  on a smaller spatial scale than delivered by the
  atmospheric model
• Land is very heterogeneous all the boundary
  condition data needed for modeling it are
  available on 1-km or finer.
• None of the current CAM formulations provide the
  scaling linkages needed.

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Some things are being done but they are
unsatisfactory

• Running 10 (or more) mosaic element of nearly
  equal area to do pdf integrations
• Convective precipitation distributed among these
  elements through assumption of expected intensity
  and exponential pdfs to randomly determine the
  mosaics with rain and its intensity
• Try to correct for correlations between rain
  areas and surface radiation received

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10 PFTs Land Grid

• Current CLM formulation allows up to 4 PFTs in a
  grid and the entire grid could be filled with the
  same land cover type.
• The new approach divides grid into 10 PFTs.

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Some things obviously important cant currently
be done

• Couple spatial subgrid heterogeneity and
  consequences for buoyancy generation to
  precipitation and cloud amounts, fractions,
  spaces scales, cloud convective velocities (e.g.
  to determine drop size distributions)
• Why? There is no spatial scaling information in
  the current parameterizations for BL, clouds, and
  moist convection, beyond an estimate of the BL
  height

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Conclusion What does the land modeling need?

• Atmospheric subgrid parameterizations for BL
  convection and clouds and deep convective
  precipitation that maintain a description of the
  horizontal spatial scales involved, and
  preferably taking input from the land model.
• Better diagnostics for quantities most affecting
  the land model such as a climatology of effective
  entrainment velocities