Landatmosphere coupling strength in a GCM

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Land-atmosphere coupling strength in a GCM

• David Lawrence1,
• Chris Taylor2, and Julia Slingo1

1 Centre for Global Atmospheric
Modelling University of Reading Reading, UK 2
Centre for Ecology and Hydrology Wallingford, UK
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Land-atmosphere interaction

• Land-atmosphere coupling strength
• The extent to which a precipitation-induced
  soil moisture anomaly influences the overlying
  atmosphere and thereby subsequent precipitation
  events.
• The coupling can be due to a variety of
  mechanisms
• local recycling
• modification to the large-scale circulation
• development of the boundary layer
• wet soil (low Bowen ratio) ? shallower and more
  moist boundary layer and a lower level of free
  convection increase potential for convective
  activity

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Land-atmosphere coupling strength diagnostic

• W(rite) - 16-member ensemble forced with June
  1 initial conditions from 16-year control run
  with climatological SSTs
• R(ead) - 16-member ensemble where surface
  values (soil moisture and temperature, canopy
  water content, and surface temperature) are
  discarded and replaced at every timestep with
  values from one of the W members

O measure of time series similarity between
ensemble members
R - SST and land BCs
W - SST
OP(W) 0.1
OP(R) 0.85
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Impact of land BCs on 3-day precipitation OP(R)
OP(W) Koster et al. (2002)
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Land-atmosphere coupling strength in HadAM3/MOSES2
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Composite diurnal cycle for wet () and dry ()
top soil layer (Eastern US)
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Summary

• HadAM3/MOSES2 exhibits relatively weak
  land-atmosphere coupling compared to other GCMs
• Standard