The influence of the diurnal cycle on the soil moisture precipitation feedback

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The influence of the diurnal cycle on the soil
moisture precipitation feedback

• David Lawrence

National Center for Atmospheric Research Boulder,
CO, USA
Chris Taylor CEH, Wallingford, UK Keith Oleson
NCAR, Boulder, CO
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Soil moisture Precipitation feedback
Definition The extent to which a
precipitation-induced soil moisture anomaly
influences the overlying atmosphere and thereby
the evolution of weather and the generation of
precipitation.
Photo by D. Fritz
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Land-atmosphere coupling strength diagnostic
(Koster et al. 2002, 2005)
W(rite) - 16-member ensemble forced with June 1
initial conditions from each year of a 16-year
climatological SST control run. Soil moisture
from W1 experiment recorded at each
timestep. S(oil moisture) - 16-member ensemble
where, at every timestep, simulated soil moisture
is discarded and replaced with values from W1
experiment.
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Global Land-Atmosphere Coupling Experiment (
)
Koster et al. 2005
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Summary of Issues Raised by GLACE

• What is real-world coupling strength?
• What is soil moisture-precipitation feedback
  mechanism?
• Can models reproduce observable relationships
  like Soil Wetness vs Latent Heat Flux or Lifting
  Condensation Level?
• What processes contribute to model differences
  in coupling strength (land surface, boundary
  layer, convection)?
• How does a GCMs inherent coupling strength
  affect its simulations of climate variability,
  climate change, land cover change, etc.?

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Summary of Issues Raised by GLACE

• What is real-world coupling strength?
• What is soil moisture-precipitation feedback
  mechanism?
• Can models reproduce observable relationships
  like Soil Wetness vs Latent Heat Flux or Lifting
  Condensation Level?
• What processes contribute to model differences
  in coupling strength (land surface, boundary
  layer, convection)?
• How does a GCMs inherent coupling strength
  affect its simulations of climate variability,
  climate change, land cover change, etc.?

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Soil moisture Precipitation coupling strength
OP(S) ? OP(W)
Global mean SM P coupling strength
CAM3
HadAM3
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Primary factors influencing soil moisture
precipitation feedback
1. The ability of soil moisture to affect
evaporation
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Soil wetness vs normalized latent heating
(LH / RNET)
ARM Extended Facility Average
LH / RNET
Soil Wetness
Dirmeyer et al. 2006
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Primary factors influencing soil moisture
precipitation feedback
1. The ability of soil moisture to affect
evaporation (HadAM3 CAM3)
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Primary factors influencing soil moisture
precipitation feedback
1. The ability of soil moisture to affect
evaporation (HadAM3 CAM3) 2. The ability of
evaporation (or more generally surface fluxes) to
affect precipitation
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Indirect soil moisture ? precipitation feedback
Theory developed in Betts and Ball (1995), Betts
et al. (1996), Eltahir (1997), and Schär et al.
(1999) supported by observations from FIFE, 1-d
models, and regional climate models.
LH SH SW LW RNET MSE /
m3 BL air
LH SH SW LW RNET MSE /
m3 BL air
wet
, cool, dark soil
dry
, warm, bright soil
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FIFE Diurnal cycle composites by soil moisture
(Betts and Ball 1995)
Evaporative fraction LH / (LHSH)
Boundary layer depth
dry
wet
Do HadAM3 and/or CAM3 capture these diurnal
timescale soil moisture-boundary
layer relationships?
12 LT
wet
dry
12 LT
Net surface radiation
Potential temperature specific humidity (T-q)
wet
dry
dry
wet
12 LT
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Diurnal cycle composites for wet and dry soils
(S-2 expt) HadAM3(Midwest US 35oN, 95oW)
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Diurnal cycle composites for wet and dry soils
(S-2 expt)CAM3(Midwest US 35oN, 95oW)
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Time of local precipitation maximum (JJA)
Observed non-MCS convective precipitation peaks
between 2pm and 3pm for most tropical and
subtropical locations
HadAM3
CAM3
Peaks 1 5 pm
Peaks 9 am - 1 pm
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Diurnal cycle composites JJA(Midwest US 35oN,
95oW)

• HadAM3 (weak coupling) Convection occurs too
  early in day, only allowing a short time for a
  difference in SM to influence boundary layer
  development
• CAM3 (strong coupling) Convection and
  boundary layer schemes are overly sensitive to
  perturbations in surface fluxes

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The soil moisture precipitation feedback A
diagnostic and modelling challenge

• Requires accurate partitioning of net radiation
  into latent and sensible heat fluxes (land
  surface including hydrology and vegetation, and
  PBL schemes)
• Reasonable response of boundary layer
  properties to surface forcing (PBL, convection,
  and cloud schemes)
• Correct relationship between boundary layer
  instability and shallow and deep moist convection
  (PBL, convection, and cloud schemes)
• As always, there is a clear need for more long
  observational time series of soil moisture,
  fluxes, and boundary layer properties in varied
  climates
• Think beyond individual model schemes and
  consider their interaction
• Integrated Treatment
  of Convection

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Coupling strength (?P, ?E) vs daily max ?E,
daily max BLHT, and daily mean LH
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Coupling strength (?P, ?E) vs daily max ?E,
daily max BLHT, and daily mean LH
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Coupling strength (?P, ?E) vs daily max ?E,
daily max BLHT, and daily mean LH
HadAM3
HadAM3
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Soil wetness vs cloud base
GLACE models and ARM sites
(Dirmeyer et al 2005)
GLACE Models
Cloud Base (PSURF-PLCL)
ARM Extended Facility Average
Soil Wetness