Vegetation Phenology and the Hydrological Cycle of Monsoons

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Vegetation Phenology and the Hydrological Cycle
of Monsoons

• David Lawrence
• and Julia Slingo

CGAM University of Reading Reading, UK
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Motivation

• Early West African monsoon onset in HADAM3
• Evaluate sensitivity of HADAM3 and MOSES 2 to
  seasonal cycle of vegetation. Is seasonally
  varying vegetation important?

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Leaf Area Index climatological seasonal
cycle9-yr ISLSCP II dataset
LAI
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Seasonal and interannual variability
One of the most important outstanding aspects of
land-atmosphere interactions is the feedbacks
associated with seasonal and interannual
vegetation cycles (Hutje, 1998).
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Vegetation-atmosphere interaction
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Vegetation phenology into MOSES 2 Leaf Area
Index

• LAI directly or indirectly controls
• canopy height
• surface roughness
• canopy water capacity
• surface albedo

LAI annual cycle (West Africa)
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Vegetation phenology into MOSES 2 Surface albedo

• Updated soil albedo based on CCRS-ERBE albedo
• Included linear dependence of albedo on soil
  moisture

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Surface albedo verification (West African
monsoon region)
Surface albedo
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Model runs

• 25-yr runs of HADAM3 with MOSES 2 (first 5-years
  thrown out for spin-up)
• Climatological SSTs
• Standard climate resolution (3.75o lon x 2.5o
  lat)
• LAI prescribed -
• LAI-Phen LAI varies across season
• LAI-Mean LAI set to annual mean value

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Area exhibiting sensitivity to phenologyNumber
of months per yearLAI-Phen statistically
different from LAI-Mean
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Surface water balance (West African monsoon)
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Surface water balance (West African monsoon)
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Surface water balance (South Asian monsoon)
LAI
Precipitation
10 mm yr-1 (2)
Evaporation
4 mm yr-1 (1)
Soil moisture content
Runoff
13 mm yr-1 (9)
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Surface water balance
West African monsoon
Evaporation
Soil moisture fraction layer 1
LAI
South Asian monsoon
Evaporation
Soil moisture fraction layer 1
LAI
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Sensitivity of evaporation to surface soil
moisture
LAI (LAI-Phen LAI-Mean) gt 1
LAI (LAI-Phen LAI-Mean) lt -1
Significant at 95
Significant at 95
Evaporation difference (mm/month)
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Resistance to evaporation
Rsoil 100 (SMF1 / SMFcrit)2 Rsoil
Resistance to evaporation from soil SMF1
Layer 1 soil moisture fraction SMFcri
Critical layer 1 soil moisture fraction
Rsoil 0 Lake Rsoil 80 well-watered
vegetation
West African monsoon
South Asian monsoon
Resistance to evaporation from soil
Resistance to evaporation from soil
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Summary

• MOSES 2 land-surface scheme has been updated so
  that realistic vegetation phenology, based on
  satellite data, can be prescribed. Phenology
  includes seasonal evolution of LAI, surface
  albedo, and roughness length.
• Globally, soil moisture, dry season daily maximum
  temperatures, evaporation, and sub-surface
  runoff, show significant sensitivity to the
  incorporation of vegetation phenology.
  Precipitation, however, does not.
• In the West African monsoon region, reduced
  evaporation in spring, when LAI is low and
  therefore access to sub-surface moisture stores
  is restricted, leads to an increase in total soil
  moisture content. The soil moisture increase is
  sustained until the end of the monsoon season,
  leading to enhanced sub-surface autumn runoff.
• Unrealistically low resistances to evaporation
  from soil may be reducing the full impact of
  seasonally varying vegetation in the model.

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Wet-dry soil moisture composite
Composite West African monsoon evolution based on
soil wetness in winter/spring prior to monsoon
season
Precipitation
LAI-Mean Wet SM Dry SM
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Future Work

• Evaluate model
• Sensitivity to resistance to evaporation in bare
  soil
• Include canopy heat capacity
• Interannual variability
• Run model with observed SSTs over long time
  period to evaluate whether sensitivity to
  phenology is related to soil moisture state
• Analysis of completed runs
• Examine surface energy balance
• Look at variability, diurnal cycle, extreme
  events

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MOSES II - Surface tilingFractional surface
cover (IGBP)
Broadleaf Trees
Need leaf Trees
C3 Grass
C4 Grass
Shrub
Urban
Bare Soil
Inland Water