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CGAMUniversity of Reading
Reading, UK
Vegetation Phenology and the Hydrological Cycle of Monsoons
David Lawrence and Julia Slingo
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?
Leaf Area Index climatological seasonal cycle9-yr ISLSCP II dataset
LAILAI
One of the most important outstanding aspects of land-atmosphere interactions is the feedbacks associated with
seasonal and interannual vegetation cycles (Hutje, 1998).
Seasonal and interannual variability
Vegetation-atmosphere interaction
evaporation surface sub-surface canopy
Impacts: hydrological cycle surface energy budget
wind
roughness
momentum flux
albedo or
surface energy budget
LAI Minimum: MOSES 2 – ISLSCP 2 (UM grid)
Vegetation phenology into MOSES 2: Leaf Area IndexLAI annual cycle (West Africa)
LAI Maximum: MOSES 2 – ISLSCP 2 (UM grid)
LAI directly or indirectly controls • canopy height• surface roughness• canopy water capacity• surface albedo
Vegetation phenology into MOSES 2: Surface albedo
MOSES 2 albedo with phenannual mean
MOSES 2 albedo – ERBE albedoannual mean
CCRS-ERBE albedoJuly - January
MOSES 2 albedo with phenJuly - January
MOSES 2 albedo w/ new soil + SM dependJuly - January
MOSES 2 albedo w/ new soil + SM dependannual mean
MOSES 2 (w/ soil, SM) – CCRS-ERBE climannual mean
Updated soil albedo based on CCRS-ERBE albedoIncluded linear dependence of albedo on soil moisture
Surface albedo verification (West African monsoon region)
Surface albedo Annual mean albedo
Model runs
LAI-Mean
LAI-Phen • 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
Area exhibiting sensitivity to phenologyNumber of months per year
LAI-Phen statistically different from LAI-Mean
Latent Heat Flux Soil Moisture Content
PrecipitationDaily Max 2m Air Temperature
Surface water balance (West African monsoon)
P = Es + Ev + Ec + Rsurf + Rsub + dSM/dt
P = PrecipitationEs = Soil evaporation
Ev = Transpiration
Ec = Canopy evaporation
Rsurf = Runoff surface
Rsub = Runoff sub-surface
dSM/dt = Change in soil moisture storage
Evaporation 17 mm yr-1 (5%)
Precipitation 13 mm yr-1 (3%)
LAI
Surface water balance (West African monsoon)dSM/dt
Soil moisture content Runoff 28 mm yr-1 (40%)
P - E 30 mm yr-1 (45%)
LAI
Surface water balance (South Asian monsoon)Evaporation
Soil moisture content Runoff 13 mm yr-1 (9%)
Precipitation 10 mm yr-1 (2%)
LAI
4 mm yr-1 (1%)
Surface water balance
West African monsoon
South Asian monsoon
LAIEvaporation Soil moisture fraction: layer 1
Evaporation Soil moisture fraction: layer 1
LAI
Sensitivity of evaporation to surface soil moistureEv
apor
atio
n di
ffere
nce
(mm
/mon
th)
LAI (LAI-Phen – LAI-Mean) > 1 LAI (LAI-Phen – LAI-Mean) < -1 Significant at 95% Significant at 95%
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 LakeRsoil ~ 80 well-watered
vegetation
West African monsoon South Asian monsoon
Res
ista
nce
to e
vapo
ratio
n fr
om s
oil
Res
ista
nce
to e
vapo
ratio
n fr
om s
oil
SummaryMOSES 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.
Wet-dry soil moisture composite
Composite West African monsoon evolution based on soil wetness in winter/spring prior to monsoon season
Precipitation
LAI-Phen Wet SM Dry SM
LAI-Mean Wet SM Dry SM
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
MOSES II - Surface tilingFractional surface cover (IGBP)
Broadleaf Trees
C3 Grass
Need leaf Trees
C4 Grass
Shrub Urban
Inland Water Bare Soil
Broadleaf Trees Needleleaf Trees
C3 Grass C4 Grass
Shrubs Bare Soil