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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 cycle 9-yr ISLSCP II dataset
LAI LAI
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One of the most important outstanding aspects of
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
evaporation surface sub-surface canopy Impacts: hydrological cycle surface energy budget albedo or surface energy budget wind roughness momentum flux
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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) LAI Minimum: MOSES 2 – ISLSCP 2 (UM grid) LAI Maximum: MOSES 2 – ISLSCP 2 (UM grid)
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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 MOSES 2 albedo with phen annual mean MOSES 2 albedo w/ new soil + SM depend annual mean CCRS-ERBE albedo July - January MOSES 2 albedo – ERBE albedo annual mean MOSES 2 (w/ soil, SM) – CCRS-ERBE clim annual mean MOSES 2 albedo with phen July - January MOSES 2 albedo w/ new soil + SM depend July - January
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Surface albedo verification (West African monsoon region)
Annual mean albedo
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Model runs LAI-Phen LAI-Mean
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 LAI-Phen LAI-Mean
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Area exhibiting sensitivity to phenology Number of months per year LAI-Phen statistically different from LAI-Mean Daily Max 2m Air Temperature Precipitation Latent Heat Flux Soil Moisture Content
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Surface water balance (West African monsoon)
Precipitation 13 mm yr-1 (3%) LAI P = Es + Ev + Ec + Rsurf + Rsub + dSM/dt P = Precipitation Es = 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%)
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Surface water balance (West African monsoon)
P - E 30 mm yr-1 (45%) LAI Surface water balance (West African monsoon) dSM/dt Soil moisture content Runoff 28 mm yr-1 (40%)
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Surface water balance (South Asian monsoon)
LAI Surface water balance (South Asian monsoon) 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 South Asian monsoon LAI LAI
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) > 1 LAI (LAI-Phen – LAI-Mean) < -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-Phen Wet SM Dry SM LAI-Mean Wet SM Dry SM
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Future Work Evaluate model Interannual variability
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 tiling Fractional surface cover (IGBP)
Broadleaf Trees Needleleaf Trees C3 Grass C4 Grass Shrubs Bare Soil Broadleaf Trees Need leaf Trees C3 Grass C4 Grass Shrub Urban Inland Water Bare Soil
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