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