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The role of soil moisture in influencing climate and terrestrial ecosystem processes

Vivek AroraCanadian Centre for Climate Modelling and AnalysisMeteorological Service of Canada

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Outline

• The structure of the climate model.• How soil moisture is modelled as a prognostic variable.• The role of soil moisture in determining various climate

system processes.• Introduction of vegetation as a dynamic component of

the climate system.• Vegetation processes that are affected by soil moisture.• Soil moisture requirements.

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The structure of the climate model

3.75° resolution

Global climate models (GCMs) solve mathematical equations describing the complex coupling of atmosphere, land surface and the oceans.

Atmosphere 31 levels

Ocean 29 levels

Dynamic sea ice

Interactive clouds

Interactive landsurface (3 levels)

10 mb

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CanopyCanopy

The four sub-areas Fgs

Fcs Fcg Fgvegetation-snowcovered

vegetation-covered bare soil snow-covered

Fc

Snow

P

rcgrcses

rsg

ec

PP

et

eg eg

ro

rg

G r o u n d

P

P

Structure of the Canadian land surface scheme (CLASS)

0.25 m0.10 m0.10 m

3.75 m

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0.25 m0.10 m0.10 m

3.75 m

The structure of the land surface scheme

Infiltration Evapotranspiration

Runoff

Both evapotranspiration and runoff are non-linear functions of soil moisture

Liquid and frozen soil moisture contents are calculated prognostically

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Land-atmosphere interactions and feedbacks

Soil moisture

Latent heat flux(evapotranspiration)

Sensible heat flux

Precipitation

Atmospheric moisture

CloudinessBoundary layer thicknessand heating

Insolation

Net Radiation

Albedo

Atmosphere

Land

Ground heat flux

Surface/Soil temperature

Outgoing long wave radiation

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Land-atmosphere interactions and feedbacks

Atmosphere

LandSoil moisture

EvapotranspirationPrecipitation

What is the strength of this feedback loop?How is it modulated by soil moisture?How does vegetation affects this loop?

8 of 18Relationship between soil moisture and the state of the atmosphere

Soil moisture index (Surface Layer)

Rel

ativ

e hum

idity

(RH

)

Soil moisture index (Surface Layer)

Low

clo

ud c

ove

r (L

CC)

Net lo

ngw

ave

radiatio

n LW

net

(W/m

2)

Betts A. K., P. Viterbo (2005), Land-surface, boundary layer, and cloud-field coupling over the southwestern Amazon in ERA-40, J. Geophys. Res., 110, D14108, doi:10.1029/2004JD005702.

Results from ECMWF reanalysis over southwestern Amazon Basin

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Observations from ARM facility sites in Oklahoma and Kansas.

Relationship between soil moisture and the state of the atmosphere

Dirmeyer, P.A., R.D. Koster, and Z. Guo, Do Global Models Properly Represent the Feedback between Land and Atmosphere? J. Hydrometeorology, 7(6), 1177–1198.

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G = photosynthesis, GPPAS = carbon allocated to stem from leavesAR = carbon allocated to roots from leavesRgL= Growth respiration for leaves, and

similarly for stem (S) and roots (R).RmL= Maintenance respiration for leaves

and similarly for stem (S) and roots (R)RhD = Heterotrophic respiration from the

litter or debris poolRhH = Heterotrophic respiration from the

soil carbon (humus) poolCD→H = carbon transferred from the litter to

the soil carbon pool

Structure of the Canadian Terrestrial Ecosystem Model (CTEM)

RhD

Stem

, CS

Roots, CR

G

AS

AR

RgL RmL

RgS

RmS

RgR RmR

Litter Pool, CD

Leaf litter, DL

Stem litter, DS

Root mortality, DRSoil Carbon Pool, CH

RhH

CD H

Leaves, CL

C3 Grass8

C4 Grass9

C4 Crop7

C3 Crop6

Broadleaf Dry Deciduous5

Broadleaf Cold Deciduous4

Broadleaf Evergreen3

Needleleaf Deciduous2

Needleleaf Evergreen1

CTEM 1.0 Plant Functional Types

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CLASS

Leaf Area Index , LAI(used in water and energy balance calculations)

Vegetation Height(determines roughness length and thus drag)

Root distribution profile and rooting depth(determines fraction of roots in each soil layer and thus transpiration)

Determines canopy heat capacity

Determines net radiation and net downward SW flux.

CTEM

Leaf biomass

Stem biomass

Root biomass

Canopy mass

Albedo

Since vegetation biomass is time

variant and model climate dependent

All these vegetation attributes also become time variant and climate dependent (except for the land cover which has to be specified).

CTEM 1 doesn’t model the competition between PFTs and thus the fractional coverages of PFTs in a grid cell has to be specified.Depends on the specified

land cover maps, which may be constant or time

varying e.g. with increasing crop cover.

CTEM provides CLASS all vegetation related attributes

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Land-atmosphere interactions and feedbacks

Soil moisture

Latent heat flux(evapotranspiration)

Sensible heat flux

Precipitation

Atmospheric moisture

CloudinessBoundary layer thicknessand heating

Insolation

Net Radiation

Albedo

Atmosphere

Land

Ground heat flux

Surface/Soil temperature

Outgoing long wave radiation

Photosynthesis Fire

Vegetation biomass, height and LAI

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How CTEM grows vegetation?

for a Broadleaf Cold Deciduous plant functional type

in Germany

Annual vegetation growth

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How CTEM grows vegetation?

for a Broadleaf Cold Deciduous plant functional type

in Germany

Annual cycle of leaf area index

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for a Broadleaf Cold Deciduous plant functional type

in Germany

Annual cycle of leaf area index

How CTEM grows vegetation?

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How CTEM grows vegetation?

Simulated annual cycle of leaf area index

Annual cycle of satellite-derived normalized difference vegetation index (NDVI)

Model-simulated annual cycle of NDVI (derived from grid-averaged LAI, m2/m2)

NDVI-LAI relationship from Buermann, W., et al. (2002): Analysis of a multi-year global vegetation leaf area index data set, J. Geophys. Res., 107(D22), 10.1029/2001JD000975.

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In near future, combined analysis of soil moisture and states of vegetation and atmosphere is

planned

• Compare model simulated and observation-based behaviourof and relationship between …

• Soil moisture, • Vegetation state defined in terms of LAI or NDVI, and• State of the atmosphere

• Observation-based information about states of vegetation (satellite-based NDVI) and atmosphere (reanalysis) is available.

• But no information is available for root zone soil moisture at large spatial scales.

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Summary

• Soil moisture affects the state of the atmospheric boundary layer (ABL) by influencing the partitioning of net radiation into latent and sensible heat fluxes.

• In a coupled vegetation-climate model, soil moisture additionally influences the state of vegetation and in particular the LAI which also interacts with the ABL.

• Quantification of soil-vegetation-atmosphere feedbacks is important to understanding the behaviour of coupled models, but hindered by the lack of observation-based root zone soil moisture data.