IPILPS Workshop ANSTO 18-22 April 2005 IsoTrans: Isotopes in the boundary layer Alastair Williams.

IPILPS Workshop

ANSTO 18-22 April 2005

IsoTrans:Isotopes in the boundary layer

Alastair Williams

IntroductionIntroduction• IsoTrans (Isotopic Tracers in Atmospheric Transport):

ANSTO “mother” Project for IPILPSBroader scope

•Purpose of presentation: Introduction to IsoTrans (very short)How is IsoTrans contributing to the improvement of

surface and boundary layer representations in models?Evaluation and development of LSSs in isotope-enabled

hydroclimate models (main subject of current workshop)How can SWI obs “add value” to our understanding of moisture

exchange in plant canopies, particularly ET partitioning?Natural radionuclides and turbulent mixing in the lower

atmosphere

Modelling

Process

studies

Sources & sinks

Physical processes

Predictive modelling

Current models do not reproduce well the complex cycles of exchange, mixing and

transport in the lower atmosphere

Problem

IsoTrans: DriversIsoTrans: DriversEffective Environmental Management Strategies need …

Informed predictions of mixing and movement

Sources & sinks

Physical processes

Predictive modelling

IsoTrans: DriversIsoTrans: DriversEffective Environmental Management Strategies need …

Informed predictions of mixing and movement

• International scientific community (1) Inability to accurately reproduce diurnal cycle a

severe limiting factor for weather and climate models (2) Need new methods to accurately track down the

origins and dynamics of atmospheric pollution (3) Lack of independent methods for evaluation of

numerical weather and climate prediction models

Modelling & Prediction Panel

IAEA

Nuclear tools applied to contemporary atmospheric issues

0 200kmMelbourne

Sydney

Brisbane

1000km

100km

IsoTrans: 3 Foci, 3 ScalesIsoTrans: 3 Foci, 3 Scales

(3) Isotopes at the land surface• Stable water isotopes• Land surface processes• Diurnal observations• Model evaluations• Major river basins,

including the MDB

(1) Local mixing• Vertical mixing processes in the

lower atmosphere• Towers and aircraft• Sydney area

(2) Regional transport• Wider Sydney region /

Eastern Sea Board• Pollution sources & dynamics• Horizontal array of surface

measurements

IsoTrans Process StudiesIsoTrans Process Studies• IsoTrans Task 3 (IPILPS)• How can SWI observations “add value” to our understanding

of moisture exchange in plant canopies, particularly evapo-transpiration partitioning?Discuss the Keeling approach for estimating the

transpired component of ET in vegetation canopiesExamine turbulent transport within vegetation canopiesAnalyze SWI behaviour in Tumbarumba air spacePresent first guess at ET partition for Tumbarumba

• Thanks to David Griffith (Wollongong Uni) for providing the vertical D data, and Helen Cleugh / Ray Leuning for providing the met data

Use of SWIs to Partition ETUse of SWIs to Partition ET

1000*;where

:""isotopeofFlux

:fluxTotal

000

xs

xs

xx

xx

xE

xT

xE

xET

xE

xT

xE

xET

ET

TT

TxTE

xEET

xET

xT

xE

xET

ixi

xi

xi

TEET

R

RR

c

cR

RR

RR

F

Ff

FRFRFRFFF

FRwcFx

FFF

Concept: simple mix of 2 fluxes with distinct isotopic signatures (): evap (frac) and transp (non-frac)

T, E: composition of contributing sources (measured / calculated)

ET: “effective” combined source

FET: from EC FT

How to estimate ET?

Keeling (1958)Keeling (1958)• Carbon isotope ratio closely follows concn in diurnal

time series over different vegetated surfaces• Mutual variation suggests simple 2-part mixing

(air and plants)

““Keeling” Analysis (1)Keeling” Analysis (1)

xET

m

xET

xaa

xm

ETxETa

xam

xm

ETxETa

xam

xmi

xi

xi

xET

xa

xm

ETam

CC

CCC

CRCRCRCRC

CCCx

CCC

1

:""isotopeWater

: waterTotal

2-part mixing model (ambient + combined ET)

Cm, mx: measured

Ca, ax: background

component from atmosphere

CET, ETx: combined

component from evap and transp

Linear relation if Ca, ax and ET

x constant, with intercept ETx

““Keeling” Analysis (2)Keeling” Analysis (2)• Versatile (temporal

& vertical gradients)• Problems:

Extrapolated intercept susceptible to large errors

Breakdown of assumptions

Yakir and Sternberg (2000)

1. Simple mixing of two major sources/sinks (atmos & ET)

2. No sources/sinks other than evap & transp (eg. dew, fog)

3. Relative contribution of all subsources remains fixed (eg. “non-fractionating” transpiration assumption true only when averaged over whole day: Harwood et al. 1998)

Diurnal variation of 18O of transpired water vapour for leaves on day 1 () and day 2 ( ,,) indicating the vapour pressure deficit (VPD) status and general trend over the day (solid line).

Harwood et al. (1998)

Yepez et al. (2003)Yepez et al. (2003)• Vertically-distrib D and 18O in semi-arid savanna woodland• Upper/lower profiles: analysed total and understory flux• Post-monsoon: transp 85% total, grass 50% understorey ET• Total ET 3.5mm/d = 2.5 (70%) tree trans + 0.5 (15%) grass

Williams et al. (2004)Williams et al. (2004)• Vert distrib D Morocco olive orchard following 100mm irrig• Keeling vs sap flow (v. hard to get representative data)• Trans/soil evap by isotope method within 4%/15% sap flow• Transpiration: pre-irrig 100%, post-irrig 70-85%

Complex CanopiesComplex Canopies• How can use of isotopes “add value” to understanding of ET

from a complex canopy/ecosystem such as Tumbarumba?

Atmospheric Boundary LayerAtmospheric Boundary Layer• First need to understand turbulent mixing processes in the

canopy, and interactions with atmospheric boundary layer

(Stull, 1988)

ABL Structure and TurbulenceABL Structure and Turbulence

Day

Night

(Holtslag and Duynkerke, 1998)

(Wyngaard, 1990)

Vegetation CanopiesVegetation Canopies• “The essential differences

between turbulence in the canopy air space and that in the boundary layer above result from the sources and sinks of momentum and scalars that are spread through the canopy” (Kaimal and Finnigan, 1994)

• Canopy turbulence is dominated by the large eddies that form in the intense shear layer confined to the crown or upper part of the canopy

Wind in Vegetation CanopiesWind in Vegetation Canopies• Similar behaviour over large

range of obs/model canopies

• Wind-shear max canopy top

• Attenuation below, foliage density determines rate

• Canopy turbulence strongly inhomogeneous in vertical

• All momentum absorbed in upper part of canopy (c.f. constant stress layer above)

• Large momentum gradient required to sustain steady air flow against aerodynamic drag of foliage

7/3/05 Tumbarumba

0

10

20

30

40

50

60

70

0.00 1.00 2.00 3.00 4.00 5.00

Windpeed (m/s)

He

igh

t (m

)

0

3am

6am

9am

12

3pm

6pm

9pm

Night(calm)

Day (gradient + inflection)

Turbulence in Vegetation CanopiesTurbulence in Vegetation Canopies• Skewness

Measure of turbulent intermittencyZero in surface layerCanopy: SKu +ve & SKw=-veTurbulence is dominated by intermittent

downward moving gusts (large eddies)

• Spectral peaksCanopy: peak positions constant“Large eddies” extend through

whole depth of foliage and into the air above

(Kaimal and Finnigan, 1994)3 3

u uSK u

Turbulence in Vegetation CanopiesTurbulence in Vegetation Canopies• TKE budget

Shear prodn peaks near canopy topWake prodn high in upper thirdTurbulent transport: sink of TKE at

canopy top, source in lower canopyLower canopy TKE not locally produced:

imported from above by “large eddies”Dissipation much higher than free stream:

wake and waving terms convert dominant large scale motions to smaller eddies

• Canopy turbulence dominated by canopy-scale “large eddies” Cool dry gusts displacing warm moist canopy air at all levels Counter-grad fluxes; non-local mixing; turb transport; distributed sources Surface layer flux-profile mixing relationships (“K-theory”) are inapplicable

in vegetation canopies

(Kaimal and Finnigan, 1994)

Turbulence in TumbarumbaTurbulence in Tumbarumba

• Quiescent at night• Strong in daytime (9:00-15:00): ABL convective motions

7/3/05 Tumbarumba

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0:00

3:00

6:00

9:00

12:00

15:00

18:00

21:00

0:00

Time

SD

Win

d d

irn

70

m

(de

g)

Temperature in Vegetation CanopiesTemperature in Vegetation Canopies• NightNight: :

lower canopy unstable strat - enhanced mixing

upper canopy stable strat (no turb - dew formation possible)

Tumbarumba: slightly stable (suppresses mixing)

• DaytimeDaytime::crown max (sun on foliage),

with stable strat below. But +ve (counter-grad) flux, so bimodal

Intermittent mixing by large eddies + quiescent periods

Tumbarumba: rapid increase of whole profile in morning; unstable for remainder of day

7/3/05 Tumbarumba

0

10

20

30

40

50

60

70

0.000 5.000 10.000 15.000 20.000

Potential Temp (Celsius)

He

igh

t (m

)

midnight

3am

6am

9am

noon

3pm

6pm

9pm

Humidity in Vegetation CanopiesHumidity in Vegetation Canopies• Night:Night:

Tumbarumba. Saturated (>80% at 70m, colder below), with slow decrease of whole profile: dew/fog

• Morning:Morning:Tumbarumba. Rapid increase of

whole profile: dew/fog re-evap as temp incr + transpiration “kicks in”

• Day:Day:Negative gradient + progressive

decrease of whole profile: dry air intrusion

Transpiration (secondary maximum in crown)

Large values near ground: surface moisture in leaf litter after rain

7-3-05 Tumbarumba

0

10

20

30

40

50

60

70

0.5 0.7 0.9 1.1 1.3 1.5

[H2O] (%)

He

igh

t (m

)

0-3am

3-6am

6-9am

9am-12

12-3pm

3-6pm

6-9pm

9pm-0

Precipitation 1-20 March 2005Precipitation 1-20 March 2005

Isotope obsthunderstorm

Moisture in air (%) v Time of day (Tumbarumba - 7 Mar 05)

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

2.000

H2O

%

0.4

4

10

26

34

42

70

Humidity in Vegetation CanopiesHumidity in Vegetation Canopies

Night: saturated (fog/dew dries air)

Morning warming: fog/dew re-evaporates + transpiration “kicks in”

Spread: fog/dew re-evaporates from top down

Afternoon: dry air intrusion + transpiration

Evening: mixing stops, temperature drops

Surface moisture in leaf litter after rain

07/03/2005Profile

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00

Hour of day

H2

O %

-180

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

de

l-D

p

er

mil

H2O

del-D

Isotopes in Vegetation CanopiesIsotopes in Vegetation Canopies

Humidity gradients only in afternoon

Isotope gradients all day

Isotopes in Vegetation CanopiesIsotopes in Vegetation Canopies

• NightNight. +ve grad: condensation onto surface/plants (temp dep). +ve grad: condensation onto surface/plants (temp dep)• MorningMorning. Re-evap of (heavy) dew/fog + transp + soil evap. Re-evap of (heavy) dew/fog + transp + soil evap• AfternoonAfternoon. -ve grad: transp + soil evap + mixing from above. -ve grad: transp + soil evap + mixing from above

7-3-05 Tumbarumba

0

20

40

60

-140 -130 -120 -110 -100 -90 -80del-D

He

igh

t (m

)0-3am

3-6am

6-9am

9am-12

12-3pm

3-6pm

6-9pm

9pm-0

Night Afternoon

Morning

Transp. ~ -40 o/ooSoil evap. ~ -95 o/ooAtmos. ~ -150 o/oo

Vertical Keeling AnalysisVertical Keeling AnalysisTumbarumba 7/3/05

-160.0

-140.0

-120.0

-100.0

-80.0

-60.0

-40.0

-20.0

0.0

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800

1/[H2O]

del

D

0-3am

3-6am

6-9am

9am-12

12-3pm

3-6pm

6-9pm

9pm-0

Transp. ~ -40 o/oo

Soil evap. ~ -95 o/oo

Atmos. ~ -150 o/oo

Tumbarumba 7/3/2005

-120

-100

-80

-60

-40

-20

0

20

0:00

6:00

12:00

18:00

0:00

time

de

l-D

in

terc

ep

t

total understory • Intercept from Keeling plots: D

ET

• Guesses for D source values:Soil evap -950Transpiration -40

• Total FT(%):n/a at night20% morning (dodgy)80% afternoon

• Understorey60% at night (no!)20% morning (dodgy)50% afternoon

Tumbarumba Keeling AnalysisTumbarumba Keeling Analysis

Tumbarumba 7/3/05

-200

20406080

100120

0:00

6:00

12:00

18:00

0:00

time

FT

(%

)

total understorey

• r2 values: only high in afternoon

Tumbarumba 7/3/05

0

0.2

0.4

0.6

0.8

1

0:00

6:00

12:00

18:00

0:00

time

r2

total understorey

Tumbarumba Keeling AnalysisTumbarumba Keeling Analysis

7/3/05 Tumbarumba: all 70m data

-140.0

-120.0

-100.0

-80.0

-60.0

-40.0

-20.0

0.0

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800

1/[H2O]

Time-varying Keeling AnalysisTime-varying Keeling Analysis

Transp. ~ -40 o/oo

Soil evap. ~ -95 o/oo

Atmos. ~ -150 o/oo

Intercept -66.6

R2=0.762

ConclusionsConclusions

• Vertically varying SWI data can be used to “add value” to our understanding of moisture exchange in plant canopies, particularly the partitioning of evapotranspiration

• The combination of time-varying and vertically-varying mixing analyses (Keeling+better?) of both D and 18O promises to be a very powerful tool for analysing ET in complex ecosystems such as Tumbarumba

• But …• Need to understand the “whole picture” in terms of the

airflow/turbulence regime within and above the canopy, so supporting meteorological data is essential.