aircraft and tower eddy covariance flux measurements

aircraft and tower eddy covariance flux aircraft and tower eddy covariance flux

measurementsmeasurements

SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)

B. Gioli (IBIMET CNR, Italy)

http://www.ibimet.cnr.it/

1. TOWER FLUX MEASUREMENTS

2. AIRCRAFT FLUX MEASUREMENTS

3. FLUX MANIPULATION EXPERIMENT


Layout

1. TOWER FLUX MEASUREMENTS

- principles of the eddy covariance micrometeorological tecnique;

- how to use turbulence to measure fluxes

-tecnique assumptions and limitations


wind & turbulence


wind & turbulence


air motion is 3D and made by 'fluctuations'

+CO2+CO2 -CO2-CO2

surface exchange

CO2 sink surface

downdraft (w-) updraft (w+)


c1, c1, w1w1

c2, c2, w2w2

surface exchange (one eddy)

CO2 sink surface



Flux associated to one eddy = c1w1 + c2w2

Fluctuations of vertical wind and scalarsc’ = c - cw’ = w - w

+CO2+CO2 -CO2-CO2

CO2 sink surface


N

kkk ccww

NcwcwFlux

11

1),cov(''

surface exchange (general)

''

''

''

''

* wuu

qwE

wcH

cwNEE

p

Fluxes from a typical EC tower(at typically 30 min resolution)

Net Ecosystem Exchange

Sensible heat flux

Latent heat flux = evapotransp.

Momentum flux = friction velocity


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ora

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rom

oli

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ora del giorno

w m

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NEE

H, LE


EC hypothesis

1. ~Flat terrain 2. Homogeneity

3. Stationarity4. 'enough' turbulence


Sdz wt

dz ux

dz wz

dz

I II III IV V

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ss

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Technology: 3D wind (fast) measurement


Technology: CO2 & H2O (fast) measurements

Open Path IRGA (Infra Red Gas Analyzer)


Isola di PianosaIsola di Pianosa

InstallationsNorunda, SveziaNorunda, Svezia


FluxNet


Complicating factors: 1. frequency response:do we measure ALL the flux by eddy covariance ?

Rn = SW↓ +LW ↓ - SW↑ -LW↑ (net radiation)

H + LE + G = Rn

H: sensible heat flux (EC measured)

LE: latent heat flux = evapotranspiration (EC measured)

G = soil heat flux (measured)


Vaccari et al 2003

Cospectra W-CO2 (Li7500)

-0.100

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Frequency [Hz]

nS

(n)

cospectra of CO2 and W

high freq. ~ parts of a secondlow freq. ~ minutes


Complicating factors: 1. frequency response:do we measure ALL the flux by eddy covariance ?

Complicating factors: 2. footprint estimation

where the measured flux comes from ?


Footprint area = area contributing to the observed flux

Footprint area extends upwind the observation point, and is a function of:

• Atmospheric stability

• Wind speed and direction

• Surface roughness

Footprint area is estimated trough models (analytic, stochastic, lagrangian back-trajectory)

Complicating factors: 3. CO2 flux partioning


• approaches exist to partition NEE into GPP and Reco, based on night time fluxes to dirve a respiration response to temperature and soil water content (Reichstein et al 2003)

NEE = GPP – Reco

NEE = Net Ecosystem ExchangeGPP = Gross Primary Productivity

Reco = Ecosystem respiration

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2. AIRCRAFT FLUX 2. AIRCRAFT FLUX MEASUREMENTSMEASUREMENTS


Sky Arrow ERA

Attitude GPS Net Radiation

PAR Radiation

Surface T

Dew Point T

Videocamera

Pressure Sphere

T Fast Response

T Low Response

Novatel GPS

IRGA

GPS

ElectronicsSwitch BOX

Mobile Flux Platform (MFP)Mobile Flux Platform (MFP)

Measurement of 3D winds @ 50 Hz

CO2, H2O, T fast sensors


(relative) wind measurement

differential & static pressures

Dynamic pressure


SkyArrow ERA: wind measurement principle


Aircraft motion measurement

Sistema alternativo (NOAA, IATA): Attitude GPS + accelerometri

Attitude 3D GPS a 4 antenne (10 Hz)

accelerometri (50 Hz)SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)

Pressures50 HzGPS

Ground GPS

PositionVelocity

10Hz

Actual 3D wind retrieval

Accelerometers 50Hz

Attitude GPSAttitude10Hz

PositionVelocityAttitude50Hz

Actual 3D wind50Hz


System calibration

IRGA Gas Analyzer

Pressure Ports


The productsThe products

3D wind + scalar (T, H2O, CO2) fast measurements (50 Hz)

(eddy covariance tecnique)

Surface fluxes along flight track(u*, H, LE, fCO2)


Flux transect over NL (Gioli et al 2006)

Tower data – 30 min continuous data

Aircraft data – ‘Spatial’ Fluxes

Data characteristics


Footprint concept & aircraft fluxes

SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy) SSOS- Summer School on Optical Sampling

(7-13 July 2011, Trento, Italy)

Validation of aircraft flux measurements


Vellinga et al 2010

Regional C-budgets


Validation of RS-based surface schemes at regional scale


Some conclusions part 1 & 2

• eddy covariance widely used to measure surface fluxes across biomes, at high temporal resolution

• limitations arise from potential flux loss, & not perfect conditions (orography, inhomogeneities, low turbulence at night...)

• EC measures NEE, while GPP needs to be retrieved trough Reco estimation at nigh time (difficult...)

• EC can be succefully applied from aircraft platform and regional scale fluxes measured


3. FLUX 3. FLUX MANIPULATION MANIPULATION

EXPERIMENTEXPERIMENT


Surface energy balance

Rn = SW↓ +LW ↓ - SW↑ -LW↑ (net radiation)

Rn = H + LE + G

H: sensible heat flux (EC measured)

LE: latent heat flux = evapotranspiration (EC measured)

G = soil heat flux (measured)


Study area

FLUX MANIPULATION EXPERIMENTFLUX MANIPULATION EXPERIMENT

Rn

HLE

G

H?

LE ?

G

CONTROL TREATMENT

(antitranspirant)Measurements:

H, LE, G (eddy towers)

Ts (IR camera on aircraft)

VIS-NIR (ground + aircraft) SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)

can we observe a decrease in LE ?

can we observe an increase in H ?

can we observe an increase in Ts ?

how is VIS-NIR reflectance affected ?

how is photosynthesis affected ?

Can we observe change in energy partioning by eddy covariance & remote sensing ?


4TE

T

bmax

Measuring Ts with thermal remote sensing

• IR camera detects 7.5 – 12 micron

• emissivity needed to estimate Ts

Wien's displacement law

Stefan–Boltzmann law


Eddy covariance can be used to measure both resistances:

H, LE Penman Monteith equation rc (stomatal resistance)

u, u* aerodynamic resistance

H = Cp (Ts – Ta) / Rtot

Rtot = Rsto + Rnsto (total resistance)Rsto = resistance for water to be transpired trough stomataRnsto = other resistance (aerodynamic)

/)/)/)()((( aaaspnc rEreecGRr

Using Ts to assess surface energy balance

Feedbacks in the coupled land-atmosphere system

van Heerwaarden et al, 2009

positive feedback

negative feedback

at plot scale:

atmosphere surface (plot)

at larger scale:

atmosphere surface (region)

why is surface energy balance important ?

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aircraft and tower eddy covariance flux measurements

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