aircraft and tower eddy covariance flux measurements ssos- summer school on optical sampling (7-13...
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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)
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1. TOWER FLUX MEASUREMENTS
2. AIRCRAFT FLUX MEASUREMENTS
3. FLUX MANIPULATION EXPERIMENT
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Layout
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1. TOWER FLUX MEASUREMENTS
- principles of the eddy covariance micrometeorological tecnique;
- how to use turbulence to measure fluxes
-tecnique assumptions and limitations
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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wind & turbulence
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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wind & turbulence
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
air motion is 3D and made by 'fluctuations'
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+CO2+CO2 -CO2-CO2
surface exchange
CO2 sink surface
downdraft (w-) updraft (w+)
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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c1, c1, w1w1
c2, c2, w2w2
surface exchange (one eddy)
CO2 sink surface
downdraft (w-) updraft (w+)
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Flux associated to one eddy = c1w1 + c2w2
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Fluctuations of vertical wind and scalarsc’ = c - cw’ = w - w
+CO2+CO2 -CO2-CO2
CO2 sink surface
downdraft (w-) updraft (w+)
N
kkk ccww
NcwcwFlux
11
1),cov(''
surface exchange (general)
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''
''
''
''
* 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 del giorno
w m
-2 H LE
NEE
H, LE
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EC hypothesis
1. ~Flat terrain 2. Homogeneity
3. Stationarity4. 'enough' turbulence
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Sdz wt
dz ux
dz wz
dz
I II III IV V
h
ss
h
s
h
s
hm m m m
0 0 0 0 ' '
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Technology: 3D wind (fast) measurement
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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Technology: CO2 & H2O (fast) measurements
Open Path IRGA (Infra Red Gas Analyzer)
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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Isola di PianosaIsola di Pianosa
InstallationsNorunda, SveziaNorunda, Svezia
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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FluxNet
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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)
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Vaccari et al 2003
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Cospectra W-CO2 (Li7500)
-0.100
0.000
0.100
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0.01 0.1 1 10 100
Frequency [Hz]
nS
(n)
cospectra of CO2 and W
high freq. ~ parts of a secondlow freq. ~ minutes
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Complicating factors: 1. frequency response:do we measure ALL the flux by eddy covariance ?
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Complicating factors: 2. footprint estimation
where the measured flux comes from ?
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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)
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Complicating factors: 3. CO2 flux partioning
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
• 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
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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
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Mobile Flux Platform (MFP)Mobile Flux Platform (MFP)
Measurement of 3D winds @ 50 Hz
CO2, H2O, T fast sensors
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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(relative) wind measurement
differential & static pressures
Dynamic pressure
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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SkyArrow ERA: wind measurement principle
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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)
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Pressures50 HzGPS
Ground GPS
PositionVelocity
10Hz
Actual 3D wind retrieval
Accelerometers 50Hz
Attitude GPSAttitude10Hz
PositionVelocityAttitude50Hz
Actual 3D wind50Hz
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System calibration
IRGA Gas Analyzer
Pressure Ports
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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)
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
Flux transect over NL (Gioli et al 2006)
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Tower data – 30 min continuous data
Aircraft data – ‘Spatial’ Fluxes
Data characteristics
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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)
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Validation of aircraft flux measurements
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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Vellinga et al 2010
Regional C-budgets
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Validation of RS-based surface schemes at regional scale
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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
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3. FLUX 3. FLUX MANIPULATION MANIPULATION
EXPERIMENTEXPERIMENT
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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)
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Study area
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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)
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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 ?
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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
SSOS- Summer School on Optical Sampling (7-13 July 2011, Trento, Italy)
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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
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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 ?