Measurements of Mass and Energy Exchange using Aircraft-based Sensors

R.L. Desjardins, D. Worth, Mauder, M., Metzger, S., and R. Srinivasan

15th EMS Conference, Sept. 2015, Sofia, Bulgaria

2

Length

Aircraft

Tower

Chamber

102 m 103 m

1 h

104 h

103 h

100 h

Time

10 m 104 m 105 m

Models

Spatial and temporal scales of mass and energy exchange

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Flux Measurement Platform – The Twin Otter Aircraft

Laser Altimeter

Side-looking video camera

Satellite simulatorGreeness indicator

Gas analyzers (CO2, CH4, H2O, O3)

Duct pressure, temperature sensors

Litton 90 inertial reference system

IntakeREA system

Data recorder

Accelerometers rate gyros

Global positioning system antenna

Console, keyboard & navigation system controls

Dew point sensor

Rosemount 858 (,, airspeed, altitude)

Video camera

Altitude gyro

Radio altimeter

(CO2, CH4, N2O, VOC, agrochemicals)

4

Reynolds:

Flux:

,xxx ,0x yxyxxy .

qwqwwqF

.

x

x

x0w

EC technique is a useful tool because it permits to quantify fluxes for an entire ecosystem without disturbing it. It is based on assumptions of stationarity and horizontal homogeneity. The temporal mean

may not be spatially representative due to large-scale organized structures.

The Eddy Covariance Method

0 10 20 30 40

Time (with tower) or distance (with aircraft)

vert

ical

win

d v

eloc

ity

scal

ar

vertical wind velocityscalar

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Relaxed Eddy Accumulation (REA)

• Alternate to eddy covariance technique to measure fluxes of trace gases for which fast-response analyzers are not operational (N2O)

• Air samples from updrafts and downdrafts are collected in two separate reservoirs for later analysis

• In EA, sample flow rate is proportional to w; this requirement is ‘relaxed’ in REA (i.e., full flow into up or down reservoir depending on the direction of the vertical wind)

Desjardins et al. 2000

• Alternate to eddy covariance technique to measure fluxes of trace gases for which fast-response analyzers are not operational (N2O)

• Air samples from updrafts and downdrafts are collected in two separate reservoirs for later analysis

• In EA, sample flow rate is proportional to w; this requirement is ‘relaxed’ in REA (i.e., full flow into up or down reservoir depending on the direction of the vertical wind)

Desjardins et al. 2000

F w w ' ' = A U p D ow n

Vent (Dead band)

PTFESample Bag

DC Power supply

3-wayValve

Mass-FlowController

2-mFilter

Reliefvalve

DiaphragmPump 12 l/min

Inlet

UP

DOWN

¼” PTFEtubing

CO2 Fluxes (kg CO2 ha-1 h-1)

Measurements of carbon dioxide absorption over a 15 km x 15 km grassland site using

the NRC Twin Otter aircraft. The data is superimposed on a satellite image.

Measurements of carbon dioxide absorption over a 15 km x 15 km grassland site using

the NRC Twin Otter aircraft. The data is superimposed on a satellite image.

CO2 flux measurements during FIFE, 1989

Evapotranspiration measured using the Twin Otter aircraft over the Konza Prairies during FIFE

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Tower and Aircraft Flux Measurements sensible and latent heat

QE QE QE

QH QH QH

SGP project 1997

SGP project 1997

USA

0 50 100 150 200

Tower (W m-2)

0

50

100

150

200

Air

cra

ft (W

m-2)

Whole run, 12 km

Segment, 2.8 km

AC / Tower Comparison of Sensible Heat Fluxes (line 8)

1:1

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Lack of energy budget closure: Implications for sensible heat, CO2 and H2O flux measurements?

From the basic energy balance equation,

Qn – QG – ΔQS = QE + QH

However, experimentally it has generally been found that on a short time scale (hours)

Qn – QG – ΔQS > QE + QH

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turb

ulen

t ene

rgy

fluxe

s: Q

H + Q

E (W

m-2

)

available energy: -Q*s – QG (W m-2)

22 European sites: residual of 20% of the available energy on average

(Wilson et al. 2002)

22 European sites: residual of 20% of the available energy on average

(Wilson et al. 2002)

Lack of Energy Budget Closure

Energy Balance at 6 European forest sites (Aubinet et al., 2000)

Are we underestimating the CO2 flux as well?Are we underestimating the CO2 flux as well?

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Mesoscale Circulation

T < [T], w < [w]

QH > 0

30min

0

1s

t

H w w T TN

QH

An example of the long term impact of surface heterogeneity on mass and energy exchange- 56 passes (Desjardins et al. 1997)

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Desjardins, R.L., MacPherson, J.I., Mahrt, L., Schuepp, P.H., Pattey, E., Neumann, H., Baldocchi, D., Wofsy, S., Fitzjarrald, D., H. McCaughey and D.W. Joiner. 1997. Scaling up flux measurements for the boreal forest using aircraft-tower combinations. J. Geophys. Res. 102: 29,125-29,134.

wavelength

Edd

y flu

xMesoscale contribution Turbulent flux

Handling of nonstationary conditions

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Candle Lake – Wavelet analysis

Why wavelet analysis?

1. Does not require stationarity and homogeneity (in contrast to Fourier analysis)

2. Gives quantitative information, where in space and on what wavelength flux contributions occur

3. Allows to distinguish between small-scale turbulence and mesoscale fluxes (< > 2 km)

4. Allows to compute fluxes at a relatively small spatial resolution ( 1 km) without neglecting flux contributions from longer wavelengths

5. With wavelet analysis the residual term is substantially reduced

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Candle Lake – Wavelet analysis

Analysis of low-frequency flux contributions as reason for the underestimation of eddy-covariance fluxes: Wavelet analysis of aircraft measurements

Canada

115 km

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Candle Lake – Wavelet cross-scalogram

Flight 1 BOREAS 1041 – 1116 CST25 May 1994

Distance (km)

Loga

rithm

ic w

aven

umbe

r (m

)

Mauder, M., R. L. Desjardins, and I. MacPherson. 2007. Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: 10.1029/2006JD008133.

Positive flux contribution

Near zero flux

Negative flux contribution

Legend

How important is mesoscale transport in the surface layer?

Candle Lake Runs (BOREAS/BERMS) at 30 m measurement height

20 flights analyzed 5 – 20% mesoscaleflux contribution (2 km)

Mauder, M., R. L. Desjardins, and I. MacPherson. 2007. Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: 10.1029/2006JD008133.

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Mesoscale flux contributions (i.e., wavelength > 2 km) in % of the flux averaged over the entire flight track

Date time (CST) H λE CO2 flux O3 flux

25-May-1994 1041 - 1116 11% 10% 10% 10%

25-May-1994 1118 - 1152 15% 13% 11% 8%

25-May-1994 1154 - 1228 17% 14% 9% 13%

27-May-1994 1328 - 1403 −2% 5% −5% 2%

01-Jun-1994 1300 - 1333 12% 14% 14% 22%

06-Jun-1994 1057 - 1130 15% 17% 12% 13%

06-Jun-1994 1133 - 1211 8% 12% 9% 7%

21-Jul-1994 1611 - 1646 13% 12% 11% 5%

23-Jul-1994 1056 - 1126 6% 14% 9% 13%

25-Jul-1994 1220 - 1251 10% 16% 17% 25%

25-Jul-1994 1515 - 1548 11% 17% 17% 18%

25-Jul-1994 1631 - 1702 11% 30% 19% 37%

27-Jul-1994 1106 - 1136 23% 12% 11% 10%

08-Sep-1994 1413 - 1444 8% 10% 2% 8%

Mauder, M., R. L. Desjardins, and I. MacPherson. 2007. Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: 10.1029/2006JD008133.

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Morewood

Casselman

N

0 5 km5 km

Morewood

Casselman

N

0 5 km5 km

N

0 5 km5 km

Measuring nitrous oxide emissions

soy

cereals

pasture/grass

alfalfa

forest

corn

town

LEGEND

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Regional N2O fluxes during and right after snowmelt at the Eastern Canada study sites in 2001

-25

0

25

50

75

100

125

15-M

ar

25-M

ar

4-A

pr

14-A

pr

24-A

pr

4-M

ay

14-M

ay

24-M

ay

3-Ju

n

13-J

un

N2O

Em

issi

on

s (g

N2O

-N h

a-1 d

-1) Casselman

Morewood

Vent (Dead band)

PTFESample Bag

DC Power supply

3-wayValve

Mass-FlowController

2-mFilter

Reliefvalve

DiaphragmPump 12 l/min

Inlet

UP

DOWN

¼” PTFEtubing

Canada

Mixed farmlande.g. REA can be used to measure the regional (≈50-100 km2) flux of N2O from agricultural land.

Each data point represents the average of 3 samples, collected during two consecutive 10 km flight legs (total flight distance for one data point is ≈ 20 km).

After accounting for 20% of the indirect N2O emissions (not considered by models such as DNDC) cumulative N2O emission estimates between the DNDC model and measurements were comparable.

e.g. REA can be used to measure the regional (≈50-100 km2) flux of N2O from agricultural land.

Each data point represents the average of 3 samples, collected during two consecutive 10 km flight legs (total flight distance for one data point is ≈ 20 km).

After accounting for 20% of the indirect N2O emissions (not considered by models such as DNDC) cumulative N2O emission estimates between the DNDC model and measurements were comparable.

Source: Desjardins, R.L., Pattey, E., Smith, W.N., Worth, D., Grant, B., Srinivasan, R., MacPherson, J.I., and Mauder, M. 2010. Multiscale estimates of N2O emissions from agricultural lands. Special Issue of Agriculture and Forest Meteorology 150 (6) 817-824.

The NRC Twin Otter

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Instrumented nose boom

in-flight REA sample collection & post-flight REA sample analysisusing Picarro G1301

CH4 Analyzer (G2301) and real-time display

CH4 emission estimates at a regional scale (2011)

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Summary

•Aircraft-based flux measuring facility based on EC and REA techniques

•A powerful tool to measure mass and energy exchange over a wide range of ecosystems

•Presented flux measurements for CO2, CH4, N2O, H2O and O3

• Energy budget closure – a poor validation tool.

•Mesoscale flux contributions have a similar magnitude as the flux underestimation by tower-based systems

•Aircraft-based flux measuring facility based on EC and REA techniques

•A powerful tool to measure mass and energy exchange over a wide range of ecosystems

•Presented flux measurements for CO2, CH4, N2O, H2O and O3

• Energy budget closure – a poor validation tool.

•Mesoscale flux contributions have a similar magnitude as the flux underestimation by tower-based systems