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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
3
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
5
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
8
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
10
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
11
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?
12
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)
13
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
15
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
16
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
17
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.
19
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.
20
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
21
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
22
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)
24
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
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