The vertical CO2 flux in the atmospheric boundary layer (ABL) is investigated with a Doppler differential absorption lidar (DIAL). The instrument was operated next to the WLEF instrumented tall tower in Park Falls, Wisconsin during three days and nights in June 2007. Profiles of turbulent CO2 mixing ratio and vertical velocity fluctuations are measured by in-situ sensors and Doppler DIAL. Time and space scales of turbulence are precisely defined in the ABL. The eddy-covariance method is applied to calculate turbulent CO2 flux both by lidar and in-situ sensors. We show preliminary mean lidar CO2 flux measurements in the ABL with a time and space resolution of 6 h and 1500 m respectively. The flux instrumental errors decrease linearly with the standard deviation of the CO2 data, as expected. Although turbulent fluctuations of CO2 are negligible with respect to the mean (0.1 %), we show that the eddy-covariance method can provide 2-h, 150-m range resolved CO2 flux estimates as long as the CO2 mixing ratio instrumental error is no greater than 10 ppm and the vertical velocity error is lower than the natural fluctuations over a time resolution of 10 s

Andrews, Arlyn

Beyon, Jeffrey Y.

Davis, Kenneth J.

Gilbert, Fabien

Hilton, Timothy W.

Ismail, Syed

Koch, Grady J.

Singh, Upendra N.

NASA Technical Reports Server

15th Coherent Laser Radar Conference
Turbulent CO2 flux measurements by lidar: length scales, results and
comparison with in-situ sensors
Fabien Gibert, 1,2 Grady J. Koch, 3 Jeffrey Y. Beyon,4 Timothy W. Hilton,1 Kenneth J. Davis,1 Arlyn Andrews,5
Syed Ismail ,3 Upendra N. Singh3
1 The Pennsylvania State University, Department of Meteorology, PA, USA
2 IPSL-LMD, Ecole Polytechnique, Palaiseau, France, fabien.gibert@lmd.polytechnique.fr
3 NASA Langley Research center, Hampton, VA, USA
4 California State University, Los Angeles, CA, USA
5 NOAA Earth System Research Laboratory, Boulder, CO, USA
1. Introduction
A mechanistic understanding of the global carbon cycle
requires quantification of terrestrial ecosystem CO 2
fluxes over regional scales. Different attempts, i.e. “top-
down” or “bottom-up” methods, to infer CO 2 fluxes
from the small (0.1 – 1 km) to larger scales (10 – 1000
km) need experimental verification and instrumentation
to do so. Range resolved CO2 flux measurements will
help significantly our understanding of the vertical
transport of CO2 at, firstly, the ABL – free troposphere
interface. Indeed the entrainment zone is usually not
reached by the tall towers and such data are inexistent.
Particular interest is in the morning and evening
transition, but also night time vertical transport.
Secondly, the biosphere – ABL interface can be studied.
While the results here are from a ground-based
experiment, an airborne lidar would be powerful
instrument to address the issue of CO2 surface flux
variability at different scales linked to the stratification
of the atmosphere and the spatial heterogeneity of the
surface (structure of the canopy).
Lidar has the ability to make direct range resolved flux
measurements using an eddy-covariance method.
Previous measurements of the flux of a scalar in the
atmospheric boundary layer (ABL) have already been
reported with latent flux using a combination of ground-
based 1 or airborne 2 Doppler and DIAL lidars.
Two micrometer Differential Absorption Lidar (DIAL)
systems for CO2 mixing ratio measurements have
already been developed at the NASA Langley Research
Center and at the Institut Pierre Simon Laplace,
Laboratoire de Météorologie Dynamique. 3-5 They
demonstrated a good precision of ~ 1 % with rather
large time (~ 30 min) and space resolution (~ 1 km)
though. These former Doppler lidars use heterodyne
detection which also provides radial velocity
measurements. Using this double ability, i.e.
concentration and velocity measurements, we present
here a preliminary study of CO 2 flux measurements by
lidar using the eddy-covariance method. Rather than a
geophysical study of CO2 flux in the ABL, the goal of
this paper is to present the requirements for the design
of a dedicated future ground-based or airborne Doppler
DIAL system for accurate CO 2 flux measurements.
Section 2 describes the experimental site in Wisconsin,
the NASA Doppler DIAL and in-situ sensors. Section 3
shows details of lidar CO2 flux calculations. Taking
advantage of a useful synergy between in-situ and lidar
instruments, section 4 presents the characteristics of
night and daytime turbulent CO 2 and velocity
fluctuations in the whole ABL. Section 5 shows a
preliminary comparison of CO2 fluxes calculated using
the eddy-covariance method and in-situ and lidar data.
2. Study site and instrumentation
The field experiment took place in June 23, 2007 at the
WLEF tall tower site in the Chequamegon National
Forest in northern Wisconsin (45.95°N, 90.27°W, 472
m above sea level) (Fig. 1). The region is a heavily
forested zone of low relief. The tower is a 447 m tall
television transmitter. Two minute mean CO2 mixing
ratios are sampled at six levels (11, 30, 76, 122, 244,
and 396 m) by two infrared gas analyzers (IRGA)
(LiCor Model Li-6251) to give CO2 profiles. Turbulent
winds, virtual potential temperature and H 2O mixing
ratio are also measured by three sonic anemometers and
other IRGAs at 30, 122 and 396 m above the ground. A
ground based meteorological station provides also
others observations such as net radiation and surface
pressure, temperature and moisture.
https://ntrs.nasa.gov/search.jsp?R=20090025444 2019-08-30T07:14:27+00:00Z
Fig.1: WLEF tower experimental site, Park Falls, Wisconsin,
USA.
The NASA Langley 2-µm Doppler DIAL was
positioned underneath the tower, approximately 40-m
away from the tower’s centreline. The 2-µm lidar
transmitter is a 90-mJ, 140-ns, 5-Hz pulsed Ho,Tm:YLF
oscillator injection seeded by continuous-wave (CW)
lasers. The pulsed laser has been described in detail in
other publications; 3, 6 its relevant parameters are
summarized in Table 1.
Tab. 1.Experimental set up
Pulse energy	 90 mJ
Pulse repetition rate for a 2.5 Hz
wavelength pair On-Off
Pulse width/ Line width
Off-line/ On-line
Spectral locking (On-line)
Telescope aperture
Detection	 heterodyne
140 ns/ 3 MHz
2053.410/ 2053.242 n
±1.9 MHz
100 mm
Dual balanced InGaAs
photodiodes
Signal digitization	 8 bits/ 500 MHz
Signal processing Estimators - Squarer (power)
- Levin-like (power,
velocity)
15th Coherent Laser Radar Conference
3. Turbulent CO2 flux measurements by
lidar
To infer a CO2 flux estimate using the eddy-covariance
method we need (as for in-situ data) high frequency
measurements of CO 2 and velocities. We are looking for
a correlation between the fluctuations of CO 2 mixing
ratio and vertical velocities due to turbulence only. The
CO2 EC flux is given by:
FCO2 = w '.PCO2 ' (1)
where and are respectively for the vertical and
time resolution of lidar CO 2 and velocities
measurements.
Signal averaging can be used to decrease the
instrumental error on flux measurements as long as the
final time and space resolution of the lidar fits within the
time and space integral scale of CO 2 turbulent flux.
Otherwise, biases on lidar flux measurements may
occur.
4. Space and time integral scales of
turbulence
In this section, we investigate the turbulence
characteristics of in-situ and lidar observations using
covariance techniques. The autocovariance (ACV) is
used to separate signal variance due to space correlated
atmospheric processes from uncorrelated instrumental
noise. For the atmospheric variable c (x):
ACVc (X) = c(x)' c(x + X )' ,	 (2)
where c(x) = c + c' (x) with c' (x) is for the space-
dependant fluctuation and X the space lag represents
the mean in the range gate used to calculate the
autocovariance.
For lidar measurements, Eq. (2) becomes:
ACVc (X) = c(x)' c(x + X )' where is for both time
and space lidar averaging. As we used a ground-based
instrument, the horizontal ACV is a function of time.
Knowing that ACVc (0) = 6, = 6^ inst + 62 and using
a Fourier transform to determine o2 	 we canc,inst
measure
catm
. Then, we can define the autocorrelation
function (ACR) by: ACRc (X) = ACV (X )/ 6 ,^ atm . The
integral of this function is called the integral scale (IS).
The first maximum of this integral is usually chosen to
be the IS. 1,7,B The IS is related to the dominant eddy size
and enables us to determine the space and time scales of
turbulence.
15th Coherent Laser Radar Conference
Fig. 2: (a) Horizontal autocovariance (ACV) and (b) integral
of autocorrelation (ACR) of hourly vertical velocity from the
sonic anemometer at 396 m at 10 Hz (black lines) and the
Doppler lidar at 375 m (grey lines) during the 06/14 night (0-1
h = dashed lines) and day (12h30-13h30 = solid lines). The
light grey lines are for sonic anemometer data interpolated to
lidar time resolution of 40/ s (c) Horizontal hourly integral
scale of vertical velocity ( l (x ) ) for WLEF sonic anemometers
at 30, 122 and 396 m and for the lidar at 375 m. l (x ) is the
maximum in the integrals of ACR(x ) . Lidar l(x ) are corrected
from bias due to lidar time averaging.
Figure 2 shows a horizontal daytime integral scale of
about 50 s whereas during the night, the integral scale
varies over a wide range from 10 s to more than 1 min,
depending on the occurrence of dynamic perturbations
such as nocturnal jets or subsidence. A similar study has
been made in the vertical direction. We find a vertical
integral scale of ~ 150 m in the convective boundary
layer and lower than 100 m during the quiet nights.
5. Results and validation with in-situ
sensors
Figure 3 displays a synoptic view of the three days of
measurements June 14-16, 2007. Lidar measurements
(first range gate at 150 m) concern the residual layer
during the night and the convective boundary layer
(CBL) during the day. 06/14 and 06/16 days are
characterized by weak wind speed conditions with V < 5
m.s- 1 whereas June 15 is particularly windy with V ~ 10
m.s- 1. The three CBLs are characterized by cumulus
clouds and large entrainment zones (especially for June
14 and 15). The June 15 night is disturbed by a strong
nocturnal jet with V > 10 m.s- 1 .
Fig. 3: (a) Off-line Carrier to Noise Ratio (CNR) (b) Vertical
velocity (w) and horizontal wind (c) speed (V) and (d)
direction (dirV) as a function of the local time.
Fig. 4: (a) In-situ CO 2 mixing ratio measurements at 11, 30,
76, 122, 244 and 396 m. (b) In-situ eddy-covariance CO 2 flux
at 396 m (grey line) and 1.5-km-ABL-mean lidar CO 2 flux
estimates using the eddy-covariance technique on 150-m-80-s
(orange dot) and 300-m-160-s (purple square) rolling
averaged lidar CO2 mixing ratio and vertical velocity
measurements. The error bars represent the standard deviation
of range-resolved fluxes in the 1.5 km vertical range gate (0.3
– 1.5 km) divided by the square root of the number of
samples. The blue dashed line is for the in-situ water vapor
eddy-covariance flux at 396 m used to correct H2O bias on
CO2 flux measurements.
15th Coherent Laser Radar Conference
Downward slanting structures in the aerosol profile lead
us to suspect additional significant subsidence motion
during June 14 night. During June 16 night, a
thunderstorm gave some rain around 3 h.
Figure 4b shows CO2 eddy-covariance flux
measurements inferred from the in-situ sensors at 396 m
and the 2-µm Doppler DIAL. The mean flux lidar
measurements are estimated in a temporal and vertical
range gate of ~ 6 h and 1500 m (from 300 m to 1800 m),
respectively. The lidar flux results displayed in Figure
14b are calculated from different time and space
resolution of CO2 mixing ratio and vertical velocity
estimates, 150 m – 80 s (dots) and 300 m – 160 s
(squares) in order to reduce as much as possible the
statistical error on CO2 mixing ratio measurements. The
flux measurements are corrected from the bias due to
parasitic H2O absorption. During the daytime, although
large statistical errors exist, lidar CO 2 flux estimates are
negative like the in-situ measurements. A CO2 uptake by
the vegetation creates a sink in the surface layer,
corresponding to a negative CO2 flux at the bottom of
the CBL. In addition, NOAA airborne measurements
around the WLEF site (46.00±0.05°N, 90.17±0.03°W)
report a free troposphere CO2 mixing ratio of 384.5 ±
0.4 ppm from 2200 m up to 3900 m on June, 11, 2007.
Therefore, the free troposphere represents a source of
CO2 for the ABL and we expect a negative flux of CO 2
at the top of the CBL. A mean ABL negative eddy-
covariance CO2 flux during the daytime is thus expected
and verified with the measurements. During the night
time, lidar flux measurements are in agreement with the
in-situ measurements within the error bars. The increase
of CO2 for the different levels below 244 m shows the
build-up of CO2 in the nocturnal layer due to a positive
CO2 surface flux linked to the vegetation respiration. At
the top of the residual layer, because of larger free
tropospheric CO2 mixing ratio than in the residual layer,
entrainment flux is expected to be negative. These
considerations explain the in-situ negative CO2 flux
measured during 06/15 and 06/16 nights, both due to
intrusion of free tropospheric air in the residual layer.
Acknowledgments. This research was funded by the
NASA Instrument Incubator Program and NASA Laser
Risk Reduction Program. We thank R. Strand and J.
Ayers of the Wisconsin Educational Communications
Board for hosting the lidar at the WLEF tower.
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Turbulent CO2 Flux Measurements by Lidar: Length Scales, Results and Comparison with In-Situ Sensors

Turbulent CO2 Flux Measurements by Lidar: Length Scales, Results and Comparison with In-Situ Sensors

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