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Advection of carbon dioxide in the presence of storm systems over a
northern Wisconsin forest
Michael D. Hurwitz and Daniel M. Ricciuto
Kenneth J. Davis, Weiguo Wang, Chiuxiang Yi, Martha P. Butler
The Pennsylvania State University, University Park, PA
Peter S. Bakwin
NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO
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Abstract:
Mixing ratios of CO2 often change abruptly in the presence of inclement weather and low-pressure
systems. We used water vapor mixing ratio, temperature, wind speed, and wind direction data to
infer that the abrupt changes in CO2 mixing ratios at a site in northern Wisconsin, USA, are due to
deep tropospheric mixing, horizontal transport, or a combination of both processes. We examined
four different scenarios: the passage of a summer cold front, a summer convective storm, an early
spring frontal passage, and a late autumn low pressure system. We observed synoptic scale
horizontal transport of CO2, as well as deep tropospheric mixing. Deep tropospheric mixing caused
CO2 mixing ratios to change rapidly. In one summer convective event, CO2 mixing ratios rose more
than 22 ppm in just 90 seconds. Synoptic scale transport is also evident in the presence of storm
systems and frontal boundaries. In the cases that we examined, synoptic scale transport changed
CO2 mixing ratios as much as 15 ppm in a one-hour time period. We selected events that represent
extremes in the rate of change of boundary layer CO2 mixing ratios, excluding venting of a shallow,
stable boundary layer. The rapid changes in CO2 mixing ratios that we observed imply that large
mixing ratio gradients must exist, often over rather small spatial scales, in the troposphere over
North America.
I. Introduction
Carbon dioxide mixing ratios in the atmosphere over North America have diurnal and
seasonal cycles resulting from respiration and photosynthesis. Typical seasonal variations in mean
CO2 mixing ratios in the continental atmospheric boundary layer (ABL) are about 20 ppm (Bakwin
et al., 1998). Highest mean atmospheric boundary layer carbon dioxide mixing ratios in the wellmixed planetary boundary layer at WLEF occur in February, and the lowest occur in July. The
difference in mean mixing ratio between February and July is about 20 ppm (Bakwin et al., 1998),
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(Davis et al., submitted). On shorter time scales, CO2 mixing ratios in the ABL may change very
rapidly, and are associated with passage of disparate air masses separated by a frontal system.
Similar dramatic changes also occur with venting of the nocturnal boundary layer, and are driven
by diurnal changes in the rate of vertical mixing in the ABL. (Yi et al.,2001;Yi et al., submitted) In
the northern coterminous USA the air mass on the west side of a cold frontal system typically
originates in Canada, hence the CO2 changes across the cold frontal system may help to
demonstrate the range of CO2 mixing ratios present across the northern portion of continental North
America (Pettersen, 1956). When strong convection is also associated with the cold front passages,
deep vertical mixing brings CO2 mixing ratios typical of the upper troposphere to the surface.
An understanding of the degree of variation in CO2 mixing ratios in the continental ABL in
space and time will aid in the design of observational networks used to determine regional to
continental CO2 budgets. Quantifying regional to continental CO2 budgets at synoptic to seasonal
time scales will require a synthesis of measurements and models of ecophysiological processes,
boundary-layer dynamics and synoptic meteorology. This paper examines changes in CO2 mixing
ratio associated with synoptic weather systems.
2. Study Site and Methods
The study was carried out at the 447-m-tall WLEF-TV tower (45o 55' N, 90o 10' W) in the
Chequamegon National Forest, 14 km east of Park Falls, Wisconsin, USA. The site and
measurements have been described by Bakwin et al. (1998). Mixing ratios of CO2 were measured
continuously at 11, 30, 76, 122, 244, and 396 m above the ground. Temperature, wind, water vapor
mixing ratio, and turbulent fluxes of CO2, momentum, latent heat, and sensible heat were measured
at 30, 122, and 396 m (Berger et al., 2001). During the events examined in this paper, rapid changes
in CO2 mixing ratio were coincident with changes in temperature and water vapor. The changes
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occurred at all observation heights, making it probable that these events encompassed the entire
depth of the ABL. This also excludes instrumental error as a source of the abrupt CO2 changes.
Because of occasional failure of sonic anemometers at certain levels, we interpret data from
varying levels, depending on data availability.
3 Results and Analysis:
3.1 July 14, 1998: summer cold front
Figure 1 shows changes in meteorological conditions during passage of a typical midsummer cold front. The frontal passage caused the wind to shift from the southwest to the northnortheast. The sharp drop in temperature after the frontal passage is concurrent with the wind shift
as well as a substantial drop in the water vapor mixing ratio. The carbon dioxide mixing ratio rose
more than 22 ppm in just 90 seconds with the frontal passage, and remained elevated for several
days before slowly decreasing to the mixing ratios found prior to the frontal passage.
At the time of frontal passage, there was no precipitation but there was a 15-minute period
of strong downdrafts, with vertical velocity between –0.5 m s-1 and –1.0 m s-1 at 30 m (Figure 1f).
This indicates deep vertical mixing associated with the frontal passage.
Figure 2 shows mixing ratios of CO2 in the free troposphere at Niwot Ridge, Colorado
(40.05°N, 105.58°N), as well as data from aircraft flights over Carr, Colorado (40.90°N,
104.80°W), and flights above the WLEF tower in Wisconsin during 1998. Niwot Ridge is located
at 3475 m above sea level (ASL) in the Rocky Mountains, and air is sampled during down-slope
winds which bring free tropospheric air to the site. The free troposphere is located above the ABL,
typically beginning between 1000 meters and 3000 meters above ground. ( Yi et. al., submitted) In
order to get the best estimate for CO2 in the free troposphere from the aircraft data, we used
measurements from the highest altitudes that were available, which were 7000 m ASL at Carr and
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5000 m ASL over the WLEF tower. This minimized the likelihood that our reported tropospheric
CO2 values might be contaminated by residual layers that commonly lie just above the ABL.
The differences in the CO2 between the three sites were relatively small, often less than 2
ppm, and rarely as high as 4 ppm. In mid-July of 1998 free troposphere CO2 mixing ratios were
about 366 ppm at all three sites, very close to the value that was recorded at 30 m above the ground
at WLEF immediately after the frontal passage on 14 July (Figure 1). In the several hours after the
frontal passage, CO2 at 396 m above the ground dropped to 352 ppm (Figure 3), which was likely
caused by horizontal transport of CO2 (Yi et al., 2000). Increasing CO2 at 30 m and 122 m several
hours after the frontal passage are not caused by sustained downdrafts, but are a result of high static
stability of the nocturnal boundary layer (Yi et al., 2001), which trapped CO2 from respiration of
the soils and plants near the ground.
It is clear that the dramatic change in CO2 mixing ratio that we observed associated with the
frontal passage on 14 July, 1998, can not be attributed to sudden changes in biological sources or
sinks. In order to understand fully the significance of such a rapid change in carbon dioxide mixing
ratio, it is useful to look at typical CO2 surface flux values, and compare them to the respiration
flux that would have been necessary to create the drastic change that was evident during the frontal
passage on July 14, 1998. Typical summer respiration rates near WLEF are about 5 mol m-2 s-1
(Davis et al., 2002; Yi et al., 2000), a 22 ppm change in CO2 in 90 seconds would require a surface
flux of at least 10000 mol m-2 s-1, assuming a boundary layer depth of 1500 meters. This is not
biologically feasible.
3.2 July 26, 2000: summer squall ahead of a cold front
On 26 July, 2000, a dramatic rise in CO2 was observed at WLEF about 15 hours prior to the
passage of a trough (Figure 4). A wind shift, temperature drop, and CO2 increase occurred
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concurrently with passage of a squall between 0400 and 0500 UTC. About 10 mm of rain fell at
WLEF during the squall, indicating that it was relatively intense. It is likely that this is a case in
which both horizontal and vertical transport of CO2 occurred. Although vertical velocity values at
WLEF do not show any sustained, vigorous downdrafts during the squall, a drop in water vapor
mixing ratio indicates that the air most likely had origins in the free troposphere. Westerly winds
associated with the squall may have transported air from the free troposphere. The period during
which there were westerly winds is initially accompanied by a drop in the water vapor mixing ratio
and by wind speeds that reached 15 m s-1. Downward transport of momentum is likely associated
with downward transport of CO2. The CO2 mixing ratio above the nocturnal stable layer (396 m)
remained unchanged for several hours after 0500 UTC. Subsequently, we observed modification,
presumably by a combination of biological fluxes and advection. Water vapor mixing ratios also
climbed to the pre-squall levels, but temperature remained fairly constant.
At the Willow Creek tower, 25 km southeast of WLEF, the squall passed almost 30 minutes
after moving past WLEF. The CO2 sensor at 30 m (Cook et al., submitted) at Willow Creek
registered a similar jump in CO2 (Figure 5), illustrating that this abrupt change in CO2 was not
limited to a small area. Gradients of CO2 between WLEF and Willow Creek prior to squall passage
are more difficult to interpret. Although u* values, which indicate the amount of horizontal mixing,
at WLEF were between 0.73 and 1.0 during the hours between 0000 UTC and 0400 UTC, there
was a vertical gradient of CO2 of about 7 ppm during these hours between 30 and 396 m (Figure 6).
Horizontal gradients in CO2 between WLEF and Willow Creek may be a result of local respiration
differences, and conditions in which mixing is insufficient to create CO2 mixing ratios that are
vertically and horizontally homogenous (Wang et al., in prep.).
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The CO2 mixing ratio continued to increase to nearly 375 ppm at both towers after the
squall passage. It is important to note that the squall passage occurred at night when the 30 m level
is usually within the stable surface layer (Yi et. al, 2001). After the squall passage, a stable
nocturnal boundary layer reformed, and subsequent increases in CO2 near the ground were
probably due to respiration. We determine the CO2 mixing ratio change brought by the squall
passage by looking at data from higher levels (Figure 6). CO2 mixing ratios at 30, 122 and 396 m at
WLEF were relatively consistent at 362 ppm just after the squall passage. Data from Niwot Ridge
and flights over Carr, Colorado indicate that CO2 mixing ratios in the free troposphere were
between 368 and 370 ppm in late July 2000, close to those observed at WLEF after passage of the
squall. We might expect that the CO2 mixing ratio at WLEF after the squall passage was lower than
in the free troposphere over Colorado because deep vertical mixing caused tropospheric air to mix
with ABL air, which had mixing ratios of CO2 as low as 340 ppm.
3.3 March 28,1998: spring cold front
A strong cold front on March 28, 1998, was associated with a 13 ppm change in CO2
(Figure 7). There were three distinct events of interest on this day. First, at 1000 UTC there was a
rapid drop in CO2 at all levels on the tower. The second event involved a rise in CO2 near 1600
UTC, and the third was a gradual decrease in CO2 during 1700-2100 UTC. At 1000 UTC no rain
was recorded, and vertical velocity data was not available at WLEF. A small decrease in the water
vapor mixing ratio and temperature was associated with the decrease in CO2 mixing ratio at 1000
UTC (Figure 8). The cold front passed at about 1600 UTC, and about 3 mm of rain fell during
1600-1700 UTC. Winds shifted from the southwest to the northwest as the front passed, and both
the temperature and the water vapor mixing ratio dropped (Figure 8).
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At 1000 UTC, the drop in CO2 may have been due to deep vertical mixing, shallow vertical
mixing, or possibly horizontal transport. An argument for deep vertical mixing can be made by
examining the mixing ratios of CO2 in the upper troposphere. Aircraft data obtained above Carr,
CO, and WLEF, and data from the high altitude site at Niwot Ridge, CO, show that CO2 in the free
troposphere in late March and early April of 2000 was between 367 and 371 ppm (Table 1). At
1000 UTC, CO2 mixing ratios at 396 m fell to about 371 ppm. We expect that after this deep
vertical mixing event, CO2 mixing ratios at the tower would be slightly higher than in the free
troposphere because air from the upper-troposphere will entrain surface and boundary layer air,
which have higher CO2 mixing ratios during late March. Failure of the sonic anemometers at
WLEF near the time of the vertical transport phenomenon prohibited us from obtaining accurate
wind measurements, so we have no evidence of a sustained downdraft. However, water vapor and
temperature changes at 1000 UTC are both small and gradual. If the mixing event were due to
entrainment of free troposphere air through deep vertical mixing, we might have expected a more
dramatic decline in temperature and water vapor mixing ratio.
Table 1: Mixing Ratios of CO2 in the free troposphere over North America
Location
Date
CO2 mixing ratio(ppm)
Niwot Ridge, CO
March 24, 1998
368
Niwot Ridge, CO
March 31, 1998
371.5
Carr, CO
March 25, 1998
367.2
WLEF (aircraft)
April 10, 1998
368
A more likely explanation of the change in CO2 at 1000 UTC is shallow vertical mixing.
Prior to 1000 UTC, CO2 mixing ratios at 122 and 396 m increased to nearly 380 ppm. The increase
of CO2 at 396 m indicates that the nocturnal boundary layer was deeper than 396 m. At 1000 UTC,
CO2 mixing ratios fell simultaneously at all levels to about 371 ppm. Although this value is near the
mixing ratio of the free troposphere, it is also about the CO2 mixing ratio that was present at 396 m
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between 0500 UTC and 0700 UTC. One possibility is that the nocturnal boundary layer exchanged
air with the residual layer above. A shallow mixing event would mix boundary layer and residual
layer air, but might not mix the much drier air of the free troposphere. Lack of u* data prevents us
from diagnosing the extent of the turbulence which might have mixed the residual layers and
nocturnal boundary layers. Horizontal transport may also explain our observations. A well-mixed
layer with lower CO2 mixing ratio values may have been advected into the WLEF region.
The sharp increase in CO2 near 1600 UTC was accompanied by a rapid decrease in
temperature and water vapor mixing ratio, as well as a shift in the wind direction from the
southwest to the northwest (Figure 8), associated with a cold front passage. Mixing ratios of CO2
approached 385 ppm, much higher than the free troposphere CO2 mixing ratios given in Table 1. If
deep vertical mixing were responsible for the rapid transition in CO2, we would expect all levels on
the tower to have approximately the same CO2 mixing ratio after the onset of the transition. After
the 16 UTC transition a vertical gradient of as much as 10 ppm was observed (Figure 7). A gradual
decrease in CO2 mixing ratio between the hours of 1700 UTC and 2100 UTC most likely resulted
from horizontal advection, as photosynthesis is probably too slow to alter significantly the
atmospheric boundary layer CO2 mixing ratios in late March.
3.4 November 10, 1998: Movement of a low pressure center over WLEF
On November 10, 1998, CO2 changed by about 9 ppm in a 30 min period in the presence of
a vigorous low pressure system that passed 50-100 km to the north of WLEF (Figure 9). Sea-level
pressure at the center of the low dipped to nearly 967 mb as the low passed by the tower. At the
tower winds shifted from southerly to westerly between 16 and 18 UTC. Biological phenomena
could not have produced such a rapid change in CO2; assuming an ABL height of 1 km, (Yi et
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al.,2001) mean surface flux of nearly 200 μmoles m-2 s-1 would have been necessary. This is about
two orders of magnitude higher than typical winter surface fluxes of CO2 (Davis et al., 2002).
Downward mixing of free troposphere air is a possible cause of the abrupt drop in CO2 that
occurred at about 16:45 UTC (Figure 9). CO2 measured from flight data over Carr, Colorado, and
over WLEF indicate that the CO2 mixing ratio values in the free troposphere were about 366 ppm.
Horizontal advection was another possible cause of the drop in CO2, (Yi et al.,2000) because CO2
mixing ratios in the marine boundary layer at 46oN at this time were around 368.3 ppm.
Figure 11 highlights the transition period on November 10, 1998, and presents atmospheric
data between 1600 UTC and 1900 UTC. The rapid decline in CO2 mixing ratio at 16.85 UTC
corresponds with a 2°C increase in temperature as well as a small increase in water vapor mixing
ratio. If the abrupt changes in CO2 concentration were due to deep tropospheric mixing, we would
expect that the temperature and water vapor mixing ratio would have dropped, similar to the deep
mixing event presented in Figure 1. Atmospheric soundings near Green Bay, WI, from the
University of Wyoming archives (Wyoming Weather Web) confirm that water vapor mixing ratios
decrease with height. We did not observe any periods of sustained negative vertical velocity near
the time of the decreasing CO2 concentration, further corroborating that the abrupt change of CO2
was not caused by proximal vertical transport.
A warm, moist air mass was horizontally advected into the region of WLEF as the low
pressure system moved by to the north of WLEF. The rebounding CO2 mixing ratio near 17:30
UTC corresponds to a sharp dip in temperature and water vapor mixing ratio, reflecting the
presence of a third air mass at the tower. The air mass that replaced the warm, moist air mass was
different from the air mass prior to arrival of the moist air, as both water vapor mixing ratios and
temperature were markedly lower at 18:00 UTC than at 16:00 UTC. Examination of data obtained
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at fixed points (such as towers) during periods of strong horizontal transport, like the event that
occurred on November 10, 1998, may be utilized to further understand horizontal gradients in CO2
that exist across continental North America.
4. Relating Horizontal Scale Transport of CO2 with Location of Origin
Events in which synoptic transport of CO2 are examined may be particularly useful in
assessing horizontal gradients that exist across North America. Figure 12 compares CO2 mixing
ratios at WLEF with CO2 mixing ratios at the Northern Old Black Spruce Site (NOBS) (55o 53’ N,
98o 29’W) in Manitoba (Goulden et al., 1997). What is particularly interesting from this plot is that
the difference between CO2 mixing ratio values at the two sites depends on the time of the year. In
the winter of 1998, NOBS had a lower CO2 mixing ratio than WLEF. Photosynthesis does not
occur at either location in the winter, but greater rates of respiration and emissions of CO2 from
anthropogenic activities in the region around the WLEF tower increases the CO2 during the winter.
Similarly, CO2 mixing ratios at WLEF were lower in the growing season that at NOBS in 1998,
perhaps indicating that net uptake of CO2 in summer by the forests around WLEF is much stronger
than at NOBS.
The difference in CO2 mixing ratios between WLEF and NOBS indicates a north-south
gradient of CO2 that varied seasonally during 1998. The events of July 14 and March 28, 1998,
shed light on the north-south gradients in CO2 over North America during 1998. On July 14, 1998,
we concluded that deep vertical mixing was responsible for the sharp increase in CO2; however, a
period of strong southerly winds prior to the frontal passage was accompanied by a gradual
decrease of CO2 by about 15 ppm over a three hour period. This indicates that the north-south
gradient in CO2 between WLEF and NOBS in summer of 1998 extended south of WLEF. That is,
mean CO2 mixing ratios continue to decrease south of WLEF, likely from differences in net rates of
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uptake by the biosphere. It is important to note that summer CO2 mixing ratios may not always
decrease with decreasing latitude during the summer. Aircraft data from August 2000 may suggest
that CO2 mixing ratios may be higher at lower latitudes in continental North America. North-south
gradients in CO2 may vary on both an annual and seasonal basis, and synoptic phenomena allows
us to infer the sign of these gradients (Gerbig et al., manuscript in preparation). Varying climatic
conditions across continental North America, such as drought, or excessive rainfall may play a
large role in creating and maintaining gradients in CO2 mixing ratios.
On March 28, 1998, the rapid increase in CO2 near 1500 UTC is due to synoptic transport,
and the shift in wind direction from a southerly wind to a northerly wind at the time of the abrupt
change in CO2 suggests that the air mass associated with high CO2 mixing ratios originated to the
north of WLEF. Figure 12 shows that during late March, the gradient in CO2 concentration between
NOBS and WLEF was small. The increase in CO2 associated with the frontal passage at 1500 UTC
on March 28, 1998 cannot be explained by deep vertical mixing, and the increase in CO2 mixing
ratios with northerly winds cannot be explained from the seemingly negligible north-south gradient
in CO2 obtained from examination of Figure 12. Perhaps inferring a constant gradient in CO2
mixing ratios from two towers is incorrect. East-west gradients in CO2 may also exist. The origin of
the air mass behind the cold front on March 26, 1998, has a northerly component, but CO2 mixing
ratios at NOBS are far from those observed after the cold front passage.
5. Conclusions
Frontal systems and squalls are generally associated with synoptic transport of CO2 as well
as deep vertical mixing of CO2 from the free troposphere to the surface. CO2 mixing ratios change
rapidly in these situations. The rate of change of CO2 mixing ratios excludes local biological
exchange as the cause of the rapid transitions. The deep vertical mixing that occurs during
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downdrafts and squalls allows us to infer the CO2 mixing ratio in the free troposphere from surface
measurements. We think this paper provides sufficient criteria for the identification of deep vertical
mixing. Monitoring deep vertical mixing events at various towers that measure CO2 will facilitate
the determination of CO2 mixing ratios in the free troposphere, even in the absence of direct free
troposphere observations. Identification of synoptic transport of CO2 across frontal systems allows
us to examine the most dramatic horizontal gradients of CO2. Lack of an extensive network of CO2
measurement towers prohibits us from determining the origins of the air masses using only CO2
data. However, meteorological quantities like water vapor mixing ratio, temperature, and wind
speed will allow us greater understanding of the air mass origins. Future work will likely consist of
a more thorough examination of synoptic transport across frontal systems. By observing CO2
changes across frontal boundaries over a prolonged period of time, we can start to develop a more
thorough understanding of the north-south and east-west CO2 gradients that exist in North America
on seasonal time scales.
Acknowledgements
We thank the State of Wisconsin Educational Communications Board for use of the WLEF-TV
transmitter tower, and Mr. R. Stand, chief engineer for WLEF-TV (Park Falls, WI). We also thank
Mr. R. Teclaw (U.S. Department of Agriculture Forest Service, Rhinelander, WI) for important
help with maintenance of our equipment at the tower. Finally, we thank Allison Dunn at Harvard
for providing the NOBS data. This work was supported in part by the Atmospheric Chemistry
Project of the Climate and Global Change Program of the National Oceanic and Atmospheric
Administration, and by the National Institute for Global Environmental Change of the U.S.
Department of Energy.
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results of the NOAA/CMDL program. Tellus, 50B, 401-415, 1998.
Berger BW, Davis KJ, Yi C, Bakwin PS, Zhao CL Long-term carbon dioxide fluxes from a
very tall tower in a northern forest: Flux measurement methodology. Journal of
Atmospheric and Oceanic Technology 18,529-542, 2001.
Cook, B.D., K.J. Davis, W. Wang, C. Yi, B.W. Berger, P.V. Bolstad, P.S.
Bakwin, J.G. Isebrands and R.M. Teclaw, Annual pattern of carbon exchange
and evapotranspiration of an upland hardwood forest in northern Wisconsin.
To be submitted.
Davis, K.J., P.S. Bakwin, B.W. Berger, C. Yi, C. Zhao, R.M. Teclaw and
J.G. Isebrands, Long-term carbon dioxide fluxes from a very tall tower in
a northern forest: Annual cycle of CO2 exchange. Accepted, Global Change
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Gerbig, C., J.C. Lin, S.C. Wofsy, B.C. Daube, A.E. Andrews, B.B. Stephens, P.S. Bakwin, J. Stith,
and A. Grainger, Constraining regional to continental scale fluxes of CO2 with atmospheric
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Yi, C., Davis, K.J., Bakwin, P.S., Berger, B.W., Marr, L.C. Influence of advection on
measurements of the net ecosystem-atmosphere exchange of CO2 from a very tall tower. J.
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Yi, C., K. J. Davis, P. S. Bakwin, A.S. Denning, N. Zhang, J. Ch.-H. Lin,
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Figure 1. Atmospheric changes at 30 m on the WLEF tower on July 14, 1998 during a frontal
passage. Wind direction is defined as the angle from which the wind is coming. An angle of 0 is
wind from the North, an angle of 90 is wind the East etc. Data are two-minute averages. Local
time at WLEF is six hours behind UTC.
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Figure 3. Two-minute average CO2 mixing ratios at 30, 122, and 396 m on the WLEF tower. Data
for each level is plotted every 12 minutes. Hour 24 corresponds to 0 UTC on July 15, and local
time is 5 hours behind UTC.
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Figure 2. Carbon dioxide mixing ratios at Niwot Ridge and from flights over Carr, Colorado, and
the WLEF tower in northern Wisconsin, for the year 1998.
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Figure 4. Atmospheric data from the WLEF tower on July 26, 2000. All data was recorded at 122
meters, with the exception of the CO2 concentration, which was recorded at 396 meters. (d) No
wind data were available at 396 meters, but we analyzed CO2 concentrations at this level to
minimize the influence of nighttime respiration.
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Figure 5. CO2 concentration changes at both WLEF tower and Willow Creek on July 26, 2000,
confirming the large-scale effects of the squall. Changes at Willow Creek occur after those at
WLEF, as the effects of the squall proceed from west to east.
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Figure 6. CO2 mixing ratios at 30 (dotted line), 122 (dashed line) and 396 (solid line) m above the
ground on the WLEF tower on July 26, 2000.
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Figure 7. Time series of CO2 at 30, 122, and 396 m on the WLEF tower on March 28, 1998.
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Figure 8. Atmospheric changes at 30 m at the WLEF tower on March 28, 1998, in which
horizontal and vertical transport phenomena are explored. The CO2 concentration in this figure is
about 3 ppm lower than the true value of CO2 due to calibration errors in the 5 Hz data.
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Figure 9. Time series of CO2 mixing ratios at 30, 122, and 396 m on the WLEF tower on
November 10, 1998.
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Figure 10. Atmospheric data from 30 m on the WLEF tower during a low-pressure passage on
November 10, 1998. Data is reported using 2-minute averages.
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Figure 11. Atmospheric data from 30 m on the WLEF tower during a low-pressure passage on
November 10, 1998. Data is reported using 2-minute averages.
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Figure 12. Five-day average CO2 mixing ratio during 1998 at WLEF and NOBS in Manitoba.
Measurements were taken between 11:00 and 16:00 local time at 30 m above the ground