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1.
One-dimensional Lagrangian dispersion models, frequently used to relate in-canopy source/sink distributions of energy, water
and trace gases to vertical concentration profiles, require estimates of the standard deviation of the vertical wind speed,
which can be measured, and the Lagrangian time scale, T
L
, which cannot. In this work we use non-linear parameter estimation to determine the vertical profile of the Lagrangian time
scale that simultaneously optimises agreement between modelled and measured vertical profiles of temperature, water vapour
and carbon dioxide concentrations within a 40-m tall temperate Eucalyptus forest in south-eastern Australia. Modelled temperature
and concentration profiles are generated using Lagrangian dispersion theory combined with source/sink distributions of sensible
heat, water vapour and CO 2. These distributions are derived from a multilayer Soil-Vegetation-Atmospheric-Transfer model subject to multiple constraints:
(1) daytime eddy flux measurements of sensible heat, latent heat, and CO 2 above the canopy, (2) in-canopy lidar measurements of leaf area density distribution, and (3) chamber measurements of CO 2 ground fluxes. The resulting estimate of Lagrangian time scale within the canopy under near-neutral conditions is about 1.7
times higher than previous estimates and decreases towards zero at the ground. It represents an advance over previous estimates
of T
L
, which are largely unconstrained by measurements. 相似文献
2.
Turf-grass lawns are ubiquitous in the United States. However direct measurements of land–atmosphere fluxes using the eddy-covariance
method above lawn ecosystems are challenging due to the typically small dimensions of lawns and the heterogeneity of land
use in an urbanised landscape. Given their typically small patch sizes, there is the potential that CO 2 fluxes measured above turf-grass lawns may be influenced by nearby CO 2 sources such as passing traffic. In this study, we report on two years of eddy-covariance flux measurements above a 1.5 ha
turf-grass lawn in which we assess the contribution of nearby traffic emissions to the measured CO 2 flux. We use winter data when the vegetation was dormant to develop an empirical estimate of the traffic effect on the measured
CO 2 fluxes, based on a parametrised version of a three-dimensional Lagrangian footprint model and continuous traffic count data.
The CO 2 budget of the ecosystem was adjusted by 135gCm −2 in 2007 and by 134gCm −2 in 2008 to determine the natural flux, even though the road crossed the footprint only at its far edge. We show that bottom-up
flux estimates based on CO 2 emission factors of the passing vehicles, combined with the crosswind-integrated footprint at the distance of the road, agreed
very well with the empirical estimate of the traffic contribution that we derived from the eddy-covariance measurements. The
approach we developed may be useful for other sites where investigators plan to make eddy-covariance measurements on small
patches within heterogeneous landscapes where there are significant contrasts in flux rates. However, we caution that the
modelling approach is empirical and will need to be adapted individually to each site. 相似文献
3.
Energy and CO 2 fluxes are commonly measured above plant canopies using an eddy covariance system that consists of a three-dimensional sonic anemometer and an H 2O/CO 2 infrared gas analyzer. By assuming that the dry air is conserved and inducing mean vertical velocity, Webb et al. ( Quart. J. Roy. Meteorol. Soc. 106, 85-100, 1980) obtained two equations to account for density effects due to heat and water vapour transfer on H 2O/CO 2 fluxes. In this paper, directly starting with physical consideration of air-parcel expansion/compression, we derive two alternative equations to correct for these effects that do not require the assumption that dry air is conserved and the use of the mean vertical velocity. We then applied these equations to eddy flux observations from a black spruce forest in interior Alaska during the summer of 2002. In this ecosystem, the equations developed here led to increased estimates of CO 2 uptake by the vegetation during the day (up to about 20%), and decreased estimates of CO 2 respiration by the ecosystem during the night (approximately 4%) as compared with estimates obtained using the Webb et al. approach. 相似文献
4.
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO 2 flux estimates obtained from open-path eddy-covariance systems. In our analysis, we present equations to analyse the propagation of errors through the Webb, Pearman, and Leuning (WPL) algorithm [ Quart. J. Roy. Meteorol. Soc. 106, 85–100, 1980] that is widely used to account for density fluctuations on CO 2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO 2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO 2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO 2 fluxes. We use open-path eddy-covariance fluxes measured over a black spruce forest in interior Alaska to explore several energy imbalance scenarios and their consequences for CO 2 fluxes. 相似文献
5.
The Bowen-ratio profile method for calculating total carbon dioxide, latent heat, and sensible heat flux density above a corn crop was used by measuring temperature, water vapor, and CO 2 concentrations at several heights in the aerodynamic boundary layer of the crop. The ratio (α) of sensible heat flux density to carbon dioxide flux density as well as the Bowen ratio (β) were used in the computations. The two ratios, α and β were determined graphically from the slopes of linear plots of temperature vs CO 2 concentration and vs water vapor. Each of the energy flux densities was computed from the two ratios and net radiation minus soil heat flux density. An analysis of probable error was performed on the Bowen-ratio profile method to evaluate the accuracy of the flux density estimates. Less than 10% error was found for latent heat flux density and less than 15% for carbon dioxide flux density under normal midday conditions for the instrumentation used. However, the carbon dioxide flux density error increased to over 40% when the sensible heat flux was small. 相似文献
6.
Methods of calibrating infrared CO 2 analysers for sensitivity to CO 2 and water vapour are described. Equations to correct eddy covariance CO 2 flux measurements are presented for: (i) analyser cross-sensitivity to water vapour and the effects of density fluctuations arising from atmospheric fluxes of water vapour and sensible heat, (ii) flux losses caused by signal processing and limited instrument frequency response for open- and closed-path CO 2 analysers, and (iii) flux losses resulting from damping of concentration fluctuations in a tube used to sample air for closed-path CO 2 analysers. Examples of flux corrections required for typical instruments are presented. 相似文献
7.
Abstract Fluxes of temperature, water vapour, O 3, SO 2 and CO 2 were estimated from the measurement of their variances, taken over a wetland region in northern Ontario (Canada) during the summer of 1990 and over a deciduous forest when it was fully leafed during the summer of 1988 and when it was leafless during the winter of 1990. A set of flux‐variance relations was employed, including empirical forms of universal functions that could be adjusted with some constants. Results from the present study show that these constants needed to be adjusted with site‐specific data in order to achieve a closer agreement between estimated and observed fluxes. Best estimates were obtained for the fluxes of temperature and water vapour and it was found that the flux estimates of O 3, SO 2 and CO 2 correlated better with water vapour than with temperature. For these trace gases the flux‐variance method yielded estimates of dry deposition velocities that were either comparable with or larger than those obtained from a resistance analogue model. Both methods yielded values that overestimated the observed dry deposition velocities. The employment of the flux‐variance method in an operational network would require the use of fast‐response sensors and a practical method for reducing the noise level of the measured variances. 相似文献
8.
It is important to improve estimates of large-scale carbon fluxes over the boreal forest because the responses of this biome to global change may influence the dynamics of atmospheric carbon dioxide in ways that may influence the magnitude of climate change. Two methods currently being used to estimate these fluxes are process-based modeling by terrestrial biosphere models (TBMs), and atmospheric inversions in which fluxes are derived from a set of observations on atmospheric CO 2 concentrations via an atmospheric transport model. Inversions do not reveal information about processes and therefore do not allow for predictions of future fluxes, while the process-based flux estimates are not necessarily consistent with atmospheric observations of CO 2. In this study we combine the two methods by using the fluxes from four TBMs as a priori fluxes for an atmospheric Bayesian Synthesis Inversion. By doing so we learn about both approaches. The results from the inversion indicate where the results of the TBMs disagree with the atmospheric observations of CO 2, and where the results of the inversion are poorly constrained by atmospheric data, the process-based estimates determine the flux results. The analysis indicates that the TBMs are modeling the spring uptake of CO 2 too early, and that the inversion shows large uncertainty and more dependence on the initial conditions over Europe and Boreal Asia than Boreal North America. This uncertainty is related to the scarcity of data over the continents, and as this problem is not likely to be solved in the near future, TBMs will need to be developed and improved, as they are likely the best option for understanding the impact of climate variability in these regions. 相似文献
9.
Carbon dioxide (CO 2) is an important greenhouse gas that influences regional climate through disturbing the earth’s energy balance. The CO 2 concentrations are usually prescribed homogenously in most climate models and the spatiotemporal variations of CO 2 are neglected. To address this issue, a regional climate model (RegCM4) is modified to investigate the non-homogeneous distribution of CO 2 and its effects on regional longwave radiation flux and temperature in East Asia. One-year simulation is performed with prescribed surface CO 2 fluxes that include fossil fuel emission, biomass burning, air–sea exchange, and terrestrial biosphere flux. Two numerical experiments (one using constant prescribed CO 2 concentrations in the radiation scheme and the other using the simulated CO 2 concentrations that are spatially non-homogeneous) are conducted to assess the impact of non-homogeneous CO 2 on the regional longwave radiation flux and temperature. Comparison of CO 2 concentrations from the model with the observations from the GLOBALVIEW-CO 2 network suggests that the model can well capture the spatiotemporal patterns of CO 2 concentrations. Generally, high CO 2 mixing ratios appear in the heavily industrialized eastern China in cold seasons, which probably relates to intensive human activities. The accommodation of non-homogeneous CO 2 concentrations in the radiative transfer scheme leads to an annual mean change of–0.12 W m –2 in total sky surface upward longwave flux in East Asia. The experiment with non-homogeneous CO 2 tends to yield a warmer lower troposphere. Surface temperature exhibits a maximum difference in summertime, ranging from–4.18 K to 3.88 K, when compared to its homogeneous counterpart. Our results indicate that the spatial and temporal distributions of CO 2 have a considerable impact on regional longwave radiation flux and temperature, and should be taken into account in future climate modeling. 相似文献
10.
Single-tower eddy-covariance measurements represent the complete surface flux of a scalar only under idealized conditions.
Therefore, we often find an underestimation of energy fluxes expressed as a lack of energy balance closure at many sites.
In this study, a multi-tower approach to measure atmospheric energy fluxes based on spatial averaging is evaluated and possible
mechanisms causing a lack of energy balance closure are analysed, focussing on daytime data only. It is shown that the multi-tower
technique is also unable to measure the entire flux for our site, likely because the assumption of horizontal homogeneity
is violated. Heterogeneity-induced and buoyancy-driven quasi-stationary circulations are probably the dominant processes causing
the underestimation of energy fluxes. A dependence of the energy balance residual on stability is found, with residuals close
to zero for stable stratification, a maximum under unstable to near-neutral conditions and still relatively large residuals
for stronger instability. Assuming the processes transporting energy and CO 2 are similar, the implications on long-term CO 2 flux measurements are analysed. Accordingly, the resulting selective systematic error of cumulative net ecosystem exchange
estimates for agricultural regions such as ours can be of the order of more than 100%, since mainly the fluxes during periods
of net CO 2 uptake are underestimated while periods of net CO 2 release are much less affected by this bias. Further investigations about this issue are highly warranted. 相似文献
11.
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO 2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions
(friction velocity greater than 0.1 ms –1), and uses measurements of mean wind speed, and standard deviations of temperature and CO 2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible
heat flux and CO 2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness
sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a
simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat
and CO 2 flux estimates that are well correlated with the measured values. 相似文献
12.
The flux footprint is the contribution, per unit emission, of each element of a surface area source to the vertical scalar flux measured at height z
m
; it is equal to the vertical flux from a unit surface point source. The dependence of the flux footprint on crosswind location is shown to be identical to the crosswind concentration distribution for a unit surface point source; an analytic dispersion model is used to estimate the crosswind-integrated flux footprint. Based on the analytic dispersion model, a normalized crosswind-integrated footprint is proposed that principally depends on the single variable z/z
m
, where z is a measure of vertical dispersion from a surface source. The explicit dependence of the crosswind-integrated flux footprint on downwind distance, thermal stability and surface roughness is contained in the dependence of z on these variables. By also calculating the flux footprint with a Lagrangian stochastic dispersion model, it is shown that the normalized flux footprint is insensitive to the analytic model assumption of a self-similar vertical concentration profile.The National Center for Atmospheric Research is funded by the National Science Foundation. 相似文献
13.
Measured and projected increases in carbon dioxide content of the atmosphere point towards a significant global warming. The
regional effects of such a warming will be of primary importance in determining the social and economic consequences. Four
methods of arriving at tentative regional scenarios are discussed and illustrated by application to Australia and New Zealand.
Methods used include numerical modelling, extreme warm and cold year ensembles, dynamical/empirical reasoning and palaeoclimatic
reconstructions from the Hypsithermal. A surprising degree of consistency is revealed between the various approaches to a
scenario for a CO 2-warmed Earth and the climatic conditions which prevailed during the Hypsithermal. The best overall analogy to a CO 2-warmed Earth seems to be this epoch, especially as recent evidence suggests it to be one of higher CO 2 concentrations. High priority should be given to further investigations using numerical models which include an interactive
dynamic ocean and hydrologic cycle including variable cloudiness, as well as more detailed reconstruction of climatic conditions
during the Hypsithermal in areas sensitive to any circulation changes. 相似文献
14.
An integrated canopy micrometeorological model is described for calculating CO 2, water vapor and sensible heat exchange rates and scalar concentration profiles over and within a crop canopy. The integrated model employs a Lagrangian random walk algorithm to calculate turbulent diffusion. The integrated model extends previous Lagrangian modelling efforts by employing biochemical, physiological and micrometeorological principles to evaluate vegetative sources and sinks. Model simulations of water vapor, CO 2 and sensible heat flux densities are tested against measurements made over a soybean canopy, while calculations of scalar profiles are tested against measurements made above and within the canopy. The model simulates energy and mass fluxes and scalar profiles above the canopy successfully. On the other hand, model calculations of scalar profiles inside the canopy do not match measurements.The tested Lagrangian model is also used to evaluate simpler modelling schemes, as needed for regional and global applications. Simple, half-order closure modelling schemes (which assume a constant scalar profile in the canopy) do not yield large errors in the computation of latent heat (LE) and CO 2 (F
c
) flux densities. Small errors occur because the source-sink formulation of LE and F
c are relatively insensitive to changes in scalar concentrations and the scalar gradients are small. On the other hand, complicated modelling frames may be needed to calculate sensible heat flux densities; the source-sink formulation of sensible heat is closely coupled to the within-canopy air temperature profile. 相似文献
15.
We have examined the potential of using a closed-path sensor to accurately measure eddy fluxes of CO 2. Five inlet tubeflow configurations were employed in the experimental setup. The fluxes of CO 2 were compared against those measured with an open-path sensor. Sampling air through an intake tube causes a loss of flux, due to the attenuation of CO 2 density fluctuations. Adjustments need to be made to correct for this loss and to account for density effects due to the simultaneous transfer of heat and water vapor. Theory quantifying these effects is discussed.The raw CO 2 flux measured with the closed-path sensor was smaller than that measured with the open-path sensor by about 15% (on average) for the turbulent tubeflow configurations with a short (3 m) intake tube, by 31% for turbulent tubeflow with a longer (6 m) intake tube and by 24% for laminar tubeflow. The difference was, in part, caused by tube attenuation of the CO 2 density fluctuations and inadequate sensor time response. The elimination of the flux adjustment for the simultaneous transfer of sensible heat (i.e., the attenuation of ambient temperature fluctuations in the intake tube) generally accounted for the rest of this difference.The raw flux measured with the closed-path sensor was corrected for frequency response and density effects. Except in the case of laminar tubeflow, the corrected closed-path flux agreed consistently with the corrected open-path flux within a few percent (<5%). These results suggest that closed-path sensors, with appropriate corrections, can be used to measure CO 2 flux accurately. Recommendations are included on selecting an optimum flow configuration to minimize the effect of sampling air through a tube.Published as Paper No. 9938, Journal Series, Nebraska Agricultural Research Division. 相似文献
16.
Using a climate model with a sophisticated land surface scheme, simulations were conducted to explore the impact of increases in leaf-level carbon dioxide (CO 2) on evaporation, temperature and other land surface quantities. Fifty-one realizations were run, for each of four Januarys and four Julys for CO 2 concentrations at leaf-level of 280, 375, 500, 650, 840 and 1,000 ppmv. Atmospheric CO 2 concentration was held constant at 375 ppmv in all experiments. Statistically significant decreases in evaporation and increases in temperature occur in specific regions as leaf-level CO 2 is increased from 280 to 375 ppmv. These same areas expand geographically, and the magnitude of the changes increase as leaf-level CO 2 is increased further suggesting that changes are caused by the increase in leaf-level CO 2 and are not internal model variability. As leaf-level CO 2 is increased further, larger areas of the continental surface are affected by increasing amounts and a statistically significant change in precipitation is seen. The increase in leaf-level CO 2 from 280 ppmv to 375 ppmv causes statistically significant changes in the evaporation over 12% of continental surfaces in July. This increases to 25% at 500 ppmv, 35% at 650 ppmv, 41% at 840 ppmv and 47% at 1,000 ppmv. This affects temperature and rainfall by similar amounts, generally in coincident regions. An analysis of these results over key regions shows that the probability density functions of the latent heat flux and temperature are affected non-uniformly. There is a shift in the latent heat flux probability density function to lower values, mainly through the reduction in the upper tail of the distribution. The temperature probability density function shifts to higher values, mainly through an increase in the upper tail of the distribution indicating that the impact is focussed on extremes. Given that there are a suite of well evaluated land surface models that include the biogeochemical effects of increasing CO 2 we suggest that the inclusion of such a model should be a recommended component of climate models used in future assessment reports by the Intergovernmental Panel on Climate Change. 相似文献
17.
The seasonality in cave CO2 levels was studied based on (1) a new data set from the dynamically ventilated Comblain-au-Pont Cave (Dinant Karst Basin, Belgium), (2) archive data from Moravian Karst caves, and (3) published data from caves worldwide. A simplified dynamic model was proposed for testing the effect of all conceivable CO2 fluxes on cave CO2 levels. Considering generally accepted fluxes, i.e., the direct diffusive flux from soils/epikarst, the indirect flux derived from dripwater degassing, and the input/output fluxes linked to cave ventilation, gives the cave CO2 level maxima of 1.9 × 10−2 mol m−3 (i.e., ∼ 440 ppmv), which only slightly exceed external values. This indicates that an additional input CO2 flux is necessary for reaching usual cave CO2 level maxima. The modeling indicates that the additional flux could be a convective advective CO2 flux from soil/epikarst driven by airflow (cave ventilation) and enhanced soil/epikarstic CO2 concentrations. Such flux reaching up to 170 mol s−1 is capable of providing the cave CO2 level maxima up to 3 × 10−2 mol m−3 (70,000 ppmv). This value corresponds to the maxima known from caves worldwide. Based on cave geometry, three types of dynamic caves were distinguished: (1) the caves with the advective CO2 flux from soil/epikarst at downward airflow ventilation mode, (2) the caves with the advective soil/epikarstic flux at upward airflow ventilation mode, and (3) the caves without any soil/epikarstic advective flux. In addition to CO2 seasonality, the model explains both the short-term and seasonal variations in δ13C in cave air CO2. 相似文献
18.
Eddy fluxes of CO 2 estimated using a sonic anemometer and a closed-path analyser were, on average, 16% lower than those obtained with the same anemometer and an adjacent open-path CO 2 analyser. Covariances between vertical windspeed and CO 2 density from the closed-path analyser were calculated using data points for CO 2 that were delayed relative to anemometer data by the time required for a parcel of air to travel from the tube inlet to the CO 2 sensor. Air flow in the intake tube was laminar. Densities of CO 2 that had been corrected for spurious fluctuations arising from fluctuations in temperature and humidity were used in the flux calculations. Corrections for the cross-sensitivity of CO 2 analysers to water vapour were also incorporated. Spectral analysis of the corrected CO 2 signal from the closed-path analyser showed that damping of fluctuations in the sampling tube at frequencies f > 0.1 Hz caused the apparent loss in flux. The measured losses can be predicted accurately using theory that describes the damping of oscillations in a sampling tube. High-frequency response of the closed-path system can be improved substantially by ensuring turbulent flow in the tube, using a combination of high volumetric flow rate and small tube diameter. The analysis of attenuation of turbulent fluctuations in flow through tubes is applicable to the measurement of fluxes of other minor atmospheric constituents using the eddy covariance method. 相似文献
19.
We examined the annual exchange of CO 2 between the atmosphere and moist tussock and dry heath tundra ecosystems (which together account for over one-third of the low arctic land area) under ambient field conditions and under increased winter snow deposition, increased summer temperatures, or both. Our results indicate that these two arctic tundra ecosystems were net annual sources of CO 2 to the atmosphere from September 1994 to September 1996 under ambient weather conditions and under our three climate change scenarios. Carbon was lost from these ecosystems in both winter and summer, although the majority of CO 2 evolution took place during the short summer. Our results indicate that (1) warmer summer temperatures will increase annual CO 2 efflux from both moist and dry tundra ecosystems by 45–55% compared to current ambient temperatures; (2) deeper winter snow cover will increase winter CO 2 efflux in both moist and dry tundra ecosystems, but will decrease net summer CO 2 efflux; and (3) deeper winter snow cover coupled with warmer summer temperatures will nearly double the annual amount of CO 2 emitted from moist tundra and will result in a 24% increase in the annual CO 2 efflux of dry tundra. If, as predicted, climate change alters both winter snow deposition and summer temperatures, then shifts in CO 2 exchange between the biosphere and atmosphere will likely not be uniform across the Arctic tundra landscape. Increased snow deposition in dry tundra is likely to have a larger effect on annual CO 2 flux than warmer summer temperatures alone or warmer temperatures coupled with increased winter snow depth. The combined effects of increased summer temperatures and winter snow deposition on annual CO 2 flux in moist tundra will be much larger than the effects of either climate change scenario alone. 相似文献
20.
It is found that the surface bog temperature and bog water level position are major factors that determine CO 2 emission from the bog massif. The emission intensity increases with increasing temperature and with a bog water level decrease. The dependence of CO 2 emission is shown to be different for each bog microlandscape. Monthly mean CO 2 emissions are given for the central bog massif in the years with different water content. Based on modeling, it is shown that in the case of a low bog water level the CO 2 flux is directed to the atmosphere (the emission value exceeds the gas amount consumed for photosynthesis), while in the case of a high level, it is taken up by the growing plants. 相似文献
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