共查询到20条相似文献,搜索用时 27 毫秒
1.
An experimental micrometeorological set-up was established at the CARBOEURO-FLUX site in Tharandt, Germany, to measure all relevant variables for the calculation of the vertical and horizontal advective fluxes of carbon dioxide. The set-up includes two auxiliary towers to measure horizontal and vertical CO 2 and H 2O gradients through the canopy, and to make ultrasonic wind measurements in the trunk space. In combination with the long-term flux tower an approximately even-sided prism with a typical side-length of 50 m was established. It is shown that under stable (nighttime) conditions the mean advective fluxes have magnitudes on the same order as the daily eddy covariance (EC) flux, which implies that they play a significant, but not yet fully understood, role in the carbon budget equation. The two advective fluxes are opposite and seem to cancel each other at night (at least for these measurements). During the day, vertical advection tends to zero, while horizontal advection is still present implying a flow of CO 2 out of the control volume. From our measurements, a mean daily gain of 2.2 gC m –2 d –1 for the horizontal advection and a mean daily loss of 2.5 gC m –2d –1 for the vertical advection is calculated for a period of 20 days. However the large scatter of the advective fluxes has to be further investigated. It is not clear yet whether the large variability is natural or due to measurement errors and conceptual deficiencies of the experiment. Similar results are found in the few comparable studies. 相似文献
2.
Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II
were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal
turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing
period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective
fluxes, primarily of CO 2. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed
and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget,
different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting
advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based
upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal
turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change
plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst
horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime
energy budget closure and might change ecosystem respiration fluxes considerably. 相似文献
3.
Observations of vegetation–atmosphere exchange of carbon dioxide (CO 2) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO 2 concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity ( $\emph{w}$ ). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean $\emph{w}$ . Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE. 相似文献
4.
A system measuring the horizontal and vertical advection was devised and installed in a sloping forest at the Vielsalm site, Belgium. The measurements showed that under stable conditions a flow regime established below the canopy: air flowed horizontally along the slope and entrained the air above the canopy vertically. This movement occurs during stable nights characterised by strongly negative net radiation. It creates negative air concentration gradients in both the vertical and horizontal directions. The advection fluxes associated with these movements are opposite and of a similar order of magnitude. This implies that the horizontal advection cannot be ignored in the carbon budget equation at night. Unfortunately, the large variability of, and considerable uncertainty about, advection fluxes does not enable one to produce estimates of the source term from these equations. Advection measurement systems should be improved in order to enable such estimates to be made. Particular attention should be paid to the estimation of the vertical velocity above the canopy and to the vertical profiles of the horizontal velocity and horizontal CO 2 gradient below the canopy. 相似文献
5.
A network of remote and in-situ sensors was deployed in a Paris suburb in order to evaluate the mesoscale evolution of the
daily cycle of CO 2 and related tracers in the atmospheric boundary layer (ABL) and its relation to ABL dynamics and nearby natural and anthropogenic
sources and sinks. A 2-μm heterodyne Doppler differential absorption lidar, which combines measurements of, (1) structure
of the atmosphere, (2) radial velocity, and (3) CO 2 differential absorption was a particularly unique element of the observational array. We analyse the differences in the diurnal
cycle of CO, CO 2, lidar reflectivity (a proxy for aerosol content) and H 2O using the lidar, airborne measurements in the free troposphere and ground-based measurements made at two sites located few
kilometres apart. We demonstrate that vertical mixing dominates the early morning drawdown of CO and aerosol content trapped
in the former nocturnal layer but not the H 2O and CO 2 mixing ratio variations. Surface fluxes, vertical mixing and advection all contribute to the ABL CO 2 mixing ratio decrease during the morning transition, with the relative importance depending on the rate and timing of ABL
rise. We also show evidence that when the ABL is stable, small-scale (0.1-km vertical and 1-km horizontal) gradients of CO 2 and CO are large. The results illustrate the complexity of inferring surface fluxes of CO 2 from atmospheric budgets in the stable boundary layer. 相似文献
6.
A dataset from two campaigns conducted at the Vielsalm experimental site in Belgium was used as a basis for discussing some
methodological problems and providing intermediate results on estimating CO 2 advection. The analysis focused on the horizontal [CO 2] gradient and on the vertical velocity w, the variables most affected by uncertainty. The sampling error for half-hourly horizontal [CO 2] gradients was estimated to be 1.3 μmol mol −1. Despite this important random error for half-hour estimations of [CO 2], the mean horizontal [CO 2] gradients in advective conditions were shown to be representative at the ecosystem scale and to extend only to the lowest
part of a drainage sub-layer, which developed in the trunk space. By contrast, under daytime conditions, this gradient was
shown to be more sensitive to local source heterogeneities. The estimation of the short-term averaged vertical velocity (
was the greater source of error when computing advection terms. The traditional correction methods used to obtain
are discussed and a (co)sine correction is tested to highlight the instrumental origin of the offset in w. A comparison of measurements by sonic anemometers placed close together above the canopy showed that the uncertainty on
was 0.042 m s −1, which is of the same order of magnitude as the velocity itself. In addition, as the drainage sub-layer is limited to the
lowest part of the canopy, the representativeness of
is questionable. An alternative computation using the divergence of the horizontal wind speed in the trunk space produced
a
estimation that was four times lower than the single-point measurement. However, this value gives a more realistic estimate
of the vertical advection term and improves the CO 2 budget closure at the site. 相似文献
7.
A field study of surface-air exchange of carbon, water, and energy was conducted at a mid-latitude, mixed forest on non-flat terrain to investigate how to best interpret biological signals from the eddy flux data that may be subject to advective influences. It is shown that during periods of Southwest winds (sector with mild topography), the eddy fluxes are well-behaved in terms of energy balance closure, the existence of a constant flux layer, consistency with chamber observations and the expected abiotic controls on the fluxes. Advective influences are evident during periods with wind from a steep (15%) slope to the Northeast of the tower. These influences appear more severe on CO 2 flux, particularly in stable air, than on the energy fluxes. Large positive flux of CO 2 (> 23 mol m -2 s -1) occurs frequently at night. The annual sum of the carbon flux is positive, but the issue about whether the forest is a source of atmospheric carbon remains inconclusive.Attempts are made to assess vertical advectionusing the data collected on a single tower. Over the Southwestsector, vertical advection makes a statistically significant but small contribution to the 30-min energy imbalance and CO 2 flux variations. Contributions by horizontal advection may be larger but cannot be verified directly by the current experimental method. 相似文献
8.
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
9.
Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September
2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem
sites, the CO 2 fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused
by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of
a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental
conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative
approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer
perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the
methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to
identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable
minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found
to be 0.15 m s −1. The obtained CO 2 fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m −2 s −1 and fell to nighttime minimum values of 3 μmol m −2 s −1. Mean daily CO 2 emissions of 21 g CO 2 m −2d −1 were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO 2. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m −2 s −1and showed lower variances than the simultaneously measured fluxes of CO 2. 相似文献
10.
A nocturnal gravity wave was detected over a south-western Amazon forest during the Large-Scale Biosphere–Atmosphere experiment
in Amazonia (LBA) in the course of the dry-to-wet season campaign on October 2002. The atmospheric surface layer was stably
stratified and had low turbulence activity, based on friction velocity values. However, the passage of the wave, an event
with a period of about 180–300 s, caused negative turbulent fluxes of carbon dioxide (CO 2) and positive sensible heat fluxes, as measured by the eddy-covariance system at 60 m (≈30 m above the tree tops). The evolution
of vertical profiles of air temperature, specific humidity and wind speed during the wave movement revealed that cold and
drier air occupied the sub-canopy space while high wind speeds were measured above the vegetation. The analysis of wind speed
and scalars high frequency data was performed using the wavelet technique, which enables the decomposition of signals in several
frequencies allowed by the data sampling conditions. The results showed that the time series of vertical velocity and air
temperature were −90° out of phase during the passage of the wave, implying no direct vertical transport of heat. Similarly,
the time series of vertical velocity and CO 2 concentration were 90° out of phase. The wave was not directly associated with vertical fluxes of this variable but the mixing
induced by its passage resulted in significant exchanges in smaller scales as measured by the eddy-covariance system. The
phase differences between horizontal velocity and both air temperature and CO 2 concentration were, respectively, zero and 180°, implying phase and anti-phase relationships. As a result, the wave contributed
to positive horizontal fluxes of heat and negative horizontal fluxes of carbon dioxide. Such results have to be considered
in nocturnal boundary-layer surface-atmosphere exchange schemes for modelling purposes. 相似文献
11.
Turbulent fluxes obtained using the conventional eddy covariance approach result in erratic results with large time fluctuations
in extremely stable conditions. This can limit efforts to estimate components of the nocturnal energy budget and respiratory
CO 2 fluxes. Well-organized fluxes that show a clear dependence on turbulent intensity were obtained when multiresolution decomposition
was used to estimate turbulent exchanges. CO 2, heat and water vapour fluxes were observed at a site in the eastern Amazon basin that had been cleared for agricultural
purposes. Temporal scales of the carbon transfer were determined and shown to be similar to those of latent heat, but as much
as three times larger than those of sensible heat. CO 2 eddy diffusivities at the temporal scales on which most of the vertical CO 2 exchange occurs are shown to be 50 times larger than the eddy diffusivity for heat. A process associated with the vertical
scale of the scalar accumulation layer is suggested to explain these different scales and turbulent diffusivities of carbon
and sensible heat transfer. For an appreciable range of turbulence intensities, the observed vertical turbulent carbon exchange
is insufficient to account for the locally respired CO 2 estimated independently. Evidence that shallow drainage currents may account for this is given. 相似文献
12.
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
13.
Summary A simple eddy correlation system is presented that allows on-line calculation of latent and sensible heat fluxes. The system is composed of a three dimensional propeller anemometer, a thermocouple and a capacitance relative humidity sensor. Results from two contrasting sites demonstrate the capability of the system to measure turbulent fluxes under varying conditions. A dry mixed (dominantly coniferous) forest in hilly terrain in Austria is compared to a well irrigated, heavily transpiring, deciduous pecan orchard in the Southwest of the US. The US site shows insufficient closure of the energy balance that is attributed to non-turbulent fluxes under advective conditions in a stable boundary layer (Blanford et al., 1991) while the Austrian site exhibits almost perfect closure with the use of the very same instruments when the boundary layer is convective and advection is negligible.With 4 Figures 相似文献
15.
To assess annual budgets of CO 2 exchange betweenthe biosphere and atmosphere over representativeecosystems, long-term measurements must be made overecosystems that do not exist on ideal terrain. How tointerpret eddy covariance measurements correctlyremains a major task. At present, net ecosystemCO 2 exchange is assessed, by members of themicrometeorological community, as the sum of eddycovariance measurements and the storage of CO 2 inthe underlying air. This approach, however, seemsunsatisfactory as numerous investigators are reportingthat it may be causing nocturnal respiration fluxdensities to be underestimated.A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology
91: 39–50) for assessing net ecosystem-atmosphere CO 2 exchange(N e) over non-ideal terrain. Itincludes a vertical advection term. We apply thisequation over a temperate broadleaved forest growingin undulating terrain. Inclusion of the verticaladvection term yields hourly, daily and annual sums ofnet ecosystem CO 2 exchange that are moreecologically correct during the growing season.During the winter dormant period, on the other hand,corrected CO 2 flux density measurements of anactively respiring forest were near zero. Thisobservation is unrealistic compared to chambermeasurements and model calculations. Only duringmidday, when the atmosphere is well-mixed, domeasurements of N e match estimatesbased on model calculations and chamber measurements. On an annual basis, sums of N ewithout the advection correction were 40% too large,as compared with computations derived from a validatedand process-based model. With the inclusion of theadvection correction term, we observe convergencebetween measured and calculated values ofN e on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of aone-dimensional, vertical advection term into thecontinuity equation is sufficient for evaluatingCO 2 exchange over tall forests in complexterrain. There is an indication that the neglected term,( c¯/ x), isnon-zero and that CO 2 may be leakingfrom the sides of the control volume during the winter. In this circumstance, forest floor CO 2 effluxdensities exceed effluxes measured above the canopy. 相似文献
16.
Wind speed profiles above a forest canopy relate to scalar exchange between the forest canopy and the atmosphere. Many studies have reported that vertical wind speed profiles above a relatively flat forest can be classified by a stability index developed assuming wind flow above a flat plane. However, can such a stability index be used to classify vertical wind speed profiles observed above a sloping forest at nighttime, where drainage flow often occurs? This paper examines the use of the bulk Richardson number to classify wind speed profiles observed above a sloping forest at nighttime. Wind speed profiles above a sloping forest were classified by the bulk Richardson number Ri B . Use of Ri B distinguished between drainage flow, shear flow, and transitional flow from drainage flow to shear flow. These results suggest that Ri B is useful to interpret nighttime CO 2 and energy fluxes above a sloping forest. Through clear observational evidence, we also show that the reference height should be high enough to avoid drainage-flow effects when calculating Ri B . 相似文献
17.
This paper reports on measurements of sensible and latent heat and CO 2 fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent ( LE) and sensible ( H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 did LE and H become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence of H and LE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, and LE. This finding is consistent with results from several recent studies. 相似文献
18.
Topography influences many aspects of forest-atmosphere carbon exchange; yet only a small number of studies have considered
the role of topography on the structure of turbulence within and above vegetation and its effect on canopy photosynthesis
and the measurement of net ecosystem exchange of CO 2 ( Nee) using flux towers. Here, we focus on the interplay between radiative transfer, flow dynamics for neutral stratification,
and ecophysiological controls on CO 2 sources and sinks within a canopy on a gentle cosine hill. We examine how topography alters the forest-atmosphere CO 2 exchange rate when compared to uniform flat terrain using a newly developed first-order closure model that explicitly accounts
for the flow dynamics, radiative transfer, and nonlinear eco physiological processes within a plant canopy. We show that variation
in radiation and airflow due to topography causes only a minor departure in horizontally averaged and vertically integrated
photosynthesis from their flat terrain values. However, topography perturbs the airflow and concentration fields in and above
plant canopies, leading to significant horizontal and vertical advection of CO 2. Advection terms in the conservation equation may be neglected in flow over homogeneous, flat terrain, and then Nee = Fc, the vertical turbulent flux of CO 2. Model results suggest that vertical and horizontal advection terms are generally of opposite sign and of the same order
as the biological sources and sinks. We show that, close to the hilltop, Fc departs by a factor of three compared to its flat terrain counterpart and that the horizontally averaged Fc-at canopy top differs by more than 20% compared to the flat-terrain case. 相似文献
19.
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
20.
净生态系统碳通量(NEE)的计算对于准确模拟区域碳通量和大气CO 2浓度的时空变化至关重要。本文利用中尺度大气-温室气体耦合模式WRF-GHG(Weather Research and Forecasting Model with Greenhouse Gases Module),对2010年7月28日至2010年8月2日期间影响长江三角洲地区大气CO 2浓度及时空分布的各种过程进行了详尽模拟。结果表明,植被光合呼吸模型(VPRM)能模拟不同植被下垫面NEE的日变化;WRF-GHG模拟的大气CO 2浓度日变化与观测相吻合,但低估了大气CO 2浓度5~15 ppm(ppm表示10-6),这可能与人为排放源的低估、VPRM参数的不确定性以及气象场模拟的不准确性有关。太湖和植被覆盖较好的地区如浙江北部山区是该地区的主要碳汇,而城市为CO 2的主要排放源。太湖和陆地生态系统对区域内碳循环起到一定的调节作用,减缓区域大气CO 2浓度的升高。此外,局地气象条件如湖陆风对太湖周边地区大气CO 2浓度有显著影响。 相似文献
|