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1.
We explore some of the underlying assumptions used to derive the density or WPL terms (Webb et al. (1980) Quart J Roy Meteorol Soc 106:85–100) required for estimating the surface exchange fluxes by eddy covariance. As part of this effort we recast the origin of the density terms as an assumption regarding the density fluctuations rather than as a (dry air) flux assumption. This new approach, which is similar to the expansion/compression approach of Liu (Boundary-Layer Meteorol 115:151–168, 2005), eliminates the dry-air mean advective vertical velocity from the development of the WPL terms and allows us to directly compare Liu’s assumptions for deriving the WPL terms with the analogous assumptions appropriate to the original expression of the WPL terms. We suggest, (i) that the main difference between these two approaches lies in the interpretation of the turbulent exchange flux, and (ii) that the original WPL formulation is the more appropriate approach. Given the importance of the WPL terms to accurate and reliable measurements of surface exchange fluxes, a careful analysis of their origins and their proper mathematical expression and interpretation is warranted. 相似文献
2.
Surface turbulent flux measurements over the Loess Plateau for a semi-arid climate change study 总被引：2，自引：0，他引：2
In order to provide high quality data for climate change studies, the data quality of turbulent flux measurements at the station
of SACOL (Semi-Arid Climate & Environment Observatory of Lanzhou University), which is located on a semi-arid grassland over
the Loess Plateau in China, has been analyzed in detail. The effects of different procedures of the flux corrections on CO2, momentum, and latent and sensible heat fluxes were assessed. The result showed that coordinate rotation has a great influence
on the momentum flux but little on scalar fluxes. For coordinate rotation using the planar fit method, different regression
planes should be determined for different wind direction sectors due to the heterogeneous nature of the ground surface. Sonic
temperature correction decreased the sensible heat flux by about 9%, while WPL correction (correction for density fluctuations)
increased the latent heat flux by about 10%. WPL correction is also particularly important for CO2 fluxes. Other procedures of flux corrections, such as the time delay correction and frequency response correction, do not
significantly influence the turbulent fluxes. Furthermore, quality tests on stationarity and turbulence development conditions
were discussed. Parameterizations of integral turbulent characteristics (ITC) were tested and a specific parameterization
scheme was provided for SACOL. The ITC test on turbulence development conditions was suggested to be applied only for the
vertical velocity. The combined results of the quality tests showed that about 62%–65% of the total data were of high quality
for the latent heat flux and CO2 flux, and as much as about 76% for the sensible heat flux. For the momentum flux, however, only about 35% of the data were
of high quality. 相似文献
3.
In the context of CO2 surface exchange estimation, an analysis combining the basic principles of diffusion and scalar conservation shows that the
mixing ratio is the appropriate variable both for defining the (eddy covariance) turbulent flux and also for expressing the
relationship between the turbulent flux and surface exchange in boundary-layer budget equations. Other scalar intensity variables
sometimes chosen, both the CO2 density and molar fraction, are susceptible to the influence of surface exchange of heat and water vapour. The application
of a hypsometric analysis to the boundary-layer “control volume” below the tower measurement height reveals flaws in previously
applied approaches: (a) incompressibility cannot be assumed to simplify mass conservation (the budget in terms of CO2 density); (b) compressibility alone makes the analysis of mass conservation vulnerable to uncertainties associated with resultant
non-zero vertical velocities too small to measure or model over real terrain; and (c) the WPL (Webb et al. (1980) Quart J
Roy Meteorol Soc 106:85–100) “zero dry air flux” assumption is invalidated except at the surface boundary. Nevertheless, the
definition and removal of the WPL terms do not hinge upon this last assumption, and so the turbulent CO2 flux can be accurately determined by eddy covariance using gas analysers of either open- or closed-path design. An appendix
discusses the necessary assumptions and appropriate interpretations for deriving the WPL terms. 相似文献
4.
北京325米气象塔上CO2梯度观测数据质量控制与评价 总被引：2，自引：1，他引：1
近些年涡度相关系统在城市通量研究中得到了广泛应用,因城市下垫面的特殊性和复杂性,以及系统观测原理和仪器精度存在着局限性,使得结果存在10%以上偏差,故必须对原始观测数据进行质量控制和评价。本文对北京325 m气象塔七层高度上的CO2(二氧化碳)、能量等通量进行了长期观测,研究了涡度相关技术在城市环境通量观测中的适用性,设计出了一套适合于复杂城市下垫面上的涡度相关系统资料前处理和质量控制方案,并对比了不同修正方案效果。分析结果表明,城市下垫面环境中计算CO2通量的最优周期为30 min;二次坐标旋转法优于平面拟合法;频率响应修正后的感热通量提高5.21%,潜热通量和CO2通量均提高9.42%。空气密度脉动修正Liu法(Liu,2005)优于WPL法(Webb et al.,1980);湍流谱在惯性副区满足-2/3次方定律,协方差谱满足-4/3次方定律。经过这套前处理和质量控制方案,原始数据中有79%能够用于基础研究,该质量控制与评价体系可为复杂城市下垫面通量研究提供参考。 相似文献
5.
Heping LIU 《大气科学进展》2009,26(1):9-16
Corrections of density effects resulting from air-parcel expansion/compression are
important in interpreting eddy covariance fluxes of water vapor and CO2 when open-path systems
are used. To account for these effects, mean vertical velocity and perturbation of the density
of dry air are two critical parameters in treating those physical processes responsible for
density variations. Based on various underlying assumptions, different studies have obtained
different formulas for the mean vertical velocity and perturbation of the density of dry air,
leading to a number of approaches to correct density effects. In this study, we re-examine
physical processes related to different assumptions that are made to formulate the density
effects. Specifically, we re-examine the assumptions of a zero dry air flux and a zero moist
air flux in the surface layer, used for treating density variations, and their implications
for correcting density effects. It is found that physical processes in relation to the assumption
of a zero dry air flux account for the influence of dry air expansion/compression on density
variations. Meanwhile, physical processes in relation to the assumption of a zero moist air
flux account for the influence of moist air expansion/compression on density variations. In
this study, we also re-examine mixing ratio issues. Our results indicate that the assumption
of a zero dry air flux favors the use of the mixing ratio relative to dry air, while the
assumption of a zero moist air flux favors the use of the mixing ratio relative to the total
moist air. Additionally, we compare different formula for the mean vertical velocity, generated
by air-parcel expansion/compression, and for density effect corrections using eddy covariance
data measured over three boreal ecosystems. 相似文献
6.
珠海凤凰山陆气相互作用观测塔通量数据的质量控制与评价 总被引：1，自引：0，他引：1
针对广东省珠海凤凰山森林生态系统2015年的湍流原始数据,分析了各修正过程对通量的影响。结果表明:野点较多,需要考虑去除;频率响应修正对通量结果影响不大;倾斜修正的影响在3%左右;超声虚温修正对感热通量的影响变化幅度达30%;WPL修正对碳通量结果影响可达25%左右。经过对观测数据的质量评价,感热通量约有85%,潜热通量和碳通量约有80%的数据可用于通量分析。感热通量在1年中的变化幅度不大;潜热通量变化显著,尤其在季风后迅速增大;碳通量变化与森林下垫面密切相关,夏季变化幅度大,冬季变化较小。 相似文献
7.
Correction Of Eddy-Covariance Measurements Incorporating Both Advective Effects And Density Fluxes 总被引：41，自引：17，他引：24
Kyaw Tha Paw U Dennis D. Baldocchi Tilden P. Meyers Kell B. Wilson 《Boundary-Layer Meteorology》2000,97(3):487-511
Equations are presented to correct eddy-covariancemeasurements for both fluctuations in density andnon-zero mean advection, induced by convergence ordivergence of flow, and spatial source/sinkinhomogeneity, under steady-state and transientconditions. This correction collapses to theWebb–Pearman–Leuning expression ifthe mean vertical velocity is zero, and formally addsthe Webb–Pearman–Leuning expression to the correctionssuggested by Lee for conditions ofnon-zero vertical velocity and source/sink and meanscalar horizontal homogeneity. The equation requiresmeasurement of the mean vertical gradients of thescalar concentration of interest (air temperature,humidity, CO2) as well as an accurateestimation of the mean vertical velocity, in additionto the vertical eddy covariance of the scalar. Simplemethods for the approximation of sensor tilt andcomplex terrain flow angle are presented, to allowestimation of non-zero mean vertical velocities. Theequations are applied to data from a maize crop and aforest to give examples of when the correction issignificant. In addition, a term for thethermodynamic expansion energy associated with watervapour flux is derived, which implies that the sonictemperature derived sensible heat flux will accuratelyinclude this contribution. 相似文献
8.
When density fluctuations of scalars such as CO2 are measured with open-path gas analyzers, the measured vertical turbulent flux must be adjusted to take into account fluctuations
induced by ‘external effects’ such as temperature and water vapour. These adjustments are needed to separate the effects of
surface fluxes responsible for ‘natural’ fluctuations in CO2 concentration from these external effects. Analogous to vertical fluxes, simplified expressions for separating the ‘external
effects’ from higher-order scalar density turbulence statistics are derived. The level of complexity in terms of input to
these expressions are analogous to that of the Webb–Pearman–Leuning (WPL), and are shown to be consistent with the conservation
of dry air. It is demonstrated that both higher-order turbulent moments such as the scalar variances, the mixed velocity-scalar
covariances, and the two-scalar covariance require significant adjustments due to ‘external effects’. The impact of these
adjustments on the turbulent CO2 spectra, probability density function, and dimensionless similarity functions derived from flux-variance relationships are
also discussed. 相似文献
9.
Under the usual assumptions for the atmospheric surface layer, we show that air density fluctuations, particularly those due
to temperature fluctuations associated with a heat flux, result in a small mean vertical wind velocity. Because of this, there
can be a significant correction to eddy flux measurements of passive scalars, for example CO2, whose average concentration is very large compared to concentration fluctuations associated with the eddy flux. 相似文献
10.
A model was developed to predict the modification with fetch in offshore flow of mixing ratio, air–water exchange flux, and
near-surface vertical gradients in mixing ratio of a scalar due to air–water exchange. The model was developed for planning
and interpretation of air–water exchange flux measurements in the coastal zone. The Lagrangian model applies a mass balance
over the internal boundary layer (IBL) using the integral depth scale approach, previously applied to development of the nocturnal
boundary layer overland. Surface fluxes and vertical profiles in the surface layer were calculated using the NOAA COARE bulk
algorithm and gas transfer model (e.g., Blomquist et al. 2006, Geophys Res Lett 33:1–4). IBL height was assumed proportional to the square root of fetch, and estimates of the IBL growth
rate coefficient, α, were obtained by three methods: (1) calibration of the model to a large dataset of air temperature and humidity modification
over Lake Ontario in 1973, (2) atmospheric soundings from the 2004 New England Air Quality Study and (3) solution of a simplified
diffusion equation and an estimate of eddy diffusivity from Monin–Obukhov similarity theory (MOST). Reasonable agreement was
obtained between the calibrated and MOST values of α for stable, neutral, and unstable conditions, and estimates of α agreed with previously published parametrizations that were valid for the stable IBL only. The parametrization of α provides estimates of IBL height, and the model estimates modification of scalar mixing ratio, fluxes, and near-surface gradients,
under conditions of coastal offshore flow (0–50 km) over a wide range in stability. 相似文献
11.
We investigate the large-scale oceanic features determining the future ice shelf–ocean interaction by analyzing global warming
experiments in a coarse resolution climate model with a comprehensive ocean component. Heat and freshwater fluxes from basal
ice shelf melting (ISM) are parameterized following Beckmann and Goosse [Ocean Model 5(2):157–170, 2003]. Melting sensitivities to the oceanic temperature outside of the ice shelf cavities are varied from linear to quadratic
(Holland et al. in J Clim 21, 2008). In 1% per year CO2-increase experiments the total freshwater flux from ISM triples to 0.09 Sv in the linear case and more than quadruples to
0.15 Sv in the quadratic case after 140 years at which 4 × 280 ppm = 1,120 ppm was reached. Due to the long response time
of subsurface temperature anomalies, ISM thereafter increases drastically, if CO2 concentrations are kept constant at 1,120 ppm. Varying strength of the Antarctic circumpolar current (ACC) is crucial for
ISM increase, because southward advection of heat dominates the warming along the Antarctic coast. On centennial timescales
the ACC accelerates due to deep ocean warming north of the current, caused by mixing of heat along isopycnals in the Southern
Ocean (SO) outcropping regions. In contrast to previous studies we find an initial weakening of the ACC during the first 150 years
of warming. This purely baroclinic effect is due to a freshening in the SO which is consistent with present observations.
Comparison with simulations with diagnosed ISM but without its influence on the ocean circulation reveal a number of ISM-related
feedbacks, of which a negative ISM-feedback, due to the ISM-related local oceanic cooling, is the dominant one. 相似文献
12.
For measurements of eddy fluxes in the atmospheric boundary layer of gases (such as CO2) whose average concentration is very large compared to the fluctuations, corrections for air density fluctuations are required. With the boundary condition of no flux of dry air at the surface, the evaporation correction to eddy fluxes is 2.6 times larger than has been estimated with the boundary condition of no mass flux at all at the surface. The heat flux correction is also increased by a few per cent. 相似文献
13.
Large-Eddy Simulation of Atmospheric Boundary-Layer Flow Over Fluvial-Like Landscapes Using a Dynamic Roughness Model 总被引：2，自引：2，他引：0
A recently developed dynamic surface roughness model (Anderson and Meneveau, J Fluid Mech 679:288–314, 2011) for large-eddy simulation (LES) of atmospheric boundary-layer flow over multi-scale topographies is applied to boundary-layer flow over several types of fluvial-like landscapes. The landscapes are generated numerically with simulation of a modified Kardar–Parisi–Zhang equation (Passalacqua et al., Water Resour Res 42:WOD611, 2006). These surfaces possess the fractal-like channel network and anisotropic features often found in real terrains. The dynamic model is shown to lead to accurate flow predictions when the surface-height distributions exhibit power-law scaling (scale invariance) in the prevalent mean flow direction. In those cases, the LES provide accurate predictions (invariant to resolution) of mean velocity profiles. Conversely, some resolution dependence is found for applications in which the landscape’s streamwise spectra do not exhibit pure power-law scaling near wavenumbers corresponding to the LES grid resolution. 相似文献
14.
A Simple Method of Estimating Scalar Fluxes Over Forests 总被引：1，自引：0，他引：1
Nelson Luis Dias Jinkyu Hong Monique Y. Leclerc T. Andrew Black Z. Nesic P. Krishnan 《Boundary-Layer Meteorology》2009,132(3):401-414
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 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 CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible
heat flux and CO2 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 CO2 flux estimates that are well correlated with the measured values. 相似文献
15.
On the Significance of the Webb Correction to Fluxes 总被引：2，自引：0，他引：2
For establishing correct mass or energy balances at the Earth's surface, detailed and correct measurements of air constituent fluxes are needed. Flux measurements obtained from the eddy covariance technique have to pass several corrections of different relevance in order to give correct flux data. One of these corrections, the Webb correction, is analysed herein from latent heat flux and CO2 flux data recorded during two field experiments. The significance of this correction for the latent heat flux data varies with the air humidity and the Bowen ratio. The correction changes the latent heat flux values only a little, but significantly (by 2 to 3%). For other air constituents (like CO2), the Webb correction is much more important (20 to 30% of the flux). 相似文献
16.
青藏高原东部草甸下垫面涡旋相关观测数据的质量控制及评价研究 总被引：2，自引：1，他引：1
目前利用涡旋相关仪观测地表通量的方法已被广泛采用, 但由于涡旋相关法是建立在方程假设简化的基础上, 这对观测数据的质量提出了一定的要求, 所以对观测数据的处理及通量结果的质量状况分析就显得尤为重要。本文以理塘站2006年7月中到8月中的数据为例, 对其进行质量控制, 并将质量控制后的通量计算结果与原始值进行比较。质量控制后感热通量和潜热通量总和有所增长, 动量通量值则受质量控制影响较大。质量评价的结果为:动量通量、感热通量、潜热通量中高质量数据所占比例为68.2%、60.6%、63.3%。 相似文献
17.
18.
Heping Liu James T. Randerson Jamie Lindfors William J. Massman Thomas Foken 《Boundary-Layer Meteorology》2006,120(1):65-85
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO2 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 CO2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO2 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 CO2 fluxes. 相似文献
19.
Penélope Serrano-Ortiz Andrew S. Kowalski Francisco Domingo Borja Ruiz Lucas Alados-Arboledas 《Boundary-Layer Meteorology》2008,126(2):209-218
Errors in the estimation of CO2 surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol
Soc 106:85–100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65–85,
(2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements
of absolute CO2 density (ρ
c
). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance
system operating in the Sierra de Gádor in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to
reliably report ρ
c
and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively
large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in ρ
c
in a semi-arid climate with large sensible heat fluxes (H
s
) and (wet) mineral deposition. When ρc is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO2 uptake. 相似文献
20.
Buoyancy and The Sensible Heat Flux Budget Within Dense Canopies 总被引：1，自引：8，他引：1
D. Cava G. G. Katul A. Scrimieri D. Poggi A. Cescatti U. Giostra 《Boundary-Layer Meteorology》2006,118(1):217-240
In contrast to atmospheric surface-layer (ASL) turbulence, a linear relationship between turbulent heat fluxes (FT) and vertical gradients of mean air temperature within canopies is frustrated by numerous factors, including local variation
in heat sources and sinks and large-scale eddy motion whose signature is often linked with the ejection-sweep cycle. Furthermore,
how atmospheric stability modifies such a relationship remains poorly understood, especially in stable canopy flows. To date,
no explicit model exists for relating FT to the mean air temperature gradient, buoyancy, and the statistical properties of the ejection-sweep cycle within the canopy
volume. Using third-order cumulant expansion methods (CEM) and the heat flux budget equation, a “diagnostic” analytical relationship
that links ejections and sweeps and the sensible heat flux for a wide range of atmospheric stability classes is derived. Closure
model assumptions that relate scalar dissipation rates with sensible heat flux, and the validity of CEM in linking ejections
and sweeps with the triple scalar-velocity correlations, were tested for a mixed hardwood forest in Lavarone, Italy. We showed
that when the heat sources (ST) and FT have the same sign (i.e. the canopy is heating and sensible heat flux is positive), sweeps dominate the sensible heat flux.
Conversely, if ST and FT are opposite in sign, standard gradient-diffusion closure model predict that ejections must dominate the sensible heat flux. 相似文献