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1.
Radiometric measurements of hemispherical surface reflectance and long-wave irradiance are required to quantify the broadband albedo and the outgoing thermal radiation. These observations are typically integrated with eddy covariance measurements of sensible and latent heat fluxes to characterize the surface energy budget. While the aerodynamic footprint has been widely investigated, the geometry of the hemispherical radiometric footprint over plant canopies has been rarely tackled. In the present work, the size and shape of the hemispherical radiometric footprint are formalized for a bare surface and in presence of a vegetation cover. For this purpose, four idealized canopies are analyzed and the dependency of the radiometric footprint on leaf area index and canopy height is explored. Besides, the radiometric footprint is compared with the aerodynamic footprint in conditions of neutral stability. It was observed that almost 100% of the hemispherical radiometric signal originates within a distance of a few radiometer heights, while only about 50–80% of the cumulative aerodynamic signal is generated within a distance of about 20 sensor heights. In order to achieve comparable extensions of the footprint areas, hemispherical radiometric measurements should therefore be taken about 6–15 times higher than turbulent flux ones, depending on the vegetation type. The analysis also highlights that the size of the radiative footprint decreases at increasing leaf area index, whereas the aerodynamic footprint shows an opposite behavior. For the abovementioned reasons, this work may support the interpretation of energy flux measurements and the optimal design of eddy covariance stations located in heterogeneous sites.  相似文献   

2.
非均匀下垫面湍流通量观测的印痕分析   总被引：5，自引：1，他引：4  
彭谷亮  刘绍民  蔡旭晖 《大气科学》2008,32(5):1064-1070
利用2005年在北京昌平区小汤山开展的非均匀下垫面观测实验的观测数据,分别处理了涡旋相关法和大孔径闪烁仪LAS测得的感热通量,并运用印痕模型对数据进行“源区”分析。分析结果表明:在复杂地表上,涡旋相关的测量值由于混杂了其他下垫面的通量信息,不能真实地反映其观测区域的湍流特征,存在着观测误差。而LAS观测出现的低估问题则与掺混高度和“源区”有关。通过用印痕方法进行数据订正后,以上问题均得到很好的改善,订正结果与LAS观测通量的线性关系良好,两者的相关系数达到0.9。实验结果验证了印痕模型在非均匀下垫面的适用性。  相似文献   

3.
Numerical analysis of flux footprints for different landscapes   总被引：5，自引：0，他引：5  
Summary A model for the canopy – planetary boundary layer flow and scalar transport based on E- closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns – forest with clear-cuts as well as hypothetical heterogeneous relief – a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown – the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends on the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Solling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites.  相似文献   

4.
The bandpass eddy covariance method has been used to measure the turbulent flux of scalar quantities using a slow-responsescalar sensor. The method issimilar in principle to the traditional eddy correlation method but includes the estimation of high-frequency components of the flux on the basis of cospectral similarity in the atmospheric surface layer. In order to investigate the performance of the method, measurements of the water vapour flux over a forest with the bandpass eddy covariance method and the direct eddy correlation method were compared. The flux obtained by the bandpass eddy covariance method agreed with that by the eddy correlation method within ±20% for most cases, in spite of a rather slow sensor-response of the adopted hygrometer. This result supports its relevance to a long-term continuous operation, since a stable, low-maintenance,general-purpose sensor canbe utilized for scalar quantities. Oneweak point of the method isits difficulty in principle to measure the correct flux when the magnitude of the sensible heat flux is very small, because the method uses the sensible heat flux as a standard reference for the prediction of undetectable high-frequency components of the scalar flux. An advanced method is then presented to increase its robustness. In the new method, output signals from a slow-response sensor are corrected using empirical frequency-responsefunctions for the sensor,thereby extending the width of the bandpass frequency region where components of the flux are directly measured (not predicted). The advanced method produced correct fluxes for all cases including the cases of small sensible heat flux. The advanced bandpass eddy covariance method is thus appropriate for along-term measurement of the scalar fluxes.  相似文献   

5.
A stochastic trajectory model was used to estimate scalar fluxfootprints in neutral stabilityfor canopies of varying leaf area distributions andleaf area indices. An analytical second-order closure model wasused to predict mean wind speed, second moments and the dissipationrate of turbulent kinetic energy within a forest canopy.The influence of source vertical profile on the flux footprint wasexamined. The fetch is longer for surface sourcesthan for sources at higher levels in the canopy. In order tomeasure all the flux components, and thus the total flux, with adesired accuracy, sources were located at the forest floor in thefootprint function estimation. The footprint functions werecalculated for five observation levels above the canopy top. Itwas found that at low observation heights both canopy density andcanopy structure affect the fetch. The higher abovethe canopy top the flux is measured, the more pronounced is the effectof the canopy structure. The forest fetch for flux measurements isstrongly dependent on the required accuracy: The 90% flux fetchis greater by a factor of two or more compared to the 75% fetch. Theupwind distance contributing 75% of flux is as large as 45 timesthe difference between canopy height and the observation heightabove the canopy top, being even larger for low observationlevels.  相似文献   

6.
北京城市通量足迹及源区分布特征分析   总被引：2，自引：1，他引：1  
刘郁珏  胡非  程雪玲  宋宗朋 《大气科学》2014,38(6):1044-1054
城市下垫面具有不同于其他下垫面的特殊复杂性,通量的源汇分布十分不均,导致直接测量以及定量的描述城市中水汽、二氧化碳等通量变得非常困难。涡动观测系统在城市通量观测中得到广泛应用,但由于涡动协方差观测系统传感器都架设在一定的高度上,其测量结果只反映特定点下垫面或某部分下垫面的物理过程,不能说明观测结果是否具有下垫面属性的代表性,无法反映观测通量的空间变异性。足迹函数(Footprint)的产生就是为了解决这一问题,其又称源权重函数,是描述近地面层表面源或汇的空间分布和仪器观测通量值之间关系的函数。本文采用北京325 m气象塔近1年7层涡动协方差观测数据、超声数据,分析了不同风向、不同稳定度、不同高度下足迹函数所表达的通量贡献区域范围的变化规律。结果表明,在大气稳定条件下时,通量贡献区范围的大小与主风向无显著差异,而不稳定条件下计算结果与主风向无关。在不同高度下所有稳定条件下通量贡献区范围要大于不稳定条件,中性条件则介于两者之间。8 m、16 m高度上的观测结果不能完全代表城市下垫面通量贡献区,47 m以上能够代表城市下垫面通量贡献区,280 m则已经包含了郊区和城区的权重平均。同一大气稳定度条件下,高度越高通量贡献区范围越大,90%通量贡献区范围与观测高度成线性关系,这种线性关系可以预测没有观测高度或者更高处的通量贡献区范围。  相似文献   

7.
We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release. We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor height of 0.158 z i, where z i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction. Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w * = 1.37 m s−1, CBL depth of z i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on the other side of the flight track.  相似文献   

8.
Footprint Analysis For Measurements Over A Heterogeneous Forest   总被引：8，自引：2，他引：6  
The air flow and vertical distribution of sources/sinks inside aforest canopy have been taken into accountin the analysis of the contribution of sources/sinks to measured fluxes and concentrations above a forest. Thestochastic estimators for concentrations and fluxes are described and their evaluation is performed by simulationof an ensemble of fluid parcel trajectories. The influence of the forest canopy on the footprint is important forobservation levels up to a few times the forest height. The influence of along-wind turbulent diffusion, whichanalytical atmospheric surface layer (ASL) footprint models do not account for, is significant even at higherlevels. The footprint analysis has been performed to deduce the Douglas fir canopy carbon dioxide uptake from eddycovariance flux measurements above a mixed Douglas fir–beech forest during the pre-leaf periods of the beech.The scatter in the results indicates that such an analysis is limited, presumably due to horizontal inhomogenetiesin flow statistics, which were not included in trajectory simulation. The analysis, however, is useful for theestimation of the qualitative effect of the forest canopy on the footprint function.  相似文献   

9.
Summary ?During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, 1998. Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7 km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99 m-tower significantly deviate from “idealised” profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels. Received June 6, 2001; revised January 15, 2002; accepted April 4, 2002  相似文献   

10.
In recent years considerable effort has been focused on combining micrometeorological and stable isotope techniques to partition net fluxes and to study biosphere–atmosphere exchange processes. While much progress has been achieved over the last decade, some new issues are beginning to emerge as technological advances, such as laser spectroscopy, permit isotopic fluxes to be measured more easily and continuously in the field. Traditional investigations have quantified the isotopic composition of biosphere-atmosphere exchange by using the Keeling two-member mixing model (the classic Keeling plot). An alternative method, based on a new capacity to measure isotopic mixing ratios, is to determine the isotope composition of biosphere–atmosphere exchange from the ratio of flux measurements. The objective of this study was to critically evaluate these methods for quantifying the isotopic composition of ecosystem respiration (δR) over a period of three growing seasons (2003–2005) within a heterogeneous landscape consisting of C3 and C4 species. For C4 canopies, the mixing model approach produced δR values that were 4–6‰ lower (isotopically lighter) than the flux-gradient method. The analyses presented here strongly suggest that differences between flux and concentration footprint functions are the main factor influencing the inequality between the mixing model and flux-gradient approaches. A mixing model approach, which is based on the concentration footprint, can have a source area influence more than 20-fold greater than the flux footprint. These results highlight the fact that isotopic flux partitioning is susceptible to problems arising from combining signals (concentration and fluxes) that represent very different spatial scales (footprint). This problem is likely to be most pronounced within heterogeneous terrain. However, even under ideal conditions, the mismatch between concentration and flux footprints could have a detrimental impact on isotopic flux partitioning where very small differences in isotopic signals must be resolved.  相似文献   

11.
Horizontal heterogeneity can significantly affect the flux data quality at monitoring sites in complex terrain. In heterogeneous conditions, the adoption of the eddy-covariance technique is contraindicated by the lack of horizontal homogeneity and presence of advective conditions. In addition, uncertainty concerning the sources or sinks influencing a measurement compromises the data interpretation. The consideration of the spatial context of a measurement, defined by a footprint analysis, can therefore provide an important tool for data quality assessment. This study presents an update of an existing footprint-based quality evaluation concept for flux measurement sites in complex terrain. The most significant modifications in the present version are the use of a forward Lagrangian stochastic trajectory model for the determination of the spatial context of the measurements, and the determination of effective roughness lengths with a flux aggregation model in a pre-processing step. Detailed terrain data gathered by remote sensing methods are included. This approach determines spatial structures in the quality of flux data for varying meteorological conditions. The results help to identify terrain influences affecting the quality of flux data, such as dominating obstacles upwind of the site, or slopes biasing the wind field, so that the most suitable footprint regions for the collection of high-quality datasets can be identified. Additionally, the approach can be used to evaluate the performance of a coordinate rotation procedure, and to check to what extent the measured fluxes are representative for a target land-use type.  相似文献   

12.
The Validity of Similarity Theory in the Roughness Sublayer Above Forests   总被引：1，自引：0，他引：1  
Flux-gradient relationships based upon similarity theory have been reported to severely underestimate scalar fluxes in the roughness sublayer above forests, as compared to independent flux estimates (for example, eddy covariance or energy balance measurements). This paper presents the results of a unique three-month investigation into the validity of similarity theory in the roughness sublayer above forests. Eddy covariance and flux-gradient measurements of carbon dioxide (CO2) exchange were compared above a mixed deciduous forest at Camp Borden, Ontario, both before and after leaf senescence. The eddy covariance measurements used a Li-Cor infrared gas analyzer, and the flux-gradient (similarity theory) measurements featured a tunable diode laser Trace Gas Analysis System (TGAS). The TGAS resolved the CO2 concentration difference to 300 parts per trillion by volume (ppt) based upon a half-hour sampling period. The measured enhancement factor (the ratio of independent flux estimates, in this case eddy covariance, to similarity theory fluxes) was smaller and occurred closer to the canopy than in most previous investigations of similarity theory. Very good agreement between the eddy covariance and similarity theory fluxes was found between 1.9 and 2.2 canopy heights (hc), and the mean enhancement factors measured before and after leaf senescence were 1.10 plusmn; 0.06 and 1.24 ± 0.07, respectively. Larger discrepancies were measured closer to the canopy (1.2 to 1.4 hc), and mean enhancement factors of 1.60 ± 0.10 and 1.82 ± 0.11 were measured before and after leaf senescence, respectively. Overall, the Borden results suggest that similarity theory can be used within the roughness sublayer with a greater confidence than previously has been believed.  相似文献   

13.
不同下垫面大孔径闪烁仪观测数据处理与分析   总被引：6，自引：0，他引：6       下载免费PDF全文
大孔径闪烁仪是近年兴起的测量大尺度（500 m~10 km）地表通量的仪器。北京师范大学等单位分别于2002年、2004年在北京昌平小汤山开展了大孔径闪烁仪短期观测实验, 2006年6月又在北京密云建立了长期观测站。利用这些数据, 对大孔径闪烁仪观测数据进行处理与分析, 结果表明:闪烁仪光径高度和风速是影响观测显热通量的关键因子。当地表粗糙元的高度变化相对于光径高度不可忽略时, 零平面位移需要精确确定。波文比在湿润地表需要准确确定, 而气温、气压和动力学粗糙度则为不敏感因子。计算中所需的大气稳定度可用理查孙数判断, 也可借助日出日落时间或净辐射观测值确定。稳定条件下的普适函数目前无统一表达式, 可采用仪器说明书推荐的函数。通过几个站点闪烁仪观测显热通量与涡动相关仪测量值的比较表明:大孔径闪烁仪在均匀和非均匀地表都能得到合理的显热通量观测值。  相似文献   

14.
We describe pragmatic and reliable methods to examine the influence of patch-scale heterogeneities on the uncertainty in long-term eddy-covariance (EC) carbon flux data and to scale between the carbon flux estimates derived from land surface optical remote sensing and directly derived from EC flux measurements on the basis of the assessment of footprint climatology. Three different aged Douglas-fir stands with EC flux towers located on Vancouver Island and part of the Fluxnet Canada Research Network were selected. Monthly, annual and interannual footprint climatologies, unweighted or weighted by carbon fluxes, were produced by a simple model based on an analytical solution of the Eulerian advection-diffusion equation. The dimensions and orientation of the flux footprint depended on the height of the measurement, surface roughness length, wind speed and direction, and atmospheric stability. The weighted footprint climatology varied with the different carbon flux components and was asymmetrically distributed around the tower, and its size and spatial structure significantly varied monthly, seasonally and inter-annually. Gross primary productivity (GPP) maps at 10-m resolution were produced using a tower-mounted multi-angular spectroradiometer, combined with the canopy structural information derived from airborne laser scanning (Lidar) data. The horizontal arrays of footprint climatology were superimposed on the 10-m-resolution GPP maps. Monthly and annual uncertainties in EC flux caused by variations in footprint climatology of the 59-year-old Douglas-fir stand were estimated to be approximately 15–20% based on a comparison of GPP estimates derived from EC and remote sensing measurements, and on sensor location bias analysis. The footprint-variation-induced uncertainty in long-term EC flux measurements was mainly dependent on the site spatial heterogeneity. The bias in carbon flux estimates using spatially-explicit ecological models or tower-based remote sensing at finer scales can be estimated by comparing the footprint-weighted and EC-derived flux estimates. This bias is useful for model parameter optimizing. The optimization of parameters in remote-sensing algorithms or ecosystem models using satellite data will, in turn, increase the accuracy in the upscaled regional carbon flux estimation.  相似文献   

15.
Summary The modification of the flow structure arising from the removal of large patches of trees in a managed forest plantation near Gainesville, Florida is described. Using wavelet analysis of turbulence measurements taken above a forest canopy hundreds of meters downwind from the forest gap and well outside the footprint, the present paper examines changes in flow characteristics and demonstrates that the presence of the nearby clearcut introduces extraneous coherent events passing by the eddy-covariance flux measurement system.  相似文献   

16.
Two almost identical eddy covariance measurement setups were used to measure the fogwater fluxes to a forest ecosystem in the “Fichtelgebirge” mountains (Waldstein research site, 786 m a.s.l.) in Germany. During the first experiment, an intercomparison was carried out with both setups running simultaneously at the same measuring height on a meteorological tower, 12.5 m above the forest canopy. The results confirmed a close agreement of the turbulent fluxes between the two setups, and allowed to intercalibrate liquid water content (LWC) and gravitational fluxes. During the second experiment, the setups were mounted at a height of 12.5 and 3 m above the canopy, respectively. For the 22 fog events, a persistent negative flux divergence was observed with a greater downward flux at the upper level. To extrapolate the turbulent liquid water fluxes measured at height z to the canopy of height hc, a conversion factor 1/[1+0.116(z−hc)] was determined. For the fluxes of nonvolatile ions, no such correction is necessary since the net evaporation of the fog droplets appears to be the primary cause of the vertical flux divergence. Although the net evaporation reduces the liquid water flux reaching the canopy, it is not expected to change the absolute amount of ions dissolved in fogwater.  相似文献   

17.
涡动相关仪和大孔径闪烁仪观测通量的空间代表性   总被引：13，自引：3，他引：10       下载免费PDF全文
在对涡动相关仪和大孔径闪烁仪足迹模型进行敏感性分析的基础上,利用北京密云站2006年8月至2007年12月期间的观测资料,应用足迹模型对观测通量的空间代表性做了初步的分析.结果表明:涡动相关仪和大孔径闪烁仪的源区对风向、Monin-Obukhov长度,空气动力学粗糙度和观测高度/有效高度等因子比较敏感.密云站涡动相关仪的源区白天主要分布在仪器的西南与南面,而夜间则在东北与北面.大孔径闪烁仪的源区为西南一东北向分布.涡动相关仪各月源区形状不同,但大致分布在南北长1000 m、东两宽850 m的范嗣内,而LAS各月源区为西南一东北向分布,长约2060 m,最宽处约为620 m.对涡动相关仪通量有贡献的下垫面主要为园地(67%)和耕地(19%).其中园地的通量贡献比例在夏、秋季比较大,冬、春季稍小,而耕地则相反.大孔径闪烁仪的主要通量贡献源区为园地、耕地和居民地,通量贡献比例分别为49%,28%和11%.其中园地和耕地通量贡献率的变化趋势与涡动相关仪的观测结果一致,但没有涡动相关仪的变化明显.  相似文献   

18.
Summary Long-term flux measurement sites are often characterized by a heterogeneous terrain, which disagrees with the fundamental theoretical assumptions for eddy-covariance measurements. An evaluation procedure to assess the influence of terrain heterogeneity on the data quality has been developed by G?ckede et al. (2004), which combines existing quality assessment tools for flux measurements with analytic footprint modeling. In addition to micrometeorological input data, this approach requires information defining the land use structure and the roughness of the surrounding terrain. The aim of this study was to improve the footprint based site evaluation approach by using high-resolution land use maps derived by Landsat ETM+ and ASTER satellite data. The influence of the grid resolution of the maps on the results was examined, and four different roughness length classification schemes were tested. Due to numerical instabilities of the analytic footprint routine, as an additional footprint model a Lagrangian stochastic footprint routine (Rannik et al., 2003) was employed. Application of the approach on two German FLUXNET sites revealed only weak influence of the characteristics of the land use data when the land use structure was homogeneous. For a more heterogeneous site, use of the more detailed land use maps derived by remote sensing methods resulted in distinct differences indicating the potential of remote sensing for improving the flux measurement site evaluation.  相似文献   

19.
Summary Typical observations of surface energy fluxes in urban areas usually employ the eddy covariance approach with measurements from a tower at a height well above the roughness elements. Net radiation and the turbulent fluxes are directly measured, the anthropogenic flux may be estimated and the storage flux is calculated as an energy balance residual. This paper reports both measurement and modelling of energy fluxes during wintertime in suburban areas of Christchurch, New Zealand. Under settled anticyclonic conditions, a strong inversion can occur over the city which severely restricts turbulent mixing. Even after sunrise the turbulent fluxes are small, and if one assumes advection is negligible, this means the storage flux increases in importance to very high levels. This paper suggests that these high storage rates are physically unrealistic for this environment. Rather, it is likely that some energy, which is assumed to be dissipated as heat storage, is more likely lost through mesoscale advection or attributed to errors caused by unsuitable measurement techniques under conditions with low friction velocity. However, even these two processes cannot fully account for flux loss. A full study is recommended to resolve this issue and meanwhile, caution is advised when applying current research methodology to estimate storage fluxes in urban areas in stable wintertime conditions.  相似文献   

20.
The influence of a freshly logged area in a managed pine forest on the flow field is investigated by comparing sodar wind profile data over the forest canopy with the synoptic wind field extracted from North American Regional Reanalysis, National Centers for Environmental Prediction. As a consequence of the pressure gradient arising from the sharp temperature difference between the clearcut and the surrounding uncut forests, the local wind direction over the forest measured with the sodar departs dramatically from the prevailing synoptic wind direction when the latter is transverse to the clearcut-sodar direction. Sodar measurements also indicate systematic strong updrafts during daytime followed by nighttime downdrafts with wind coming from the logged area. This suggests the presence of horizontal advection carrying daytime warm air (or nighttime cool air) from the clearcut to the forested area. This paper also examines the influence of wind velocity, clearcut fetch, and solar radiation on locally generated circulations and advection. The presence of local circulations arising from contrasting neighboring surface characteristics well outside the footprint is of particular relevance for atmospheric flux sites where robust surface?Catmosphere exchange values are sought. This study highlights the high level of circumspection required at the time of identifying locations for flux sites. It also suggests vigilant monitoring of the surrounding landscape during eddy?Cflux measurements particularly in actively managed landscapes.  相似文献   

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