Carbon dioxide exchange between the atmosphere and an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, China

We used the eddy covariance method to measure the M exchange between the atmosphere and an alpine meadow ecosystem (37degrees29-45'N, 101degrees12-23'E, 3250m a.s.l.) on the Qinghai-Tibetan Plateau, China in the 2001 and 2002 growing seasons. The maximum rates Of CO2 uptake and release derived from the diurnal course Of CO2 flux (FCO2) were -10.8 and 4.4 mumol m(-2) s(-1), respectively, indicating a relatively high net carbon sequestration potential as compared to subalpine coniferous forest at similar elevation and latitude. The largest daily CO2 uptake was 3.9 g cm(-2) per day on 7 July 2002, which is less than half of those reported for lowland grassland and forest at similar latitudes. The daily CO2 uptake during the measurement period indicated that the alpine ecosystem might behave as a sink of atmospheric M during the growing season if the carbon lost due to grazing is not significant. The daytime CO2 uptake was linearly correlated with the daily photosynthetic photon flux density each month. The nighttime averaged F-CO2 showed a positive exponential correlation with the soil temperature, but apparently negative correlation with the soil water content. (C) 2004 Elsevier B.V. All rights reserved.

Short-term variation of CO2 flux in relation to environmental controls in an alpine meadow on the Qinghai-Tibetan Plateau

[1] The alpine meadow ecosystem on the Qinghai-Tibetan Plateau may play a significant role in the regional carbon cycle. To assess the CO2 flux and its relationship to environmental controls in the ecosystem, eddy covariance of CO2, H2O, and energy fluxes was measured with an open-path system in an alpine meadow on the plateau at an elevation of 3,250 m. Net ecosystem CO2 influx (Fc) averaged 8.8 g m(-2) day(-1) during the period from August 9 to 31, 2001, with a maximum of 15.9 g m(-2) day(-1) and a minimum of 2.3 g m(-2) day(-1). Daytime Fc averaged 16.7 g m(-2) day(-1) and ranged from 10.4 g m(-2) day(-1) to 21.7 g m(-2) day(-1) during the study period. For the same photosynthetic photon flux density (PPFD), gross CO2 uptake (Gc) was significantly higher on cloudy days than on clear days. However, mean daily Gc was higher on clear days than on cloudy days. With high PPFD, Fc decreased as air temperature increased from 10degreesC to 23degreesC. The greater the difference between daytime and nighttime air temperatures, the more the sink was strengthened. Daytime average water use efficiency of the ecosystem (WUEe) was 8.7 mg (CO2)(g H2O)(-1); WUEe values ranged from 5.8 to 15.3 mg (CO2)(g H2O)(-1). WUEe increased with the decrease in vapor pressure deficit. Daily albedo averaged 0.20, ranging from 0.19 to 0.22 during the study period, and was negatively correlated with daily Fc. Our measurements provided some of the first evidence on CO2 exchange for a temperate alpine meadow ecosystem on the Qinghai-Tibetan Plateau, which is necessary for assessing the carbon budget and carbon cycle processes for temperate grassland ecosystems.

青藏高原高寒灌丛生态系统二氧化碳通量研究

本论文以青藏高原东北部海北地区高寒灌丛（Alpine Shrub）生态系统为研究对象，利用微气象观测系统及涡度相关（Eddy Covariance）技术，自2003年1月1日至2005年12月31日对该类广布于青藏高原的典型高寒草地类型进行长期连续观测。在对生态系统CO2净交换（NEE）以及群落叶面积指数（LAI）、生物量等生物学指标和光合有效辐射（PAR）、温度、土壤水分、脉冲性降水事件等主要环境因子进行连续监测的基础上，重点分析和探讨了海北地区高寒灌丛生态系统净生态系统CO2交换(NEE)在时、日、月及年际尺度上的变化模式，生长季与非生长季高寒灌丛生态系统CO2净交换特征，高寒灌丛生态系统大气CO2源/汇年际差异，土壤温度、昼夜温差、光合有效辐射、脉冲性降水事件等主要环境因子影响。从而，揭示了不同时间尺度下的高寒灌丛生态系统NEE变化规律，阐明主要环境因子对生态系统NEE的影响，明确了该生态系统大气CO2源/汇状况及其季节分布模式；同时，也为青藏高原区域尺度的高寒草地生态系统CO2通量研究和碳收支的估算提供科学依据和基础数据，对进一步揭示我国乃至亚洲陆地生态系统的碳收支状况有着重要意义。主要研究结果概括为以下几个方面： 1、海北地区高寒灌丛生态系统净生态系统CO2交换时动态特征存在很大的季节性差异，暖季小时NEE变化振幅大，CO2净吸收的极值一般出现在午间，最大吸收量为1.7 g CO2 m-2 h-1左右。夜间为CO2净释放，净生态系统交换值较为稳定（0.5~ 0.9 g CO2 m-2 h-1）；冷季日变化振幅极小，除14:00~18:00时一定量CO2释放外，其余时段通量均很小。 2、从日平均净生态系统CO2交换来看，6~9月日平均NEE一般为负值（CO2净吸收），2003~2005年6~9 月间日平均NEE分别为-5.65 g CO2 m-2 d-1、-6.08 g CO2 m-2 d-1和-4.81 g CO2 m-2 d-1；而10~12月及翌年1~5月期间日平均NEE通常为正值（CO2净释放），该时段3年高寒灌丛日平均净生态系统CO2交换分别为1.91 g CO2 m-2 d-1、1.90 g CO2 m-2 d-1和2.19 g CO2 m-2 d-1。2003~2004年高寒灌丛生态系统CO2净释放维持天数分别为249 d、 254 d和264 d，2003年净释放维持天数最少，而净吸收维持天数2005年最少（101d）。2003、2004和2005年全年日平均CO2净吸收分别为0.611 g CO2 m-2 d-1、0.759 g CO2 m-2 d-1和0.167 g CO2 m-2 d-1。 3、就季节差异而言，2003、2004和2005年整个生长季节高寒灌丛平均CO2日净生态系统交换分别为-3.99 g CO2 m-2 d-1、-4.59 g CO2 m-2 d-1、-3.27 g CO2 m-2 d-1。7、8月生长季节CO2净吸收的最高，2003、2004、2005年7月和8月份高寒灌丛生态系统CO2净吸收分别为222 g CO2 m-2 和224 g CO2 m-2、355 g CO2 m-2和216 g CO2 m-2、263 g CO2 m-2和186 g CO2 m-2。在相对短暂的生长季节海北地区高寒灌丛生态系统表现出显著的大气CO2净吸收能力，2003、2004和2005年生长季节高寒灌丛生态系统CO2净吸收量分别为610 g CO2 m-2、701 g CO2 m-2和500 g CO2 m-2。相对于温度等环境因子，高寒灌丛生态系统生长季白昼NEE小时变化规律更受光合有效辐射变化的影响。 4、2003～2005年非生长季节日平均NEE分别为1.83 g CO2 m-2、2.01 g CO2 m-2和2.07 g CO2 m-2。4月和10月是非生长季节CO2净释放的最高月份，2003、2004和2005年全月净释放量为105 g CO2 m-2和77 g CO2 m-2、105 g CO2 m-2和117 g CO2 m-2及105 g CO2 m-2和138 g CO2 m-2，2003～2005年整个非生长季CO2净释放分别为CO2为388 g CO2 m-2、425 g CO2 m-2和439 g CO2 m-2。非生长季节海北地区高寒灌丛生态系统NEE小时变化与5 cm土壤温度存在极显著的正相关关联，表明在非生长季节土壤温度是影响青藏高原高寒灌丛生态系统NEE的重要环境因子。 5、从生态系统CO2源/汇特征来看，海北地区高寒灌丛生态系统2003、2004和2005年全年净CO2固定总量分别为223 g CO2 m-2 a-1、277 g CO2 m-2 a-1和61 g CO2 m-2 a-1，3年平均CO2值为187 g CO2 m-2 a-1。在为期3年的研究时段海北地区高寒灌丛生态系统表现为弱的大气二氧化碳的汇。 6、高寒灌丛群落表观光合量子产额（a）和表观最大光合速率（Pmax）受叶面积指数的影响。在6～9月份期间，由于LAI的不同，a和Pmax值差异明显，7、8月份较高而6月和9月明显较低。海北地区高寒灌丛生态系统a和Pmax值高于西藏当雄地区高寒草甸生态系统，但低于平原地区相关生态系统。 维持天数2005年最少（101d）。2003、2004和2005年全年日平均CO2净吸收分别为0.611 g CO2 m-2 d-1、0.759 g CO2 m-2 d-1和0.167 g CO2 m-2 d-1。 3、就季节差异而言，2003、2004和2005年整个生长季节高寒灌丛平均CO2日净生态系统交换分别为-3.99 g CO2 m-2 d-1、-4.59 g CO2 m-2 d-1、-3.27 g CO2 m-2 d-1。7、8月生长季节CO2净吸收的最高，2003、2004、2005年7月和8月份高寒灌丛生态系统CO2净吸收分别为222 g CO2 m-2 和224 g CO2 m-2、355 g CO2 m-2和216 g CO2 m-2、263 g CO2 m-2和186 g CO2 m-2。在相对短暂的生长季节海北地区高寒灌丛生态系统表现出显著的大气CO2净吸收能力，2003、2004和2005年生长季节高寒灌丛生态系统CO2净吸收量分别为610 g CO2 m-2、701 g CO2 m-2和500 g CO2 m-2。相对于温度等环境因子，高寒灌丛生态系统生长季白昼NEE小时变化规律更受光合有效辐射变化的影响。 4、2003～2005年非生长季节日平均NEE分别为1.83 g CO2 m-2、2.01 g CO2 m-2和2.07 g CO2 m-2。4月和10月是非生长季节CO2净释放的最高月份，2003、2004和2005年全月净释放量为105 g CO2 m-2和77 g CO2 m-2、105 g CO2 m-2和117 g CO2 m-2及105 g CO2 m-2和138 g CO2 m-2，2003～2005年整个非生长季CO2净释放分别为CO2为388 g CO2 m-2、425 g CO2 m-2和439 g CO2 m-2。非生长季节海北地区高寒灌丛生态系统NEE小时变化与5 cm土壤温度存在极显著的正相关关联，表明在非生长季节土壤温度是影响青藏高原高寒灌丛生态系统NEE的重要环境因子。 5、从生态系统CO2源/汇特征来看，海北地区高寒灌丛生态系统2003、2004和2005年全年净CO2固定总量分别为223 g CO2 m-2 a-1、277 g CO2 m-2 a-1和61 g CO2 m-2 a-1，3年平均CO2值为187 g CO2 m-2 a-1。在为期3年的研究时段海北地区高寒灌丛生态系统表现为弱的大气二氧化碳的汇。 6、高寒灌丛群落表观光合量子产额（a）和表观最大光合速率（Pmax）受叶面积指数的影响。在6～9月份期间，由于LAI的不同，a和Pmax值差异明显，7、8月份较高而6月和9月明显较低。海北地区高寒灌丛生态系统a和Pmax值高于西藏当雄地区高寒草甸生态系统，但低于平原地区相关生态系统。

西藏高原草原化嵩草草甸生态系统呼吸及碳平衡

The soil respiration and net ecosystem productivity of Kobresia littledalei meadow ecosystem was investigated at Dangxiong grassland station, one grassland field station of Lhasa Plateau Ecosystem Research Station. Soil respiration and soil heterotrophic respiration were measured at the same time by using Li6400-09 chamber in growing season of year 2004. The response of soil respiration and its components, i.e. microbial heterotrophic respiration and root respiration to biotic and abiotic factors were addressed. We studied the daily and seasonal variation on Net Ecosystem carbon Exchange (NEE) measured by eddy covariance equipments and then the regression models between the NEE and the soil temperature. Based on the researches, we analyzed the seasonal variation in grass biomass and estimated NEE combined the Net Ecosystem Productivity with heterogeneous respiration and then assessed the whether the area is carbon source or carbon sink. 1.Above-ground biomass was accumulated since the grass growth started from May; On early September the biomass reached maximum and then decreased. The aboveground net primary production (ANPP) was 150.88 g m~" in 2004. The under-ground biomass reached maximum when the aboveground start to die back. Over 80% of the grass root distributed at the soil depth from 0 to 20cm. The underground NPP was 1235.04 g m"2.. Therefore annual NPP wasl.385X103kg ha"1, i.e.6236.6 kg C ha"1. 2. The daily variation of soil respiration showed single peak curve with maximum mostly at noon and minimum 4:00-6:00 am. Daily variations were greater in June, July and August than those in September and October. Soil respiration had strong correlation with soil temperature at 5cm depth while had weaker correlation with soil moisture, air temperature, surface soil temperature, and so on. But since early September the soil respiration had a obviously correlation with soil moisture at 5cm depth. Biomass had a obviously linearity correlation with soil respiration at 30th June, 20th August, and the daytime of 27th September except at 23lh October and at nighttime of 27th September. We established the soil respiration responding to the soil temperature and to estimate the respiration variation during monsoon season (from June through August) and dry season (May, September and October). The regression between soil respiration and 5cm soil temperature were: monsoon season (June through August), Y=0.592expfl()932\ By estimating , the soil daily respiration in monsoon season is 7.798gCO2m"2 and total soil respiration is 717.44 gCC^m" , and the value of Cho is 2.54; dry season (May, September and October), Y=0.34exp°'085\ the soil daily respiration is 3.355gCO2m~2 and total soil respiration is 308.61 gCC^m", and the value of Cho is 2.34. So the total soil respiration in the grown season (From May to October) is 1026.1 g CO2IT1"2. 3. Soil heterogeneous respiration had a strong correlation with soil temperature especially with soil temperature at 5cm depth. The variation range in soil heterogeneous respiration was widely. The regression between soil heterogeneous respiration and 5cm soil temperature is: monsoon season, Y=0.106exp ' 3x; dry season, Y=0.18exp°"0833x.By estimating total soil heterotrophic respiration in monsoon season is 219.6 gCC^m"2, and the value of Cho is 3.78; While total soil heterogeneous respiration in dry season is 286.2 gCCbm"2, and the value of Cho is 2.3. The total soil heterotrophic respiration of the year is 1379.4kg C ha"1. 4. We estimated the root respiration through the balance between soil respiration and the soil heterotrophic respiration. The contribution of root respiration to total respiration was different during different period: re-greening period 48%; growing period 69%; die-back period 48%. 5. The Ecosystem respiration was relatively strong from May to October, and of which the proportion in total was 97.4%.The total respiration of Ecosystem was 369.6 g CO2 m" .we got the model of grass respiration respond to the soil temperature at 5cm depth and then estimated the daytime grass respiration, plus the nighttime NEE and daytime soil respiration. But when we estimated the grass respiration, we found the result was negative, so the estimating value in this way was not close. 6. The estimating of carbon pool or carbon sink. The NPP minus the soil heterogeneous respiration was the NEE, and it was 4857.3kg C o ha"1, which indicated that the area was the carbon sink.

Burst Detection in Turbulent Channel Flows Based on Large Eddy Simulation Databases

Reliable turbulent channel flow databases at several Reynolds numbers have been established by large eddy simulation (LES), with two of them validated by comparing with typical direct numerical simulation (DNS) results. Furthermore, the statistics, such as velocity profile, turbulent intensities and shear stress, were obtained as well as the temporal and spatial structure of turbulent bursts. Based on the LES databases available, the conditional sampling methods are used to detect the structures of burst events. A method to deterimine the grouping parameter from the probability distribution function (pdf) curve of the time separation between ejection events is proposed to avoid the errors in detected results. And thus, the dependence of average burst period on thresholds is considerably weakened. Meanwhile, the average burst-to-bed area ratios are detected. It is found that the Reynolds number exhibits little effect on the burst period and burst-to-bed area ratio.

Large-Eddy-Simulation of 3-Dimensional Rayleigh-Taylor Instability in Incompressible Fluids

The 3-dimensiqnal incompressible Rayleigh-Taylor instability is numerically studied through the large-eddy-simulation (LES) approach based on the passive scalar transport model. Both the instantaneous velocity and the passive scalar fields excited by sinu

Effects of Subgrid-Scale Modeling on Time Correlations in Large Eddy Simulation

The effects of the unresolved subgrid-scale (SGS) motions on the energy balance of the resolved scales in large eddy simulation (LES) have been investigated actively because modeling the energy transfer between the resolved and unresolved scales is crucial to constructing accurate SGS models. But the subgrid scales not only modify the energy balance, they also contribute to temporal decorrelation of the resolved scales. The importance of this effect in applications including the predictability problem and the evaluation of sound radiation by turbulent flows motivates the present study of the effect of SGS modeling on turbulent time correlations. This paper compares the two-point, two-time Eulerian velocity correlation in isotropic homogeneous turbulence evaluated by direct numerical simulation (DNS) with the correlations evaluated by LES using a standard spectral eddy viscosity. It proves convenient to express the two-point correlations in terms of spatial Fourier decomposition of the velocity field. The LES fields are more coherent than the DNS fields: their time correlations decay more slowly at all resolved scales of motion and both their integral scales and microscales are larger than those of the DNS field. Filtering alone is not responsible for this effect: in the Fourier representation, the time correlations of the filtered DNS field are identical to those of the DNS field itself. The possibility of modeling the decorrelating effects of the unresolved scales of motion by including a random force in the model is briefly discussed. The results could have applications to the problem of computing sound sources in isotropic homogeneous turbulence by LES

Large Eddy Simulation of Complex Turbulent Flows: Physical Aspects and Research Trends

In the current paper, we have primarily addressed one powerful simulation tool developed during the last decades-Large Eddy Simulation (LES), which is most suitable for unsteady three-dimensional complex turbulent flows in industry and natural environment. The main point in LES is that the large-scale motion is resolved while the small-scale motion is modeled or, in geophysical terminology, parameterized. With a view to devising a subgrid-scale(SGS) model of high quality, we have highlighted analyzing physical aspects in scale interaction and-energy transfer such as dissipation, backscatter, local and non-local interaction, anisotropy and resolution requirement. They are the factors responsible for where the advantages and disadvantages in existing SGS models come from. A case study on LES of turbulence in vegetative canopy is presented to illustrate that LES model is more based on physical arguments. Then, varieties of challenging complex turbulent flows in both industry and geophysical fields in the near future-are presented. In conclusion; we may say with confidence that new century shall see the flourish in the research of turbulence with the aid of LES combined with other approaches.

Extended self-similar scaling law of multi-scale eddy structure in wall turbulence

The longitudinal fluctuating velocity of a turbulent boundary layer was measured in a water channel at a moderate Reynolds number. The extended self-similar scaling law of structure function proposed by Benzi was verified. The longitudinal fluctuating velocity, in the turbulent boundary layer was decomposed into many multi-scale eddy structures by wavelet transform. The extended self-similar scaling law of structure function for each scale eddy velocity was investigated. The conclusions are I) The statistical properties of turbulence could be self-similar not only at high Reynolds number, but also at moderate and low Reynolds number, and they could be characterized by the same set of scaling exponents xi (1)(n) = n/3 and xi (2)(n) = n/3 of the fully developed regime. 2) The range of scales where the extended self-similarity valid is much larger than the inertial range and extends far deep into the dissipation range,vith the same set of scaling exponents. 3) The extended selfsimilarity is applicable not only for homogeneous turbulence, but also for shear turbulence such as turbulent boundary layers.

Effects Of Subgrid-Scale Modeling On Lagrangian Statistics In Large-Eddy Simulation

The application of large-eddy simulation (LES) to turbulent transport processes requires accurate prediction of the Lagrangian statistics of flow fields. However, in most existing SGS models, no explicit consideration is given to Lagrangian statistics. In this paper, we focus on the effects of SGS modeling on Lagrangian statistics in LES ranging from statistics determining single-particle dispersion to those of pair dispersion and multiparticle dispersion. Lagrangian statistics in homogeneous isotropic turbulence are extracted from direct numerical simulation (DNS) and the LES with a spectral eddy-viscosity model. For the case of longtime single-particle dispersion, it is shown that, compared to DNS, LES overpredicts the time scale of the Lagrangian velocity correlation but underpredicts the Lagrangian velocity fluctuation. These two effects tend to cancel one another leading to an accurate prediction of the longtime turbulent dispersion coefficient. Unlike the single-particle dispersion, LES tends to underestimate significantly the rate of relative dispersion of particle pairs and multiple-particles, when initial separation distances are less than the minimum resolved scale due to the lack of subgrid fluctuations. The overprediction of LES on the time scale of the Lagrangian velocity correlation is further confirmed by a theoretical analysis using a turbulence closure theory.

Derivation Of A Nonlinear Reynolds Stress Model Using Renormalization Group Analysis And Two-Scale Expansion Technique

Adopting Yoshizawa's two-scale expansion technique, the fluctuating field is expanded around the isotropic field. The renormalization group method is applied for calculating the covariance of the fluctuating field at the lower order expansion. A nonlinear Reynolds stress model is derived and the turbulent constants inside are evaluated analytically. Compared with the two-scale direct interaction approximation analysis for turbulent shear flows proposed by Yoshizawa, the calculation is much more simple. The analytical model presented here is close to the Speziale model, which is widely applied in the numerical simulations for the complex turbulent flows.

On the computation of space-time correlations by large-eddy simulation

The effect of subgrid-scale (SGS) modeling on velocity (space-) time correlations is investigated in decaying isotropic turbulence. The performance of several SGS models is evaluated, which shows superiority of the dynamic Smagorinsky model used in conjunction with the multiscale large-eddy simulation (LES) procedure. Compared to the results of direct numerical simulation, LES is shown to underpredict the (un-normalized) correlation magnitude and slightly overpredict the decorrelation time scales. This can lead to inaccurate solutions in applications such as aeroacoustics. The underprediction of correlation functions is particularly severe for higher wavenumber modes which are swept by the most energetic modes. The classic sweeping hypothesis for stationary turbulence is generalized for decaying turbulence and used to analyze the observed discrepancies. Based on this analysis, the time correlations are determined by the wavenumber energy spectra and the sweeping velocity, which is the square root of the total energy. Hence, an accurate prediction of the instantaneous energy spectra is most critical to the accurate computation of time correlations. (C) 2004 American Institute of Physics.

Computation of turbulence-generated noise by large-eddy simulation

The hybrid method of large eddy simulation (LES) and the Lighthill analogy is being developed to compute the sound radiated from turbulent flows. The results obtained from the hybrid method are often contaminated by the absence of small scales in LES, since the energy level of sound is much smaller than that of turbulent flows. Previous researches investigate the effects of subgrid sacle (SGS) eddies on the frequency spectra of sound radiated by isotropic turbulence and suggest a SGS noise model to represent the SGS contributions to the frequency spectra. Their investigations are conducted in physical space and are unavoidably influenced by boundary conditions. In this paper, we propose to perform such calculations in Fourier space so that the effects of boundary conditions can be correctly treated. Posteriori tests are carried out to investigate the SGS contribution to the sound. The results obtained recover the -7/2 law within certain wave-number ranges, but under-estimate the amplitudes of the frequency spectra. The reason for the underestimation is also discussed.

Prediction of particle distribution in isotropic turbulence by large-eddy simulation

The recent application of large-eddy simulation (LES) to particle-laden turbulence requires that the LES with a subgrid scale (SGS) model could accurately predict particle distributions. Usually, a SGS particle model is used to recover the small-scale structures of velocity fields. In this study, we propose a rescaling technique to recover the effects of small-scale motions on the preferential concentration of inertial particles. The technique is used to simulate particle distribution in isotropic turbulence by LES and produce consistent results with direct numerical simulation (DNS). Key words: particle distribution, particle-laden turbulence, large-eddy simulation, subgrid scale model.

A kinematic subgrid scale model for large-eddy simulation of turbulence generated sound

In the hybrid approach of large-eddy simulation (LES) and Lighthill’s acoustic analogy for turbulence-generated sound, the turbulence source fields are obtained using an LES and the turbulence-generated sound at far fields is calculated from Lighthill’s acoustic analogy. As only the velocity fields at resolved scales are available from the LES, the Lighthill stress tensor, serving as a source term in Lighthill’s acoustic equation, has to be evaluated from the resolved velocity fields. As a result, the contribution from the unresolved velocity fields is missing in the conventional LES. The sound of missing scales is shown to be important and hence needs to be modeled. The present study proposes a kinematic subgrid-scale (SGS) model which recasts the unresolved velocity fields into Lighthill’s stress tensors. A kinematic simulation is used to construct the unresolved velocity fields with the imposed temporal statistics, which is consistent with the random sweeping hypothesis. The kinematic SGS model is used to calculate sound power spectra from isotropic turbulence and yields an improved result: the missing portion of the sound power spectra is approximately recovered in the LES.