Forest-Air Fluxes Of Carbon, Water And Energy Over Non-Flat Terrain

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 CO₂ flux, particularly in stable air, than on the energy fluxes. Large positive flux of CO₂ (> 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 CO₂ flux variations. Contributions by horizontal advection may be larger but cannot be verified directly by the current experimental method.     

LES Study of the Energy Imbalance Problem with Eddy Covariance Fluxes

The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the `turbulent' heat flux at a grid point from the horizontally averaged `total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks.The temporally averaged `turbulent' flux based on a point measurement systematically underestimates the `total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.     

Spatial Variability of Turbulent Fluxes in the Roughness Sublayer of an Even-Aged Pine Forest

The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (_u, _w) and temperature (_T) are more planar homogeneous than their vertical flux of momentum (u_* ²) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO₂ and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u_*, especially when u_* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (I_w) and the arrival frequency of organized structures (/h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.     

Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy

Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.     

CO2湍流通量误差的修正和不确定性研究进展

目前国内外对能量和CO₂涡动相关通量的测量误差的谱修正缺乏一致性，导致各测量站之间的资料对比难以进行，给全球尺度资料的合成造成很大困难。因此在强调长期CO₂通量测量精度的同时，还应该注重确定对测量误差修正的首选方法和研究重点。该文主要介绍了两种用于修正测量仪器误差的谱方法；二维和三维的气流运动对通量误差的影响；资料处理中的误差；夜间CO₂通量的测量问题。     

Measurement of the Sensible Eddy Heat Flux Based on Spatial Averaging of Continuous Ground-Based Observations

Using the standard eddy-covariance (EC) method to quantify mass and energy exchange at a single location usually results in an underestimation of vertical eddy fluxes at the surface. In order to better understand the reasons for this underestimation, an experimental set-up is presented that is based on spatial averaging of air temperature data from a network of ground-based sensors over agricultural land. For eight days during the 34-day observational period in May and June 2007, additional contributions to the sensible heat flux of more than 50Wm⁻² were measured in the lower surface layer by applying the spatial EC method as opposed to the standard temporal EC method. Smaller but still significant additional sensible heat fluxes were detected for four more days. The additional energy is probably transported in organised convective structures resulting in a mean vertical wind velocity unequal to zero at the tower location. The results show that convective transport contributes significantly to the surface energy budget for measurement heights as low as 2–3 m. Since these structures may be quasi-stationary, they can hardly be captured by a single-location measurement. The spatial EC set-up presented here is capable of quantifying contributions to the sensible heat flux from structures up to the scale of our spatial sensor network, which covered an area 3.5 × 3.5 km. For future experiments aiming at closing the energy balance, the spatial EC method should be employed to measure both the sensible and latent heat fluxes. Experimental determination of the horizontal advection of sensible and latent heat should also be considered, since such transport must occur due to convergence and divergence related to convection.     

不同平均时间对LOPEX10资料涡动相关湍流通量计算结果影响的探讨

通过处理涡动相关系统观测的近地层湍流脉动量可以获取地—气间感热和潜热通量,然而选择不同平均时间对通量计算的结果有较大影响。采用黄土高原陆面过程野外观测试验(LOPEX10)期间获得的涡动相关系统观测资料,分析了不同平均时间对湍流通量计算的影响,并采用雷诺平均和分解方法推导了平均时间引起的通量差值的数学表达式(Flux Compensation,FC)。结果表明:(1)FC公式可以说明采用不同平均时间数据之间的关系,也可以直接计算低频涡旋对湍流通量的贡献。FC公式计算的结果与直接计算的不同平均时间通量计算之差的相关系数在0.95以上,并可以确定计算湍流通量的最佳平均时间。(2)通过采用Ogive函数确定了计算LOPEX10期间通量的最佳平均时间长度为30min,印证了利用雷诺平均和分解方法计算湍流通量补偿的准确性。(3)通过进一步的数学变换,证明了平均时间对湍流通量计算的影响直接与湍流低频变化相关,FC公式可以用来确定涡动相关观测数据的最佳平均时间,并且在获得较高时间分辨率的湍流通量数据的同时,补偿因平均时间过短而遗漏的低频信息。     

北京城区二氧化碳时空分布及湍流谱特征

利用北京325 m气象塔上安装的7层CO₂涡动相关系统在2014年12月到2015年11月的观测资料,分析了北京城区不同高度上CO₂浓度、通量时空分布及湍流谱的特征。结果表明:城市CO₂浓度日变化除了冬季都呈现双峰型,冬季由于人为碳源排放的大幅增加,双峰型不明显。每层的CO₂浓度、通量都有明显的季节变化:冬季最高,春末、夏季最低。CO₂浓度整体随高度的增加而降低。北京城区是CO₂源,CO₂通量的日变化不如CO₂浓度日变化规律明显。CO₂通量在47 m以下为负,47 m以上为正。通量在140 m以下随高度的增加而增加;140m以上随高度的增加而减少。根据对CO₂时空分布的分析可知:边界层CO₂浓度、通量强烈受到碳源、下垫面植被、大气稳定度、环境温度和天气过程等因素的影响。各变量谱与Kaimal等的研究结果接近:归一化速度谱和CO₂谱在惯性子区有－2/3的斜率,在低频区与稳定度参数（Z/L）有一定的关系。这说明复杂地形的城市下垫面的湍流谱结构与平坦地形相比没有太大的实质性差异。     

Spatial Variability of Both Turbulent Fluxes and Temperature Profiles in an Urban Roughness Layer

The spatial variability of both turbulent flow statistics in the roughness sublayer (RSL) and temperature profiles within and above the canopy layer (CL) were investigated experimentally in a densely built-up residential area in Tokyo, Japan. Using five towers with measuring devices, each tower isolated from the others by at least 200 m, we collected high-frequency measurements of velocity and temperature at a height z=1.8 z _H, where z _H, the mean building height in the area, is 7.3 m. Also, temperature profiles were measured from z=0.4 to 1.8 z _H. The ‘areal mean’ geometric parameters that were obtained for the areas within 200 m of each tower were fairly homogeneous among the tower sites. The main results are as follows: (1) The spatial variability of all RSL turbulent statistics, except the sensible heat flux, was comparable to that reported in a pine forest. Also, the variability decreased with increasing friction velocity. (2) The spatial variability of the RSL sensible heat flux was larger than that reported in a pine forest. Also, the variability depended on the time of the day and became larger in the morning. The difference among the sites was well related to the areal fraction of vegetation. (3) The spatial variability of the CL temperature profile depended on the time of the day and became larger in the morning. Nevertheless, the spatial standard deviation of CL temperature was always below 0.7 K. (4) It is suggested that the “warming-up” process in the morning when heat storage is dominant increases the spatial variation of RSL sensible heat flux and CL temperature according to the local properties around each tower and the variation decreases once there is further convective mixing in the midday     

Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest

Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.     

Characteristics of Lake Breezes and Their Impacts on Energy and Carbon Fluxes in Mountainous Areas

In mountainous lake areas, lake–land and mountain–valley breezes interact with each other, leading to an "extended lake breeze". These extended lake breezes can regulate and control energy and carbon cycles at different scales. Based on meteorological and turbulent fluxes data from an eddy covariance observation site at Erhai Lake in the Dali Basin,southwest China, characteristics of daytime and nighttime extended lake breezes and their impacts on energy and carbon dioxide exchange in 2015 are investigated. Lake breezes dominate during the daytime while, due to different prevailing circulations at night, there are two types of nighttime breezes. The mountain breeze from the Cangshan Mountain range leads to N1 type nighttime breeze events. When a cyclonic circulation forms and maintains in the southern part of Erhai Lake at night, its northern branch contributes to the formation of N2 type nighttime breeze events. The prevailing wind directions for daytime, N1, and N2 breeze events are southeast, west, and southeast, respectively. Daytime breeze events are more intense than N1 events and weaker than N2 events. During daytime breeze events, the lake breeze decreases the sensible heat flux(Hs) and carbon dioxide flux(F_{CO_2}) and increases the latent heat flux(LE). During N1 breeze events, the mountain breeze decreases Hs and LE and increases F_{CO_2}. For N2 breeze events, the southeast wind from the lake surface increases Hs and LE and decreases suppress carbon dioxide exchange.     

基于GOSAT反演的中国地区二氧化碳浓度时空分布研究

卫星遥感监测大气二氧化碳柱平均干空气体积混合比(XCO₂)是实现碳源汇全球监测的最有效手段,本文对国际上4种应用GOSAT卫星观测的短波红外反演算法进行了介绍和结果分析。首先对于4种反演产品的有效数据量的分析表明:现有单一反演产品还不足以支撑XCO₂时空分布研究。其次利用集合平均方法,综合使用4种反演产品研究了2010年中国地区XCO₂时空分布特征,结果表明:XCO₂呈现显著的地理分布和季节变化,不同地区季节变化趋势基本一致,均在春季达到最高值、夏季达到最低值,多数地区全年高于380 ppm (×10-6);在地理分布上,东部和西部地区存在较明显的差异,东部地区人口密集、工农业生产等人为活动旺盛,周边多被森林和草地覆盖,碳源汇强度大,因此XCO₂季节变化幅度较大,全年约8 ppm;中、西部地区受人类活动影响较少,植被覆盖稀疏,XCO₂全年变化仅5 ppm。     

Consequences of Incomplete Surface Energy Balance Closure for CO2 Fluxes from Open-Path CO2/H2O Infrared Gas Analysers

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.     

Three-year Variations of Water, Energy and CO$_2$ Fluxes of Cropland and Degraded Grassland Surfaces in a Semi-arid Area of Northeastern China

Based on 3 years （2003-05） of the eddy covariance （EC） observations on degraded grassland and cropland surfaces in a semi-arid area of Tongyu （44°25′N, 122°52′E, 184 m a.s.1.）, Northeast China, seasonal and annual variations of water, energy and CO2 fluxes have been investigated. The soil moisture in the thin soil layer （at 0.05, 0.10 and 0.20 m） clearly indicates the pronounced annual wet-dry cycle; the annual cycle is divided into the wet （growing season） and dry seasons （non-growing season）. During the growing season （from May to September）, the sensible and latent heat fluxes showed a linear dependence on the global solar radiation. However, in the non-growing season, the latent heat flux was always less than 50 W m^-2, while the available energy was dissipated as sensible, rather than latent heat flux. During the growing season in 2003-05, the daily average sensible and latent heat fluxes were larger on the cropland surface than on the degraded grassland surface. The cropland ecosystem absorbed more CO2 than the degraded grassland ecosystem in the growing season in 2003-05. The total evapotranspiration on the cropland was more than the total precipitation, while the total evapotranspiration on the degraded grassland was almost the same as the total annual precipitation in the growing season. The soil moisture had a good correlation with the rainfall in the growing season. Precipitation in the growing season is an important factor on the water and carbon budget in the semi-arid area.     

Comparison of Ozone Fluxes over a Maize Field Measured with Gradient Methods and the Eddy Covariance Technique

Ozone(O_3) fluxes were measured over a maize field using the eddy covariance(EC) technique and gradient methods.The main objective was to evaluate the performance of the gradient methods for measuring the O_3 flux by comparing them with the EC O_3 flux.In this study,turbulent exchange coefficients(K) calculated with three methods were compared.These methods were the aerodynamic gradient(AG) method(in which K is calculated by using wind speed and temperature gradients),the aerodynamic gradient combined with EC(AGEC) method,in which the friction velocity and other variables are based on EC measurements,and the modified Bowen ratio using the EC sensible heat flux and temperature gradient(MBR) method.Meanwhile,the effects of the measurement and calculation methods of the O_3 concentration gradient were analyzed.The results showed that:(1) on average,the transfer coefficient computed by the MBR method was 40% lower,and the coefficient determined with the AG method was 25% higher,than that determined with the AGEC method.(2) The gradient method's O_3 fluxes with the MBR,AGEC,and AG methods were 30.4% lower,11.7% higher,and 45.6% higher than the EC O_3 flux,respectively.(3) The effect of asynchronous O_3 concentration measurements on the O_3 gradient must be eliminated when using one analyzer to cyclically measure two-level O_3 concentrations.The accuracy of gradient methods for O_3 flux is related to the exchange coefficient calculation method,and its precision mainly depends on the quality of the O_3 gradient.     

Summertime Seawater Concentrations of Dimethylsulfide in the Western Indian Ocean: Reconciliation of Fluxes and Spatial Variability with Long-Term Atmospheric Observations

Dimethylsulfide (DMS) measurements in the seawater of the subtropical and the temperate western Indian Ocean were conducted for the first time from 3 December to 20 December 1997. In total, 443 surface seawater DMS determinations were performed between 24°–49° S and 50° E–77° E with a frequency of 1 sample every 10 km. An important spatial variability was observed in seawater DMS concentrations with values ranging from 0.9 to 35.8 nM. DMS maxima coincided in most cases with thermal fronts and were in reasonable agreement with mean pigment figures obtained from satellite observations. The deduced DMS fluxes are consistent with long-term observations of atmospheric DMS and rainwater concentrations of nss- SO4= and MSA measured at Amsterdam island (37° S, 77° E); then account for the differences observed in atmospheric DMS concentrations between Amsterdam island and Cape Grim, Indian Ocean monitoring stations.     

陆气双向耦合模式中全球感热和潜热通量的时空特征模拟

利用中国科学院大气物理研究所(IAP/CAS)含有动态植被过程的海-陆-气耦合模式AVIlVI-GOAIS的积分结果,与ERA-40再分析资料的感热和潜热通量场进行对比分析,结果表明:AVIM-GOALS模拟的感热和潜热通量的气候态、季节变化等特征和ERA-40一致,其中感热通量的纬向分布为双峰型,而潜热通量从1～7月...     

The Effect of Chessboard Variability of the Surface Fluxes on the Aggregated Turbulence Fields in a Convective Atmospheric Surface Layer

To what degree the variability of surface features can be identified in the turbulent signals observed in the atmospheric boundary layer is still an unresolved problem. This was investigated by conducting an analytical experiment for a one-dimensional 'chessboard'-type surface-flux distribution on the basis of local free convection scaling. The results showed that, due to their nonlinear dependency on the surface fluxes, the dimensionless gradients of the mean quantities and the dimensionless standard deviations are altered by the surface-flux variability. Furthermore, passive scalars, such as humidity, are considerably more sensitive to surface variability than the main active scalar, temperature. However, the response of the gradients of the mean quantities is fairly negligible in the range of variability studied herein as compared to that of the standard deviations, which were found to be more sensitive to the surface-flux variability. In addition, the phase difference between the active and the passive scalar flux distribution strongly affects the passive scalar turbulence. This dissimilarity between passive and active scalars, or between passive scalars when their source distributions are different, brings into question the use of variance methods for the measurement of a scalar flux, such as evaporation, over variable surfaces. The classical Bowen ratio method, which depends on the validity of the Reynolds analogy for the vertical gradients of the mean quantities, was shown to be relatively more robust. However, under conditions of strong surface variability, it can also be expected to fail.     

Climatic Indices Influencing the Long-Term Variability of Mediterranean Heat and Water Fluxes: The North Atlantic and Mediterranean Oscillations

Interannual to interdecadal precipitation (P), evaporation (E), water deficit (E-P), and total heat flux have been correlated with North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO) indices to explore the influence of large-scale atmospheric forcing on the variability of the Mediterranean water and heat budgets. Basin-averaged precipitation decrease from the mid-1960s to the late 1980s clearly corresponds to a switch from a low to a high state of both indices. The variability of E-P is not so well correlated with the atmospheric indices because of the different sensitivity of E and P that leads to correlations of opposite sign in the eastern and western sub-basins. The effectiveness of the NAO and MO indices is quite similar for P and E-P but the regional MO index has turned out to be a more successful indicator of interdecadal evaporation and net heat flux because, from the mid-1970s to the early 1990s, correlation with the NAO index decreases considerably. Because the MO centre remains relatively steady, it influences most of the Mediterranean Sea year round, so it is more suitable for monitoring long-term water and especially heat budget variability.