Flux agreement above a Scots pine plantation

Summary The surface energy exchange of 12m high Scots pine plantation at Hartheim, Germany, was measured with a variety of methods during a 11-day period of fine weather in mid-May 1992. Net radiation and rate of thermal storage were measured with conventional net radiometers, soil heat flux discs and temperature-based storage models. The turbulent fluxes discussed in this report were obtained with an interchanging Bowen ratio energy budget system (BREB, at 14 m), two one-propeller eddy correlation systems (OPEC systems 1 and 2 at 17m), a 1-dimensional sonic eddy correlation system (SEC system 3) at 15 m, all on one low tower, and a 3-dimensional sonic eddy correlation system (SEC system 22) at 22 m on the high tower that was about 46 m distant. All systems measured sensible and latent heat (H and LE) directly, except for OPEC systems 1 and 2 which estimated LE as a residual term in the surface energy balance. Closure of turbulent fluxes from the two SEC systems was around 80% for daytime and 30% for night, with closure of 1-dimensional SEC system 3 exceeding that of 3-dimensional SEC system 22. The night measurements of turbulent fluxes contained considerable uncertainty, especially with the BREB system where measured gradients often yielded erroneous fluxes due to problems inherent in the method (i.e., computational instability as Bowen's ratio approaches –1). Also, both eddy correlation system designs (OPEC and SEC) appeared to underestimate |H| during stable conditions at night. In addition, both sonic systems (1- and 3-dimensional) underestimated |LE| during stable conditions. The underestimate of |H| at night generated residual estimates of OPEC LE containing a phantom dew error that erroneously decreased daily LE totals by about 10 percent. These special night problems are circumvented here by comparing results for daytime periods only, rather than for full days. To summarize, turbulent fluxes on the low tower from OPEC system 2 and the adjacent SEC system 3 were in reasonable agreement, while the BREB system appeared to overestimate H and underestimate LE; H and LE measured by SEC system 22 on the high tower were lower than from OPEC and SEC3 on the low tower. The turbulent flux measurements tended to converge, but the data exhibit unexplained differences between days, between systems, and between locations.With 7 Figures     

The May–October energy budget of a Scots pine plantation at Hartheim,Germany

Summary This paper describes measurements of the Hartheim forest energy budget for the 157-day period of May 11 – Oct. 14, 1992. Data were collected as 30-min means. Energy available to the forest was measured with net radiometers and soil heat flux discs; sensible heat exchange between the canopy and atmosphere was measured with two One-Propeller Eddy Correlation (OPEC) systems, and latent energy (evapotranspiration orET) was determined as a residual in the surface energy balance equation. Net rediation, change in thermal storage, and sensible heat flux were verified by independent measurements during the Hartheim Experiment (HartX, May 11–12), and again during the HartX2 experiment over 20 days late in the summer (Sep. 10–29). Specifically, sensible heat estimates from the two adjacent OPEC sensor sets were in close agreement throughout the summer, and in excellent agreement with measurements of sonic eddy correlation systems in May and September. The eddy correlation/energy balance technique was observed to overestimate occurrence of dew, leading to an underestimate of dailyET of about 5%. After taking dew into account, estimates of OPECET totaled 358 mm over the 5.1-month period, which is in quite good agreement with an ET estimate of 328 mm from a hydrologic water balance. An observed decrease in forestET in July and August was clearly associated with low rainfall and increased soil water deficit. The OPEC system required only modest technical supervision, and generated a data yield of 99.5% over the period DOY 144–288. The documented verification and precision of this energy budget appears to be unmatched by any other long-term forest study reported to date.With 9 Figures     

Testing of the bandpass eddy covariance method for a long-term measurement of water vapour flux over a forest

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.     

Frequency response corrections for eddy correlation systems

Simplified expressions describing the frequency response of eddy correlation systems due to sensor response, path-length averaging, sensor separation and signal processing are presented. A routine procedure for estimating and correcting for the frequency response loss in flux and variance measurements is discussed and illustrated by application to the Institute of Hydrology's Hydra eddy correlation system.The results show that flux loss from such a system is typically 5 to 10% for sensible and latent heat flux, but can be much larger for momentum flux and variance measurements in certain conditions.A microcomputer program is included which, with little modification, can be used for estimating flux loss from other eddy correlation systems with different or additional sensors.     

Latent and sensible heat flux predictions from a uniform pine forest using surface renewal and flux variance methods

A surface renewal model that links organized eddy motion to the latent and sensible heat fluxes is tested with eddy correlation measurements carried out in a 13m tall uniform Loblolly pine plantation in Duke Forest, Durham, North Carolina. The surface renewal model is based on the occurance of ramp-like patterns in the scalar concentration measurements. To extract such ramp-like patterns from Eulerian scalar concentration measurements, a newly proposed time-frequency filtering scheme is developed and tested. The time-domain filtering is carried out using compactly-supported orthonormal wavelets in conjunction with the Universal Wavelet Thresholding approach of Donoho and Johnstone, while the frequency filtering is carried out by a band-pass sine filter centered around the ramp-occurrence frequency as proposed by other studies. The method was separately tested for heat and water vapour with good agreement between eddy correlation flux measurements and model predictions. The usefulness of the flux-variance method to predict sensible and latent heat fluxes is also considered. Our measurements suggest that the simple flux-variance method reproduces the measured heat and momentum fluxes despite the fact that the variances were measured within the roughness sublayer and not in the surface layer. Central to the predictions of water vapour fluxes using the flux-variance approach is the similarity between heat and water vapour transport by the turbulent air flow. This assumption is also investigated for this uniform forest terrain.     

Boundary-Layer Adjustment Over Small-Scale Changes of Surface Heat Flux

Four months of eddy correlation data collected over a grass field and a nearby sage brush community are analyzed to examine the adjustment of the boundary-layer structure as it flows from the heated brush to the snow-covered grass. The grass site includes a 34-m tower with seven levels of eddy correlation data. The midday heat flux over the snow-covered grass and bare ground surfaces is often downward particularly with melting conditions, while the corresponding heat flux over the brush is almost always upward. For most of these cases, a stable internal boundary layer over the snow is well defined in terms of vertical profiles of the buoyancy flux over the snow-covered grass. The stable internal boundary layer is generally embedded within a deeper layer of flux divergence corresponding to increasing upward heat flux with height above the internal boundary layer. With thin snow cover, the surface heat flux over the grass is weak upward due to heating of grass protruding above the snow so that the flow adjusts to a decrease of the upward surface heat flux in the downwind direction. This common case of an adjusting boundary layer contrasts with the formation of an internal boundary layer due to a change of sign of the surface heat in flux the downwind direction. The adjustment of the boundary layer to the decrease of the surface heat flux leads to vertical divergence of the upward heat flux in contrast to the usual heated boundary layer over homogeneous surfaces. The consequences of the cooling due to the vertical divergence of the heat flux are discussed in terms of the heat budget of the adjusting and internal boundary layers.     

Measurements Of Turbulence Transfer In The Near-Surface Layer Over The Southeastern Tibetan Plateau

Turbulent flux measurements at Qamdo site over the Tibetan Plateau during TIPEX from May 18 to June 30, 1998 are presented. Sensible heat dominated,accounting for about 66% of the available energy (the sum of net radiation and soil heat flux) prior to the monsoon(dry period), reducing to about 31%, with latent heat increased to about 56% of available energy,in the monsoon season (wet period). Surface energy budget closure on average was about 0.80 (0.85)prior to the monsoon and 0.89 (0.76) during the monsoon using eddy correlation (profile) methods. The sum of latent and sensible heat fluxes calculated from the flux-profilemethod was smaller by about 15% than that from eddy correlation. Martano's method is used toestimate the surface aerodynamic roughness length z₀ and zero plane displacement d from singlelevel sonic anemometer data, giving d = 0.12 m and z₀ = 0.08 m. The overall neutral dragcoefficient (C_DN) and scalar coefficient (C_HN) were found to be C_DN = 0.0055and C_HN = 0.0059 in the southeastern area of Tibet. Their variations with the mean wind speed at 10 m are discussed.     

广州番禺地区草地陆气相互作用观测研究

介绍了2004年在广州番禺进行的陆气相互作用观测试验。观测研究表明:新型的超声风温仪虽然带有雨滴防护沙网, 但雨滴对超声观测的误差影响显著, 尤其是对u, w方向。雨天情况下与z/L相关系数很低, 湍流强度与稳定度不存在1/3次方关系。涡动相关法和能量平衡法计算的感热通量及潜热通量比较接近, 感热、潜热通量相关系数分别达到0.8699和0.8633, 两种方法带来的误差主要发生在近地层稳定度发生明显变化的时刻, 即在午间热通量的峰值或傍晚或晚间热通量的低值, 其中能量平衡法计算的感热、潜热普遍具有较大的正负峰值。涡动相关法计算的Q_h+Q_e普遍偏小, 与可用能量R_n-Q_g多数情况下存在能量不平衡, 说明了忽略热存储项的地表能量平衡方程的局限性。番禺夏、秋季近地层各能量具有与太阳辐射相似的日变化特征, 但夏季的潜热大于感热, 而秋季则相反。近地面二氧化碳从5—8月是一个减低过程, 尔后上升到12月份浓度最高, 总体浓度值在350×10-6~400×10-6之间变化。     

淮河流域典型地面水热通量的观测分析

根据1998年淮河流域能量和水循环试验加密观测(HUBEX-IOP)获得的资料,用三种微气象方法计算该地区的感热通量和潜热通量状况,比较了不同方法所估算结果的异同,并以梯度法的资料为主分析了其变化特征.结果表明:(1)梯度法估算的潜热通量与用波文比-能量平衡方法的估算值比较接近,与涡度相关方法估算的值之间有一定的差异.(2)感热通量和波文比在梅雨(6月28日～7月4日)前较大,在后期较小.大部分辐射能用于地面蒸发.     

Dry-air boundary conditions for correction of eddy flux measurements

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.     

Comparative measurements of the energy fluxes over a pine forest

During the summer of 1974, simultaneous heat flux measurements were made over a pine forest in three ways:

1. Using the Bowen ratio - Energy Budget technique, with the Bowen ratio estimated from temperature and humidity profiles measured with two pairs of wet-and dry-bulb thermometers at six levels.
2. Using the Bowen ratio - Energy Budget technique, with the Bowen ratio estimated from temperature and humidity measurements using pairs of wet-and dry-bulb thermometers mechanically interchanged every ten minutes.
3. Using the Eddy Correlation - Energy Budget technique, with a direct measurement of the sensible heat flux made by a simple eddy correlation apparatus.

Methods (b) and (c) are shown to give similar results apart from a systematic difference of about 25% in the measured sensible heat flux. This corresponds to an underestimate of the sensible heat flux by the eddy correlation apparatus. Although sometimes consistent with the other two methods, on occasions method (a) gave results which were significantly different from both (b) and (c). When differences occurred, they tended to be systematic and persistent over individual days; but they could change magnitude and sign if the particular sensors used at each level in the profile were rearranged. The experimental program used to collect these and previous data involved the rearrangement of sensors on a regular (two day) time scale. It is shown that, when averaged over several such rearrangements, method (a) produces median values of surface resistance which are more in keeping with those produced by the other methods. This is taken to imply that data previously gathered in this way can be used to produce physically reasonable results providing they are averaged over several days. On the basis of the results presented, recommendations are made on future experimental work in forest micrometeorology.     

Estimates of sensible heat flux from observations of temperature fluctuations

Comparisons between sensible heat flux measured using eddy correlation instrumentation and estimated using the temperature fluctuation method are presented for four types of surface in West Africa. Agreement between measured and estimated values is good. Regression of estimated on measured sensible heat flux gave a mean slope of 0.98 with a mean r ² of 0.94 for bare soil, mature millet, fallow savannah and tiger bush. Estimates of heat flux from temperature fluctuations measured by an instrument mounted beneath a tethered balloon are also shown to be in close agreement with eddy correlation measurements made at the surface (regression slope = 0.98, r ² = 0.84). The results provide evidence that the ratio /_×is indeed a universal function of z/L for all the surface types considered.     

Sea surface wind stress and drag coefficients: The hexos results

Turbulent fluxes have been measured in the atmospheric surface layer from a boom extending upwind from the Dutch offshore research platform Meetpost Noordwijk (MPN) during HEXMAX (Humidity Exchange over the Sea Main Experiment) in October–November, 1986. We started out to study eddy flux of water vapour, but discrepancies among simultaneous measurements made with three different anemometers led us to develop methods to correct eddy correlation measurements of wind stress for flow distortion by nearby objects. We then found excellent agreement among the corrected wind stress data sets from the three anemometers on the MPN boom and with eddy correlation measurements from a mast on a tripod. Inertial-dissipation techniques gave reliable estimates of wind stress from turbulence spectra, both at MPN and at a nearby ship. The data cover a range of wave ages and the results yield new insights into the variation of sea surface wind stress with sea state; two alternative formulas are given for the nondimensional surface roughness as a function of wave age.     

The air density correction to eddy flux measurements

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.     

关于大气涡动热通量参数化问题研究

用NCEP/NCAR等再分析资料计算和分析表明,月平均瞬变涡动热通量和月平均温度梯度的局域相关性很低,甚至在一些纬度是负相关,更严重的是二者之间存在非线性的多值对应函数关系.如果用非局域参数化,参数化水平将有所提高,但改进并不明显.通过与分子运动论输运理论进行数学、特别是物理类比提出了考虑"风暴路径"效应的瞬变涡动热通量的参数化方案,经再分析资料的检验,结果表明它能大大提高参数化的水平.其次,还证实采用空间非局域参数化仍然是进一步提高瞬变涡动热通量参数化水平的方法之一.此外,资料分析显示中高纬度纬向平均温度梯度和纬偏涡动温度通量之间也存在负相关及多值函数关系,说明仅用温度梯度去参数化纬向平均涡动热通量难以得到好的结果,需要考虑多元参数化或非同步参数化等.     

GAME/Tibet实验中感热通量计算方法的讨论

回顾了前人对感热通量的分析研究，详细分析了计算感热通量公式的推导过程。结合GAME／Tibet那曲近地面通量观测站1998年6月6日到9月13日的观测资料验证了近地面层逆梯度热输送现象的存在；并比较了两种不同计算感热通量的方法。同时，对简化生物圈模式(SiB2)计算感热通量方案进行了分析。主要结论有：(1)由于用涡旋相关法测量得到的总体感热通量本身已经包含了非局地热(冷)泡运动的影响，所以在非均匀下垫面上，传统的闭合方案必然会“遭遇”能量不平衡。这里，非均匀下垫面可能是导致能量不平衡的关键所在。(2)在今后的有关热通量研究中，有必要仔细研究非局地热(冷)泡运动对热通量的贡献，而为了更好地理解这种贡献，应用陆面过程模式研窑能量平衡是必需的。     

The Spatial Variability of Energy and Carbon Dioxide Fluxes at the Floor of a Deciduous Forest

Fluxes of carbon dioxide, water and sensible heat were measured using three different eddy covariance systems above the forest floor of a closed deciduous forest (leaf area index approx 6). The primary objective was to examine the representativeness of a single eddy covariance system in estimating soil respiration for time scales ranging from one-half hour to more than one week. Experiments were conducted in which the eddy covariance sensors were in one of three configurations: i) collocated, ii) separated horizontally or iii) separated vertically. A measure of the variation between the three systems (CV',related to the coefficient of variation) for half-hour carbon dioxide fluxes was 0.14 (collocated systems), 0.34 (vertically separated systems at 1, 2 and 4 m above the surface), and 0.57 (systems horizontally separated by 30 m). A similar variation was found for other scalar fluxes (sensible and latent heat). Variability between systems decreased as the number of half-hour sampling periods used to obtain mean fluxes was increased. After forty-eight hours (means from ninety-six half-hour samples), CV' values for carbon dioxide fluxes were 0.07, 0.09 and 0.16 in the collocated, vertically separated and horizontally separated experiments, respectively. The time dependence of variability has implications on the appropriateness of using short-term measurements in modelling validation studies. There are also implications concerning the appropriate number of half-hour samples necessary to obtain reliable causal relationships between flux data and environmental parameters. Based on the longer-term measurements, we also discuss the representativeness of a single eddy covariance system in long-term monitoring of soil respiration and evaporation beneath forest canopies using the eddy covariance method.     

Turbulent variance characteristics of temperature and humidity over a non-uniform land surface for an agricultural ecosystem in China

This paper describes the application of the variance method for flux estimation over a mixed agricultural region in China. Eddy covariance and flux variance measurements were conducted in a near-surface layer over a non-uniform land surface in the central plain of China from 7 June to 20 July 2002. During this period, the mean canopy height was about 0.50 m. The study site consisted of grass (10% of area), beans (15%), corn (15%) and rice (60%). Under unstable conditions, the standard deviations of temperature and water vapor density (normalized by appropriate scaling parameters), observed by a single instrument, followed the Monin-Obukhov similarity theory. The similarity constants for heat (CT ) and water vapor (Cq) were 1.09 and 1.49, respectively. In comparison with direct measurements using eddy covariance techniques, the flux variance method, on average, underestimated sensible heat flux by 21% and latent heat flux by 24%, which may be attributed to the fact that the observed slight deviations (20% or 30% at most) of the similarity “constants” may be within the expected range of variation of a single instrument from the generally-valid relations.     

Aircraft Observations of Sea-Surface Turbulent Fluxes Near the California Coast

Aircraft turbulence data from the Autonomous Ocean Sampling Network project were analyzed and compared to the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parametrization of turbulent fluxes in an ocean area near the coast of California characterized by complex atmospheric flow. Turbulent fluxes measured at about 35 m above the sea surface using the eddy-correlation method were lower than bulk estimates under unstable and stable atmospheric stratification for all but light winds. Neutral turbulent transfer coefficients were used in this comparison because they remove the effects of mean atmospheric conditions and atmospheric stability. Spectral analysis suggested that kilometre-scale longitudinal rolls affect significantly turbulence measurements even near the sea surface, depending on sampling direction. Cross-wind sampling tended to capture all the available turbulent energy. Vertical soundings showed low boundary-layer depths and high flux divergence near the sea surface in the case of sensible heat flux but minimal flux divergence for the momentum flux. Cross-wind sampling and flux divergence were found to explain most of the observed discrepancies between the measured and bulk flux estimates. At low wind speeds the drag coefficient determined with eddy correlation and an inertial dissipation method after corrections were applied still showed high values compared to bulk estimates. This discrepancy correlated with the dominance of sea swell, which was a usually observed condition under low wind speeds. Under stable atmospheric conditions measured sensible heat fluxes, which usually have low values over the ocean, were possibly affected by measurement errors and deviated significantly from bulk estimates.     

Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment

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