Eddy Correlation Method Research Articles

Ａｌａｓｋａ州Ｂａｒｒｏｗの湛水ツンドラにおける２０００年度植物生育期間のＣＯ２フラックスの季節変化とＣＯ２収支

The CO2 flux of a wet sedge tundra ecosystem in the Arctic, at Barrow, Alaska, was measured applying an eddy correlation method. The seasonal change of CO2 uptake by the vegetation and the net CO2 budget through the growing period were examined for the 2000 growing season. Tundra CO2 budget analysis model(TCBM) was applied to examine the vegetation responses to the environmental variables. Photosynthesis of the vegetation was well parameterized as functions of measured solar radiation, Rsd and air temperature, Ta, and ecosystem respiration was determined as a function of soil temperature at 0.01 m depth, Ts. Estimated CO2 flux by TCBM with measured Rsd and Ta was verified by comparison with observed CO2 flux, and then was used for gap-filling of data missing or eliminated data during the quality control. Parameters of TCBM functions were in accordance with previous studies; temperature dependence of the ecosystem respiration Q10 was 2.6, peak levels of photosynthesis-light response curve ranged 0.12-2.43gCO2m-2h-1 with optimum air temperature of 0.5-14.0°C, with high levels in late July, and the initial slope of the curve ranged 0.002-0.067 gCO2 W-1h-1. Daily CO2 flux was almost 0gCO2 m-2d-1 while the snow covered the ground. Then that changed to a source of up to 1.6gCO2m-2d-1 for about a week after snowmelt. After mid June, CO2 flux was a sink until mid September; those levels showed clear seasonal trends with a maximum of −17.8gCO2 m-2d-1 occurring on August 2. The Tundra ecosystem at Barrow accumulated −335gCO2 m-2 during 122 days of growing season in 2000. Photosynthesis and ecosystem respiration calculated by TCBM were −490gCO2 m-2 and 156 gCO2m-2, respectively. The larger photosynthesis and lower respiration were brought about because of the high water level condition, with standing water completely submerging the base of the wet sedge tundra. These conditions provided high photosynthetic activity and less gases-exchange between rhizosphere and atmosphere.

Landfill odour: assessment of emissions by the flux footprint method

This paper will focus on the development of an innovative method for estimating odour fluxes and the contributing source areas from a Municipal Solid Waste (MSW) landfill site. A micrometeorological model has been developed based on the estimation of footprints of scalar odour concentration measurements in the atmospheric surface layer. The model is based on an analytical solution of the Eulerian advection-diffusion equation for vertical diffusion; model parameters include the location of the odour sensor and standard surface layer scaling factors. Lindvall Hoods are commonly used for measuring odour fluxes from ground based fugitive sources. However these cannot replicate the real atmospheric conditions. Common micrometeorological techniques for determining fluxes, such as vertical gradient measurements or eddy correlation methods, yield a flux magnitude but give practically no information about the source location. On the other hand the flux footprint describes the expansion and contraction of the required fetch under varying atmospheric stabilities. Preliminary results from the model are quite encouraging and agree well with those from Lindvall hood measurements. The accuracy of Lindvall hood measurements depends on how tightly the hood is positioned on the uneven landfill surface. It should prevent any air escaping out from the sides of the hood open to the atmosphere. Typical results show an average odour flux of ±25.91 (ou m −2 s −1) from freshly tipped wastes for an upwind fetch of 45.0 m and with the sensor at a height of 1.5 m from the ground. The hood results have a geometric mean of ±29.35 (ou m −2 s −1) based on the inlet air volume and a shade air temperature of 22.5 °C.

Validation and Sensitivity Analysis of a New Atmosphere–Soil–Vegetation Model

This paper describes details, validation, and sensitivity analysis of a new atmosphere–soil–vegetation model. The model consists of one-dimensional multilayer submodels for atmosphere, soil, and vegetation and radiation schemes for the transmission of solar and longwave radiations in canopy. The atmosphere submodel solves prognostic equations for horizontal wind components, potential temperature, specific humidity, fog water, and turbulence statistics by using a second-order closure model. The soil submodel calculates the transport of heat, liquid water, and water vapor. The vegetation submodel evaluates the heat and water budget on leaf surface and the downward liquid water flux. The model performance was tested by using measured data of the Cooperative Atmosphere–Surface Exchange Study (CASES). Calculated ground surface fluxes were mainly compared with observations at a winter wheat field, concerning the diurnal variation and change in 32 days of the first CASES field program in 1997, CASES-97. The measured surface fluxes did not satisfy the energy balance, so sensible and latent heat fluxes obtained by the eddy correlation method were corrected. By using options of the solar radiation scheme, which addresses the effect of the direct solar radiation component, calculated albedo agreed well with the observations. Some sensitivity analyses were also done for model settings. Model calculations of surface fluxes and surface temperature were in good agreement with measurements as a whole.

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 (CDN) and scalar coefficient (CHN) were found to be CDN = 0.0055and CHN = 0.0059 in the southeastern area of Tibet. Their variations with the mean wind speed at 10 m are discussed.

Some characteristics of atmospheric surface layer over a tropical inland region during southwest monsoon period

A surface layer experiment conducted at a site in the northwest part of India is described. Micrometeorological data collected from the intense field experiment conducted at a tropical inland station, Ahmedabad (23°N, 72.5°E), India during southwest monsoon period by deploying tower-based instruments were analyzed using eddy correlation method to study the characteristics of atmospheric surface layer (ASL). The normalized standard deviation of vertical wind, temperature and specific humidity were studied using Monin Obukhov (MO) similarity relations. The normalized standard deviation of vertical wind velocity ( φ w) is found to be around 1.37 for near neutral condition and it is found to be proportional to ( z/ L) 1/3 for unstable condition and ( z/ L) 1 for stable conditions. The normalized standard deviation of temperature ( φ T) and humidity ( φ q) are studied as a function of z/ L and it showed ( z/ L) −1/3 dependence for unstable condition and ( z/ L) −1 dependence for stable condition. The value of φ T for near neutral condition is found to be around 6.5, which is high compared with the values reported from other field experiments. The drag coefficient showed a strong dependence on wind velocity at low wind conditions and it is found to obey a power law of wind speed irrespective of the various stability conditions.

Comparison Between Eddy-Correlation and Inertial Dissipation Methods in the Marine Atmospheric Surface Layer

A comparison of momentum fluxes determined by the eddy-correlation method(ECM) and the inertial dissipation method (IDM) has been performed using longterm measurements over the sea. The measurements were made on the island ofOstergarnsholm in the middle of the Baltic Sea. The results show that a`classical' form of the inertial dissipation method, i.e., assuming that the transportterms are negligible, and using an effective value for the Kolmogorov constant of0.55, can be used with a mean relative difference between the two methods of about15% for -1 < z/L < 0.5 (z being height and L the Obukhov length). The IDMmethod works best for high wind speeds and neutral conditions. For low windspeeds (U < 6 m s-1) the relation between the two methods is morecomplex. IDM then gives higher values than ECM on the average (about 20%),especially for swell conditions, indicating the need for an imbalance function inthe turbulent kinetic energy budget. Calculations of the effective Kolmogorovconstant, αa, suggest a dependence upon the wave age, αa increasing with increasing wave age, where the value 0.59 fits the data well for saturated waves.

Comments on “On the Effect of Ocean Waves on the Kinetic Energy Balance and Consequences for the Inertial Dissipation Technique”

Open ocean wind stress data obtained from the Southern Ocean (Yelland and Taylor 1996, henceforth YT) were found to be of significantly lower magnitude than collocated stress estimates derived from the operational wave model, WAMcy4, used at the European Centre for Medium-Range Weather Forecasts (ECMWF). Furthermore, the observations did not exhibit the wave age dependence predicted by Janssen (1989) and others (Yelland et al. 1998). Janssen (1999, henceforth J99) suggested that these differences were due to our use of the inertial dissipation (ID) rather than eddy-correlation (EC) method to determine the stress. Janssen argued that the neglect of the pressure fluctuation term in the turbulent kinetic energy budget would cause ID-derived stress data to be biased low for higher wind speeds and suggested a correction factor. In this comment we will argue that 1) the magnitude of our ID stress data is confirmed by EC data from the open ocean, 2) the height dependence of stress estimates predicted by J99 is not observed, and 3) our neglect of the pressure fluctuation term (as calculated by J99) would have enhanced rather than diminished the wave age dependency of our data, had any been present. We conclude that the suggested bias in ID stress estimates is not significant and that J99’s criticism of the ID method is not justified. The further implication is that the WAM-derived wind stress estimates were overestimated.

Estimating evapotranspiration for a temperate salt marsh, Newcastle, Australia

AbstractEvapotranspiration was studied at a salt marsh site in the Hunter River estuary, NSW, Australia, during 1996–8. Estimates of actual evapotranspiration (Ea) were obtained for three sites using the eddy correlation method. These values were compared with results obtained with the Penman and Penman–Monteith equations, and with pan evaporation. The Penman–Monteith method was found to be most reliable in estimating daily and hourly evapotranspiration. Surface resistance values averaging 12 s m−1 were derived from the eddy correlation estimates. Recent tidal flooding and rainfall were found to decrease surface resistance and increase Ea/Ep ratios. Estimates of evapotranspiration obtained using the Penman–Monteith method were shown to be sensitive to changes in surface resistance, canopy height and the method used to estimate net radiation from incoming solar radiation. These results underline the importance of accurately estimating such parameters based on site‐specific data rather than relying on empirical equations, which are derived primarily for crops and forests. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Ozone Deposition at a Forest Site in NE Bavaria

The dry deposition of ozone to aconiferous forest in northeastern Bavaria(southern Germany) was quantified during 1999with both the eddy correlation method and a bigleaf model. The model included parameterizationsof the atmospheric transfer resistances fromdirect measurements, stomatal resistance from aplant ecological model, and an estimation of thecuticle resistance as function of leaf wetness.Early in the season, the measured and themodelled deposition fluxes were in goodagreement, although the modelled fluxes tended tounderestimate the measured ones. Thisunderestimation was more pronounced in the latesummer, when high nocturnal fluxes werefrequently measured. The model parameterizationof the cuticle and the stomatal resistances didnot allow for such high fluxes. In these cases,the 24 hour average of the measured fluxes wereup to 4.5 times higher than the modelled ones.The reasons for these large discrepancies remainunknown. However, assigning the unaccounted partof the deposition to a non‐stomatal surfacedeposition pathway, a new parameterization of therespective resistance yielded an average value of300 s m-1. It exhibited a decreasing trendthrough the vegetation period.

302 渦相関法に基づく潜熱・顕熱輸送量計測システムの開発 : 屋外計測による検討(熱工学I)

302 渦相関法に基づく潜熱・顕熱輸送量計測システムの開発 : 屋外計測による検討(熱工学I)

都市気候に及ぼす河川水の熱的影響に関する実測研究 : 隅田川における熱収支と周辺影響の検討

Water bodies in urban area have been expected to mitigate the urban heat island as well as green space. In this paper, we showed the results of observations about heat budget at river water and moving observations to clarify horizontal extent of its thermal effects. Diurnal variations of sensible heat flux estimated by several methods corresponded closely with each other, though these values didn't exceed 100W/m2 even in the peak about midday. Estimated heat storage term occupied more than 90% of net radiation during daytime. According to the numerical simulation, observed water temperature variation required equivalent heat conductivity for actual stirred water about forty times as much as that of stagnant water. This heat balance is completely different from that of green space characterized large latent heat flux. On the other hand, horizontal sensible heat flux from adjacent built-up area to the river space also estimated by eddy correlation method at riverside terrace. Turbulent heat flux across the river-edge amounted to 250W/m2 in daytime, which greatly surpassed vertical sensible heat flux at water surface. The horizontal limits of river-effect along the street crossing the river were about 300m from river-edge, and there was no clear relation between this limits and wind speed.

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

The tissue heat-balance method (Cy and the heat-dissipation method (Granier) were compared in three Scots pines and two Norway spruces in a forest in central Sweden. The Granier system measured up to 50% lower sapflow than the Cy system at high flow rates. New coefficients for the Granier system were estimated, based on sapflow density from the Cy measurements. Wi- thout compensation, natural temperature gradients may cause large errors in measurements made by the Granier system. By using a hori- zontal reference sensor, no compensation was necessary. It was also shown that radial flow patterns must be considered when calculating total tree sapflow. Transpiration of two adjacent stands, one measured by the Granier method and the other by the Cy method, sho- wed good agreement internally and with total evaporation measured by the eddy-correlation method.

On the Size Dependence of Particle Deposition

Measurements of dry deposition ofparticles 0.1 to 1.0 μm diameter to forest bythroughfall and eddy correlation methods giveresults of 1.0 cm s-1 or more, whilemechanistic models do not explain values greaterthan about 1 mm s-1. Results of othermethods involving radioactive tracers arerecalled. These data indicate values of about 5 mm s-1. The possibility of additionalmechanisms, omitted from the models, is difficultto exclude, but much of the remaining discrepancymay be due to the wide size distribution of theadventitious radioactive tracers in the atmosphere.

Sources and sinks of ozone in savanna and forest areas during EXPRESSO: Airborne turbulent flux measurements

An airborne study of ozone concentrations and fluxes in the lower layers of the atmosphere was conducted over the Central African Republic (CAR) and northern Congo in November/December 1996, within the framework of the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). The first 4 km of the atmosphere above savanna, rain forest, and the transitional area between them, were investigated with the French research aircraft Avion de Recherche Atmosphérique et de Télédétection (ARAT). Turbulent fluxes and deposition velocities of ozone were determined using the Eddy Correlation (EC) method. A specific methodology was developed to obtain accurate airborne turbulent flux measurements. This methodology is linked to the turbulence stationarity. The average values of ozone fluxes and ozone deposition velocities in the Atmospheric Boundary Layer (ABL) increase appreciably from savanna to forest. Near the ground, the ozone fluxes range between −0.115 +/−0.073 ppbv m/s above savanna and −0.350 +/−0.115 ppbv m/s above forest; for the deposition, the ranges are 0.0042 +/−0.0018 m/s and 0.015 +/−0.004 m/s. A simple empirical relationship between deposition velocity and Leaf Area Index (LAI) is proposed, giving an estimation of the deposition velocity for a whole latitudinal band. Vertical inputs of ozone to the ABL are estimated according to entrainment fluxes. The role of advection is neglected for horizontal transport of ozone in the ABL. The photochemical ozone production is deduced from the photo‐stationary state deviation, and compared to the net ozone increase in the ABL during the flights performed above the forest. A tentative ozone budget based on the aircraft measurements is proposed in the ABL of the rain forest. Around noon, the photochemical production dominates with a net production of about 10 ppbv/h.

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.

Aircraft assessment of trace compound fluxes in the atmosphere with relaxed eddy Accumulation: Sensitivity to the conditions of selection

The Relaxed Eddy Accumulation (REA) technique, implemented aboard aircraft, may be used to measure a wide variety of trace gas fluxes at a regional scale. Its principle is rather simple: air is sampled at a constant rate and the flux is calculated by multiplying a constant β (0.58 in field experiment and 0.62 in simulations) by the standard deviation of the vertical velocity and by the difference between the average concentrations of the scalar (trace gas) for updrafts and downdrafts. The storage of the chemical compound in reservoirs allows for trace gas analysis in laboratory, when in situ measurement with fast response and high sensitivity sensors are not available. The REA method was implemented on the Avion de Recherche Atmosphérique et de Télédétection aircraft during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) campaign. The main requirement for accurate flux determination is the measurement of the vertical component of wind velocity in real time. A simulation technique was developed to evaluate the performance of an aircraft REA. The influence of the time lag between the vertical velocity (W) measurement and REA control was tested, as well as the offset of W, the threshold, and the filtering imposed on W. Correction factors, used in a deployment of aircraft REA, were deduced from this study. An additional simulation was performed to evaluate the influence of spatial or temporal drifts on the scalar. The simulation showed that the REA method is not more disturbed than the Eddy Correlation method by low frequencies of physical origin, such as topography. The REA method was used during EXPRESSO for the measurement of isoprene fluxes over the wet savanna and the evergreen rain forest.

Ozone uptake by an evergreen forest canopy: temporal variation and possible mechanisms

Patterns of ozone concentration ([O3]), O3 deposition velocity (vd) and O3 flux (Fc) over an evergreen forest canopy are shown in relation to measuring method, physiological activity of the trees, and time of year. The gradient and eddy correlation methods were compared and showed similar diel vd patterns. Daytime Fc was correlated with CO2 and water vapour fluxes, while no correlation between [O3] in the range 10–70 ppb (nl l−1) and Fc was seen in this study. Fc was primarily driven by stomatal conductance, reactions with surfaces, particles and gases, and not by [O3]. On a monthly basis, [O3] was always highest in the afternoon while vd was typically higher in the morning, resulting in an equal Fc over the day. Night-time Fc was more than half of the daytime O3 flux. The data reveal the importance of emissions of nitric oxide and terpenes as O3 removal factors in evergreen forest dominated by Norway spruce.

The influence of subtropical cold fronts on the surface energy balance of a semi-arid site

The passage of subtropical cold fronts through central Australia produces the only significant mesoscale meteorological features in the region. The interaction of these cold fronts with the surface energy balance strongly affects the local weather and climate. The surface energy balance was measured at a semi-arid site in Alice Springs, central Australia, to determine how it was influenced by the passage of subtropical cold fronts. Both Bowen ratio and eddy correlation methods were used. The daytime energy balance of the site showed high net radiation that was partitioned into 75% sensible heat flux and 25% soil heat flux with little or no latent heat flux. At night there was a large net radiative loss that was balanced primarily by a loss of heat from the soil. The cold fronts predominately passed through Alice Springs at night and showed a strong surface signature. The fronts brought moister air resulting in higher water vapour pressures during their passage. The nocturnal boundary layer was often disturbed as the front passed, resulting in warm, moist air being mixed down toward the land surface. Mixing decreased the soil heat flux and increased latent heat fluxes toward the surface. Moisture that accumulated at the surface at these times was often evaporated after a return to drier conditions. During the daytime, surface signatures in soil and sensible heat fluxes were less distinct due to the strong convective mixing. Latent heat fluxes followed a similar trend to the nocturnal case.

Examples of the effects of different averaging methods on carbon dioxide fluxes calculated using the eddy correlation method

Abstract. Three hours of high frequency vertical windspeed and carbon dioxide concentration data recorded over tropical forest in Brazil are presented and discussed in relation to various detrending techniques used in eddy correlation analysis. Running means with time constants 100, 1000 and 1875s and a 30 minute linear detrend, as commonly used to determine fluxes, have been calculated for each case study and are presented. It is shown that, for different trends in the background concentration of carbon dioxide, the different methods can lead to the calculation of radically different fluxes over an hourly period. The examples emphasise the need for caution when interpreting eddy correlation derived fluxes especially for short term process studies. Keywords: Eddy covariance; detrending; running mean; carbon dioxide; tropical forest

Wind speed measurements in upper and lower boundary layer to determine regional momentum fluxes

Radiosonde profiles in the atmospheric boundary layer (ABL) with other supporting measurements obtained during NOPEX Second Concentrated Field Effort over a complex area in Sweden were analyzed to determine regional momentum fluxes. Two methods were explored, which consist of (i) the combined similarity approach (CSA) for the entire ABL derived by assuming an overlap region between the mixed layer and surface sublayer and by joining two governing equations, and (ii) the surface sublayer profile equation derived from Monin-Obukhov similarity theory. The CSA has an advantage in that it does not require the knowledge of the regional roughness length which itself is not easy to determine. The surface sublayer profile equation needs the roughness length as input. This was estimated by a model of Grant and Mason (1990) for the upwind area with the size of approximately 26km2, from surface information such as the mean roughness height and density, with assumed drag coefficient of major obstacles, Dh/2=0.89, taken from previous study in a similar experimental area. A good agreement was found between the momentum fluxes derived from the two methods, and it shows the validity of the model used in the estimation of z0 and the assumed value of Dh/2=0.89 for the NOPEX region. The momentum fluxes from the eddy correlation method with an instrument installed at 44m on a tall tower were found to be much smaller than those derived from the profile equation with the wind speed measurements at the same height. The difference appears to be due to the smaller size of the footprint area of momentum flux than that of the wind speed measurements.