Friction velocity

The eddy covariance technique, which is used in the determination of net ecosystem CO 2 exchange (NEE), is subject to significant errors when advection that carries CO 2 in the mean flow is ignored. We measured horizontal and vertical advective CO 2 fluxes at the Niwot Ridge AmeriFlux site (Colorado, USA) using a measurement approach consisting of multiple towers. We observed relatively high rates of both horizontal (F hadv ) and vertical (F vadv ) advective fluxes at low surface friction velocities (u * ) which were associated with downslope katabatic flows. We observed that F hadv was confined to a relatively thin layer (0-6 m thick) of subcanopy air that flowed beneath the eddy covariance sensors principally at night, carrying with it respired CO 2 from the soil and lower parts of the canopy. The observed F vadv came from above the canopy and was presumably due to the convergence of drainage flows at the tower site. The magnitudes of both F hadv and F vadv were similar, of opposite sign, and increased with decreasing u * , meaning that they most affected estimates of the total CO 2 flux on calm nights with low wind speeds. The mathematical sign, temporal variation and dependence on u * of both F hadv and F vadv were determined by the unique terrain of the Niwot Ridge site. Therefore, the patterns we observed may not be broadly applicable to other sites. We evaluated the influence of advection on the cumulative annual and monthly estimates of the total CO 2 flux (F c ), which is often used as an estimate of NEE, over six years using the dependence of F hadv and F vadv on u * . When the sum of F hadv and F vadv was used to correct monthly F c , we observed values that were different from the monthly F c calculated using the traditional u * -filter correction by À16 to 20 g CÁm À2 Ámo À1 ; the mean percentage difference in monthly F c for these two methods over the six-year period was 10%. When the sum of F hadv and F vadv was used to correct annual F c , we observed a 65% difference compared to the traditional u * -filter approach. Thus, the errors to the local CO 2 budget, when F hadv and F vadv are ignored, can become large when compounded in cumulative fashion over long time intervals. We conclude that the ''micrometeorological'' (using observations of F hadv and F vadv ) and ''biological'' (using the u * filter and temperature vs. F c relationship) corrections differ on the basis of fundamental mechanistic grounds. The micrometeorological correction is based on aerodynamic mechanisms and shows no correlation to drivers of biological activity. Conversely, the biological correction is based on climatic responses of organisms and has no physical connection to aerodynamic processes. In those cases where they impose corrections of similar magnitude on the cumulative F c sum, the result is due to a serendipitous similarity in scale but has no clear mechanistic explanation.

CITATIONS 10 READS 257 4 authors: Some of the authors of this publication are also working on these related projects: EVEX (EVènements EXtrêmes et érosion du trait de côte : mesures, modélisation numérique et impacts sociétaux / Extreme events and erosion of the shoreline: measurements, numerical modeling and societal impacts) View project Coupled ocean-wave model development in forecast environment (WAVE2NEMO) View project X. Bertin Université de La Rochelle a b s t r a c t

The dissolution of in-situ generated methane hydrate in undersaturated, synthetic seawater (S = 35) was investigated in a series of laboratory-based experiments at P-/T-conditions within the hydrate stability field. A controlled flow field was generated across the smooth hydrate surface to test if, in addition to thermodynamic variables, the dissolution rate is influenced by changing hydrodynamic conditions. The dissolution rate was found to be strongly dependent on the friction velocity, showing that hydrate dissolution in undersaturated seawater is a diffusion-controlled process. The experimental data was used to obtain diffusional mass transfer coefficients k d , which were found to correlate linearly with the friction velocity, u ★ . The resulting k d /u ★ -correlation allows predicting the flux of methane from natural gas hydrate exposures at the sediment/seawater interface into the bulk water for a variety of natural P, T and flow conditions. It also is a tool for estimating the rate of hydrate regrowth at locations where natural hydrate outcrops at the seafloor persist in contact with undersaturated seawater.

A simple analytical model is developed for the current induced by the wind and modified by surface wind-waves in the oceanic surface layer, based on a first-order turbulence closure and including the effect of a vortex force representing the Stokes drift of the waves. The shear stress is partitioned between a component due to shear in the current, which is reduced at low turbulent Langmuir number (La t), and a wave-induced component, which decays over a depth proportional to the dominant wavelength. The model reproduces the apparent reduction of the friction velocity and enhancement of the roughness length estimated from current profiles, detected in a number of studies. These effects are predicted to intensify as La t decreases, and are entirely attributed to non-breaking surface waves. The current profile becomes flatter for low La t owing to a smaller fraction of the total shear stress being supported by the current shear. Comparisons of the model with the comprehensive dataset provided by the laboratory experiments of Cheung and Street show encouraging agreement, with the current speed decreasing as the wind speed increases (corresponding to decreasing La t), if the model is adjusted to reflect the effects of a full wave spectrum on the intensity and depth of penetration of the wave-induced stress. A version of the model where the shear stress decreases to zero over a depth consistent with the measurements accurately predicts the surface current speed. These results contribute towards developing physically-based momentum flux parameterizations for the wave-affected boundary layer in ocean circulation models.

Shipboard erosion experiments were conducted on retrieved carbonate ooze sediments using the suction-stirring flume instrument of . Interfacial hydrodynamics and entertainment functions of currently used erosion devices. Burt, N.T., et al. (Eds.) Cohesive Sediments. Wiley, Chichester, at two differing localities (site A, 3800 m water depth; site B, 1100 m water depth) in the eastern North Atlantic.

Shading banana and other orchard crops with screens is increasing in popularity due to decreased water use, increased fruit quality and other reasons. This study focused on airflow, turbulence, ventilation rate, evapotranspiration and energy balance in a large commercial flat-roof banana screenhouse in northern Israel. Measurements were made during 14 days. An eddy covariance system, consisting of a one-dimensional sonic anemometer, a fine-wire thermocouple and a Krypton hygrometer, was deployed at 5 m height, in a location allowing a minimum fetch of 100 m in all directions. Net radiation, soil heat flux and storage, soil evaporation, air temperature and humidity and turbulent airflow characteristics were measured. Comparison between wind speed inside and outside of the screenhouse suggests that the logarithmic wind profile model is approximately valid within the screenhouse. Airflow direction inside the screenhouse was usually the same as that of the external wind. Friction velocity scaled with the mean horizontal wind speed inside the screenhouse. Integral length scale calculations showed that eddies were always narrow and long and high air velocities caused eddies to become even narrower, as expected. Average turbulence intensity within the screenhouse for all data was 0.49 AE 0.12 (AE standard deviation). Spectral energy density depended on frequency to the power of about À5/3, typical of the inertial subrange in turbulent boundary layers. Air exchange rate increased with wind speed, as expected. Evapotranspiration averaged 5.6 AE 0.47 mm day À1 , in accord with the 7-8 mm day À1 irrigation. Analysis of energy balance closure resulted in a slope of 0.94 and an intercept of 2.4 W m À2 .

This paper presents a multi-site (>20) analysis of the relative and absolute energy balance (EB) closure at European FLUXNET sites, as a function of the stability parameter , the friction velocity u * , thermallyinduced turbulence, and the time of the day. A focus of the analysis is the magnitude of EB deficits for very unstable conditions. A univariate analysis of the relative EB deficit as function of alone (both for individual sites and a synthesis for all sites), reveals that the relative EB deficit is larger for very unstable conditions ( < −1.0) than for less unstable conditions (−0.02 > ≥ −1.0). A bivariate analysis of the relative EB deficit as function of both and u * , however, indicates that for situations with comparable u * the closure is better for very unstable conditions than for less unstable conditions. Our results suggest that the poorer closure for very unstable conditions identified from the univariate analysis is due to reduced u * under these conditions. In addition, we identify that the conditions characterized by smallest relative EB deficits (elevated overall turbulence, mostly during day time) correspond to cases with the largest absolute EB deficits. Thus, the total EB deficit at the sites is induced mostly under these conditions, which is particularly relevant for evapotranspiration estimates. Further, situations with the largest relative EB deficits are generally characterized by small absolute EB deficits. We also find that the relative EB deficit does generally not correspond to the regression line of absolute EB deficit with the net radiation because there is a (positive or negative) offset. This can be understood from theoretical considerations. Finally, we find that storage effects explain a considerable fraction of the large relative (but small absolute) nocturnal EB deficits, and only a limited fraction of the overall relative and absolute EB deficits.

Data from National Centre for Atmospheric Research (NCAR) Queen Air boundarylayer flights over the Nebraska Sandhills are analyzed to investigate the effects of these low hills on boundary-layer turbulence. The Sandhills are an area of anisotropic rolling terrain with characteristic wavelengths of order 2 km and rms height variations of order 25 m. The biggest impact is found in early morning flight data where horizontal velocity perturbations appear at the same wavelengths as the terrain and variances (normalised by u 2 * , where u * is the local friction velocity) are significantly enhanced relative to standard flat terrain values. By contrast the vertical velocity variance seems less affected and terrain effects are much less evident in data from the afternoon convective boundary layer.

1] If liquid CO 2 is stored as a dense ''lake'' on the deep ocean floor, it is expected to dissolve in seawater. Ocean currents and turbulence can increase the net rate of CO 2 release by several orders of magnitude compared to molecular diffusion. However, density stratification in the seawater created by dissolved CO 2 will tend to reduce vertical mixing. A two-dimensional numerical study with a high-resolution advection-diffusion model, coupled with a general turbulence model, reveals significant modifications of the boundary layer structure above a generic CO 2 lake taken to be 500 m in length and placed in a 10 km domain that is subject to specified far-field currents in the range of 0.05-0.20 m s À1 . Dissolution rates of order 0.07 mmol cm À2 s À1 , reaching approximately 0.5 mmol cm À2 s À1 during a benthic storm, are predicted. The friction velocity is reduced above the CO 2 lake, and CO 2 concentrations corresponding to excess water densities of up to 0.5 kg m À3 occur in the lower 10 m of the water column. The persistency of this low-pH, CO 2 -enriched water layer in weakly stratified and neutral background conditions in the model suggests that gravity current dynamics are important over considerable distances and should be considered in future larger-scale models and impact studies.

In order to demonstrate the flow properties of the river bed and the design of hydraulic structures, the estimation of friction velocity is essentially required. However, existing friction velocity equations such as Log method and Power law have trouble to estimate the friction velocity because a boundary condition and various hydraulic properties are changed constantly in near the wall. In the present study, therefore, a new friction velocity equation that can minimize the parameters and reduce an error was suggested. To verify accuracy and reliability for the proposed equation, Clauser method,    method, reynolds stress method by Dr. Song were compared with the proposed method by estimated entropy parameter M for each channel. Consequently, the results show that uniform flow condition as well as non-uniform flow condition with highly accuracy nearly matched in case of accelerating non-uniform condition of R 2 =0.9621, Decelerating Non Uniform condition of R 2 =0.9274, Uniform condition of R 2 =0.8865

A previously obtained analytical solution to model the short-range dispersion of pollutants in low winds from surface releases has been used to simulate diffusion tests conducted during winter in weakly convective conditions at the Indian Institute of Technology (IIT) Delhi. The turbulence parameterization based on friction velocity has been tested to simulate diffusion experiment. Such a parameterization in this study is considered justifiable on two counts: (1) prevailing meteorological and dispersion conditions have been generally of weakly unstable type as indicated by values of Monin-Obukhov length and bulk Richardson number, (2) uncertainties associated with the application of convective velocity based similarity parameterization to simulation of diffusion experiment at IIT Delhi, resulting in significant underprediction in most of the cases (Atmos. Environ. 30 (1996a) 1137). With this parameterization, the model simulations have improved considerably and compare reasonably well with the observations. Further, the results from a simple Gaussian model have been included for comparison. This study is in continuation of the work done earlier to simulate near-source dispersion in weak winds.

Aim of the present paper is to quantify the ecosystem respiration of a mountain meadow in the Austrian Alps during the vegetation period 2002 by constraining nighttime eddy covariance measurements with ecosystem respiration derived from (i) daytime eddy covariance, (ii) ecosystem chamber and (iii) scaled up leaf and soil chamber measurements. The study showed that the discrimination of valid nighttime eddy covariance measurements based on friction velocity (u *), the so-called u *-correction, is very sensitive to the imposed quality control criteria. Excluding half-hourly nighttime data, which deviate more than 30% from the stationarity and integral turbulence tests caused the magnitude of the u *-correction to be significantly reduced. Based solely on nighttime eddy covariance data, we are currently unable to decide whether the observed high CO 2 fluxes during intermittent turbulence represent artefacts and should be screened out, or whether these reflect a genuine transport of CO 2 not accounted for by the storage term and must be retained. Evidence against the inclusion of these data is derived from soil respiration rates measured in situ and calculated inversely from the other approaches, which were significantly lower as compared to soil respiration calculated from inversion of the half-hourly nighttime data inclusive of the observations which failed to meet the specified quality control criteria. Seasonal (8 March-8 November 2002) nighttime carbon balances simulated based on the parameters derived from the remaining approaches agreed with each other to within 35%, which is of the order of the uncertainty of each individual approach.

The eddy covariance technique, which is used in the determination of net ecosystem CO 2 exchange (NEE), is subject to significant errors when advection that carries CO 2 in the mean flow is ignored. We measured horizontal and vertical advective CO 2 fluxes at the Niwot Ridge ...

1] Results of an experimental investigation of the turbulent boundary layer in airflow over evolving young wind-waves are presented. The experiments were conducted in a laboratory wind-wave flume consisting of a closed-loop wind tunnel capable of generating wind speed that may exceed 15 m/s, atop of a 5 m long wave tank. Simultaneous measurements of mean wind velocity and of instantaneous fluctuations of the horizontal and vertical air velocity components were carried out along the test section at different airflow rates and at numerous heights above the highest wave. Instantaneous surface elevation at the air sensors' location was simultaneously recorded. The friction velocities at all locations and for all airflow rates were determined by two independent methods: by fitting the logarithmic velocity profiles and by extrapolating the measured Reynolds shear stresses to mean water surface level. The variation with height and along the test section of the fluctuations of two velocity components, in the mean flow and in the vertical directions, was also studied and the results compared with flow behavior over rough and smooth plates. Wave-induced airflow parameters were then investigated by application of cross-spectral analysis. Results on the vertical extent of wave-induced boundary layer, on the phase relation between the wave-induced velocity fluctuations and the surface elevation, as well as on the wave-induced Reynolds shear stress are reported.

Shipboard erosion experiments were conducted on retrieved carbonate ooze sediments using the suction-stirring flume instrument of . Interfacial hydrodynamics and entertainment functions of currently used erosion devices. Burt, N.T., et al. (Eds.) Cohesive Sediments. Wiley, Chichester, at two differing localities (site A, 3800 m water depth; site B, 1100 m water depth) in the eastern North Atlantic.

A model for estimating the turbulent kinetic energy dissipation rate in the oceanic boundary layer, based on insights from rapid-distortion theory, is presented and tested. This model provides a possible explanation for the very high dissipation levels found by numerous authors near the surface. It is conceived that turbulence, injected into the water by breaking waves, is subsequently amplified due to its distortion by the mean shear of the wind-induced current and straining by the Stokes drift of surface waves. The partition of the turbulent shear stress into a shear-induced part and a wave-induced part is taken into account. In this picture, dissipation enhancement results from the same mechanism responsible for Langmuir circulations. Apart from a dimensionless depth and an eddy turn-over time, the dimensionless dissipation rate depends on the wave slope and wave age, which may be encapsulated in the turbulent Langmuir numberLat. For large Lat, or any Lat but large depth, the dissipation rate tends to the usual surface layer scaling, whereas when Lat is small, it is strongly enhanced near the surface, growing asymptotically as ε ∝ Lat−2 when Lat → 0. Results from this model are compared with observations from the WAVES and SWADE data sets, assuming that this is the dominant dissipation mechanism acting in the ocean surface layer and statistical measures of the corresponding fit indicate a substantial improvement over previous theoretical models. Comparisons are also carried out against more recent measurements, showing good order-of-magnitude agreement, even when shallow-water effects are important.

Wind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (u Ãt), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that u Ãt is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 m s-1. Schmidt's threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnold's threshold parameter is 0.18. Roughness lengths (z 0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to u 2 Ãt. The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 z 0 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100-175 mm, and independent of surface particle size.

Concentration fluctuation data from surface-layer released smoke plumes have been investigated with the purpose of finding suitable scaling parameters for the corresponding two-particle, relative diffusion process. Dispersion properties have been measured at downwind ranges between 0.1 and 1 km from a continuous, neutrally buoyant ground level source. A combination of SF 6 and chemical smoke (aerosols) was used as tracer. Instantaneous crosswind concentration profiles of high temporal (up to 55 Hz) and spatial resolution (down to 0.375 m) were obtained from aerosolbackscatter Lidar detection in combination with simultaneous gas chromatograph (SF 6) reference measurements. The database includes detailed crosswind concentration fluctuation measurements. Each experiment, typically of 1 2-hour duration, contains plume mean and variance concentration profiles, intermittency profiles and exceedence and duration statistics. The diffusion experiments were accompanied by detailed in-situ micrometeorological mean and turbulence measurements. In this paper, a new distance-neighbour function for surface-released smoke plumes is proposed, accompanied by experimental evidence in its support. The new distance-neighbour function is found to scale with the surface-layer friction velocity, and not with the inertial subrange dissipation rate, over the range of distance-neighbour separations considered.

This paper presents a new technique for estimating the wind friction velocity at the ocean surface from C-band radar scatterometer measurements. This technique uses physical models of ocean surface waves and electromagnetic backscattering from a rough surface at intermediate angles of incidence to generate predictions of the normalized radar cross section (NRCS, or rr ø) of the ocean surface for a given wind friction velocity and observational geometry. The ocean spectral model used in this technique has been developed specifically for this application. It combines in situ wave measurements at low wave numbers with the Phillips [1985] equilibrium spectral model. This choice of ocean wave model is supported by a set of open ocean wave measurements summarized in this paper. A suite of models, derived from both in situ and remote measurements of the sea surface, is used to characterize the directional spreading of ocean waves relative to the wind direction. The resulting two-dimensional ocean wave spectra are used with a composite surface model to predict radar backscattering from the ocean surface at Cband. These radar cross-section predictions are combined with ERS-1 scatterometer measurements in a cost function minimization scheme to yield estimates of the friction velocity vector at the ocean surface. We present examples of this technique and compare friction velocity retrievals obtained via this scheme with buoy-based measurements under a variety of wind and wave conditions. On the basis of the analysis of a limited number of cases, this technique yielded friction velocity estimates for which the magnitude was within 22% and the direction was within _+25 ø. Given that scientific applications require magnitude estimates within 10-15% and directional estimates within _+20 ø of in situ measurements, these preliminary results suggest that this is a promising approach to wind retrieval.

For the determination of biotic fluxes of carbon dioxide (CO 2 ) or other trace gases to or from a forest canopy, it is important to measure the storage of the trace gas within the forest canopy in addition to the net vertical flux above the forest canopy. However, the data continuity of within-canopy storage measurements can be poor because these measurements are subject to frequent equipment breakdowns. We here explore methods for gap-filling within-canopy CO 2 storage, using the data derived from an Amazonian rainforest (Caxiuanã). Our first approach was to estimate hourly storage from hourly CO 2 concentration measured above the canopy at the tower top. This proved unreliable, since at this hourly time scale the variations in above-canopy CO 2 are often decoupled from local changes in within-canopy storage. We then explored a second approach based on determination of the total CO 2 accumulation over a night. This was found to be adequately correlated with a time-weighted friction velocity (u *w ) averaged over a night (R 2 = 0.42). The total night-time storage was then used to model daytime depletion of CO 2 within the canopy. The gap-filling model was validated against independent data from the same site, and also applied to another tropical forest (Jarú) with similar results. The modelled storage is in good agreement with the measured storage, and by reducing susceptibility to advection error it is in some ways superior to the direct storage measurements. This suggests at the possibility of a general method for estimating storage in forest canopies, with re-calibration for each site. #

In this paper, we focus on assessing the performance of diverse turbulence closures in the simulation of dilute sediment-laden, open-channel flows. To that end, we base our analysis on a framework developed in a companion paper of this special issue, which puts forward a standard sediment transport model (SSTM), a partial two-fluid model (PTFM) and a complete two-fluid model (CTFM), in three-and one-dimensional (3D and 1D) versions. First, we propose in this paper extensions of the transport equations for the Reynolds stresses, and of the equations of the K-ω model to two-phase flows, starting from the general two-fluid model. We consider the drag force to be the predominant force amongst all the interactions between the two phases (water and sediment). Second, under the framework of models formed by the SSTM, the PTFM and the CTFM, we discuss simulation results obtained by employing the Reynolds stress model (RSM), the algebraic stress model (ASM), and the K-ε and the K-ω models (in their standard and extended versions), paired with each member of the framework. To assess the accuracy of the models, we compare numerical results with the experimental datasets of Vanoni, Trans ASCE 111:67-133, 1946; Coleman, Water Resour Res 22(10):1377-1384 Muste and Patel, J Hydraul Eng 123(9):742-751, 1997; Azuma, J Hydraul Eng 130:988-1001, 2004; Muste et al. Water Resour Res 41:W10402, 2005 . Third, we obtain from those comparisons the values of the Schmidt number that facilitate the agreement of model predictions with data. We conclude that the standard K-ε model, the ASM and the K-ω models all provide satisfactory descriptions of flow variables and sediment concentrations in open-channel flows; further, we show that the more complicated RSM does not provide much improvement in dilute sediment transport as compared to those previous models, even when it is paired with the CTFM. We also show that the inclusion of model extensions in the turbulence closures does not improve the predictions for dilute mixtures either. We find that our values for the Schmidt number agree well with Environ Fluid Mech (2009) 9:237-266 available data, and we provide an explanation for the variation of the Schmidt number with the ratio of the fall velocity and the wall-friction (shear) velocity. Finally, we corroborate that the Schmidt number is the key parameter to obtain satisfactory predictions of sediment transport in suspension.

A method is suggested for the calculation of the friction velocity for stable turbulent boundary-layer flow over hills. The method is tested using a continuous upstream mean velocity profile compatible with the propagation of gravity waves, and is incorporated into the linear model of Hunt, Leibovich and Richards with the modification proposed by Hunt, Richards and Brighton to include the effects of stability, and the reformulated solution of Weng for the near-surface region. Those theoretical results are compared with results from simulations using a non-hydrostatic microscale-mesoscale two-dimensional numerical model, and with field observations for different values of stability. These comparisons show a considerable improvement in the behaviour of the theoretical model when the friction velocity is calculated using the method proposed here, leading to a consistent variation of the boundary-layer structure with stability, and better agreement with observational and numerical data.

Using in-situ field measurements, we determined surface friction velocities (u *) and aerodynamic roughness heights (z 0) at the base of 10 dust devils. z 0 values agree closely with previous studies in this region and range from 0.1 to 1.0 cm. The peak u * coincide with the largest wind speed measurements within the dust devil and range from 0.9 to 2.4 ms À1. Surface friction velocities are sufficient to lift all types of windblown material from dust to pebble sized particles. Peak u * values for dust devils appear to be a linear function of the peak wind speed measured at $2 m height within the dust devil. This provides a simple approximation for estimating surface shear stress for dust devils.

1] Using in-situ field measurements, we determined surface friction velocities (u * ) and aerodynamic roughness heights (z 0 ) at the base of 10 dust devils. z 0 values agree closely with previous studies in this region and range from 0.1 to 1.0 cm. The peak u * coincide with the largest wind speed measurements within the dust devil and range from 0.9 to 2.4 ms À1 . Surface friction velocities are sufficient to lift all types of windblown material from dust to pebble sized particles. Peak u * values for dust devils appear to be a linear function of the peak wind speed measured at $2 m height within the dust devil. This provides a simple approximation for estimating surface shear stress for dust devils.

ABSTRACT Knowledge about ecosystem respiration is required for accurate estimates of daily, seasonal and annual carbon exchange between ecosystems and the atmosphere. Ecosystem respiration differs from photosynthesis in its diurnal and seasonal regime and its response to environmental controls. Contrary to photosynthesis, ecosystem respiration persists during nighttime. Measurements by the closed chamber technique were often used for the calculation of nighttime respiration fluxes during low-turbulence nighttime conditions, where the use of the eddy covariance method is problematic. The goal of this study is to investigate if also the closed chamber measurements could be biased during atmospheric low-turbulence conditions. This study presents CO2 flux observations (n = 602) measured at night at a boreal peatland in eastern Finland for two investigation periods during the years 2005 and 2006. We have used the closed chamber technique for measurements at flark, lawn and hummock microsites. Threshold for screening data into either well-developed or low-turbulence conditions were made using the friction velocity (u*), with u*

ABSTRACT Knowledge about ecosystem respiration is required for accurate estimates of daily, seasonal and annual carbon exchange between ecosystems and the atmosphere. Ecosystem respiration differs from photosynthesis in its diurnal and seasonal regime and its response to environmental controls. Contrary to photosynthesis, ecosystem respiration persists during nighttime. Measurements by the closed chamber technique were often used for the calculation of nighttime respiration fluxes during low-turbulence nighttime conditions, where the use of the eddy covariance method is problematic. The goal of this study is to investigate if also the closed chamber measurements could be biased during atmospheric low-turbulence conditions. This study presents CO2 flux observations (n = 602) measured at night at a boreal peatland in eastern Finland for two investigation periods during the years 2005 and 2006. We have used the closed chamber technique for measurements at flark, lawn and hummock microsites. Threshold for screening data into either well-developed or low-turbulence conditions were made using the friction velocity (u*), with u*