Tower Instruments Research Articles

Vegetation description around the savanna flux measurement site at Benfontein Nature Reserve, South Africa

A flux tower with eddy covariance instruments was recently erected at Benfontein Nature Reserve (BNR) outside of Kimberley. An understanding of the landscape within the flux tower footprint, in this case natural vegetation, is necessary to interpret the data collected by the eddy covariance instruments. The tower was erected as part of the Expanded Freshwater and Terrestrial Environmental Observation Network (EFTEON) infrastructure initiative to establish long-term monitoring platforms focused on socially relevant terrestrial landscapes and their coupled hydrological systems. We sought to describe the vegetation within the flux tower footprint to set a baseline of the vegetation in the landscape at the inception of the long-term monitoring of flux measurements at BNR. Woody vegetation was surveyed in five square 1 ha plots while the herbaceous layer was sampled across 105 circular plots following the Socio-Ecological Observatory for Southern African Woodland (SEOSAW) protocol. Woody vegetation was quantified in terms of abundance, diversity and biomass, together with Size Class Distribution (SCD) methods to describe the woody vegetation structure. The herbaceous layer was described by species count, basal cover, biomass, and diversity which was assessed using the Shannon-Weiner Diversity Index. Five tree species, dominated by Vachellia erioloba, were found in the plots. Recruits (0 and 1 m tall) were significantly more abundant than large trees (1.2–9.5 m high). The SCD for the woody vegetation yielded a Type IIIa curve i.e. populations missing one or more size classes, for all tree species except Ziziphus mucronata which yielded a Type IIIb curve representing a population missing small sized trees. The herbaceous layer was diverse, consisting of 10 grass and 32 forb species. Schmidtia pappophoroides was the dominant grass species in terms of count, basal cover and aboveground biomass. Herbaceous biomass was two times higher than that of woody vegetation, with the North plot contributing the highest biomass for herbaceous and woody vegetation. The results indicate that the vegetation around the flux tower represents a wooded grassland, which is a type of a savanna. These vegetation data will contribute to the interpretation of data collected by the flux tower instruments now and in the future. Furthermore, long-term data collection in this reserve is needed to capture the woody vegetation dynamics and its interaction with the herbaceous layer.

Eddy covariance based methane flux in Sundarbans mangroves, India

We report the initial results of the methane flux measured using eddy covariance method during summer months from the world’s largest mangrove ecosystem, Sundarbans of India. Mangrove ecosystems are known sources for methane (CH4) having very high global warming potential. In order to quantify the methane flux in mangroves, an eddy covariance flux tower was recently erected in the largest unpolluted and undisturbed mangrove ecosystem in Sundarbans (India). The tower is equipped with eddy covariance flux tower instruments to continuously measure methane fluxes besides the mass and energy fluxes. This paper presents the preliminary results of methane flux variations during summer months (i.e., April and May 2012) in Sundarbans mangrove ecosystem. The mean concentrations of CH4 emission over the study period was 1682 ± 956 ppb. The measured CH4 fluxes computed from eddy covariance technique showed that the study area acts as a net source for CH4 with daily mean flux of 150.22 ± 248.87 mg m−2 day−1. The methane emission as well as its flux showed very high variability diurnally. Though the environmental conditions controlling methane emission is not yet fully understood, an attempt has been made in the present study to analyse the relationships of methane efflux with tidal activity. This present study is part of Indian Space Research Organisation–Geosphere Biosphere Program (ISRO–GBP) initiative under ‘National Carbon Project’.

Nighttime Turbulent Events in a Steep Valley: A Nested Large-Eddy Simulation Study

Abstract This numerical study investigates the nighttime flow dynamics in Owens Valley, California. Nested high-resolution large-eddy simulation (LES) is used to resolve stable boundary layer flows within the valley. On 17 April during the 2006 Terrain-Induced Rotor Experiment, the valley atmosphere experiences weak synoptic forcings and is largely dominated by buoyancy-driven downslope and down-valley flows. Tower instruments on the valley floor record a continuous decrease in temperature after sunset, except for a brief warming episode. This transient warming event is modeled with good magnitude and temporal precision with LES. Analysis of the LES flow field confirms the event to be the result of a slope to valley flow transition, as previously suggested by researchers based on field observations. On the same night, a northerly cold airflow from the Great Basin is channeled through a pass on the eastern valley sidewall. The current plunges into the stable valley atmosphere, overshooting the altitude of its neutral buoyancy, and generating a large-scale oscillatory motion. The resulting cross-valley flow creates strong vertical shear with the down-valley flow in the lower layers of the atmosphere. A portion of the cross-valley flow is captured by a scanning lidar. The nested LES is in good agreement with the lidar-recorded radial velocity. Furthermore, the LES is able to resolve Kelvin–Helmholtz waves, and ejection and sweep events at the two-layer interface, which lead to top-down vertical mixing.

Assessing the uncertainties in flux measurements

Climate and environmental studies require knowledge of the exchange of carbon, water vapor, and energy between the atmosphere and terrestrial ecosystems. These exchanges are measured by tower instruments at numerous sites around the world using a technique known as eddy covariance. However, there are few studies estimating the errors in those flux tower networks by comparing their measurements with measurements from independent instruments. Schmidt et al. assessed the errors in the AmeriFlux network by comparing measurements made by the towers to measurements made using portable instruments during 84 site visits between 2002 and 2012. They found that, after statistical outliers were removed, the network had mean relative instrumental errors of less than 10% for all variables measured. The lowest errors were for meteorological variables, while the highest errors were for latent heat and carbon dioxide flux measurements. They note that many of the errors, such as calibration problems, were corrected at the sites. Overall, the authors say the study confirms the high quality and value of the AmeriFlux network data for studying fluxes between the atmosphere and the terrestrial surface. (Journal of Geophysical Research‐Biogeosciences, doi:10.1029/2012JG002100, 2012)

Empirical assessment of uncertainties of meteorological parameters and turbulent fluxes in the AmeriFlux network

Terrestrial ecosystem‐atmosphere exchange of carbon, water vapor, and energy has been measured for over a decade at many sites globally. To minimize measurement and analysis errors, quality assurance data have been collected over short periods along‐side tower instruments at AmeriFlux research sites. Theoretical and empirical error and uncertainty values have been reported for various aspects of the eddy covariance technique but until recently it has not been possible to constrain network level variation based on direct comparison of side‐by‐side measurements. Paired observations, although rare in practice, offer a possibility to obtain real‐world error estimates for flux observations and corresponding uncertainties. In this study, we report the relative instrumental errors from the AmeriFlux quality assurance and quality control (QA/QC) site intercomparisons of 84 site visits (2002–2012). Relative errors, including random and systematic instrumental errors, are presented for meteorological and radiation variables, gas concentrations, and the turbulent fluxes. The lowest relative errors (<2%) were found for the meteorological parameters, while the largest relative errors were found for latent heat and CO2 fluxes. The mean relative instrumental error for CO2 flux averaged −8.2% (underestimation by the tower instruments). Sensible and latent heat fluxes exhibited mean errors of −1.7% and −5.2%, respectively. Deviation around the mean was also largest for the turbulent fluxes, approaching 20%. Because the data collected during QA/QC site visits are used to identify and correct errors, our results represent a conservative estimate of instrumental errors in the AmeriFlux database. Overall, the presented results confirm the high quality of the network data and underline its status as a valuable data source for the research community.

Model simulation of gravity waves triggered by a density current

AbstractIn this study, starting from an observational case of internal gravity waves (IGWs) generated at the top of a drainage flow during the SABLES2006 field campaign, we aim to reproduce the IGWs and their origin through mesoscale meteorological modelling. We used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km), testing the model capabilities to simulate the IGWs through a WRF fixed physics package option but two different planetary boundary layer schemes, the Mellor‐Yamada‐Janjić (MYJ) and the Yonsei University (YSU). The comparison between model simulations and measurements from a 100 m meteorological tower reveals that the MYJ scheme simulation gives much better results, as it better represents the main features of the density current measured by the tower instruments, although the event is predicted to occur sooner than it is observed to occur. The study has also shown the capacity of this scheme to detect the oscillations in temperature and specific humidity generated by the arrival of the density current. In contrast, the YSU scheme captures the arrival of the current on time but it fails to correctly track its properties and therefore it does not reproduce the gravity waves with the current arrival. In addition, wave parameters calculated from model outputs (MYJ) using the wavelet method reveal waves with longer periods and longer wavelengths (T = 20–22 min and λ = 8–10 km) than those calculated from measurements (T = 9.2 min and λ = 3.5 km) using the same technique. Copyright © 2012 Royal Meteorological Society

Stability and turbulence in the atmospheric boundary layer: A comparison of remote sensing and tower observations

When monitoring winds and atmospheric stability for wind energy applications, remote sensing instruments present some advantages to in‐situ instrumentation such as larger vertical extent, in some cases easy installation and maintenance, measurements of vertical humidity profiles throughout the boundary layer, and no restrictions on prevailing wind directions. In this study, we compare remote sensing devices, Windcube lidar and microwave radiometer, to meteorological in‐situtower measurements to demonstrate the accuracy of these measurements and to assess the utility of the remote sensing instruments in overcoming tower limitations. We compare temperature and wind observations, as well as calculations of Brunt‐Väisälä frequency and Richardson numbers for the instrument deployment period in May–June 2011 at the U.S. Department of Energy National Renewable Energy Laboratory's National Wind Technology Center near Boulder, Colorado. The study reveals that a lidar and radiometer measure wind and temperature with the same accuracy as tower instruments, while also providing advantages for monitoring stability and turbulence. We demonstrate that the atmospheric stability is determined more accurately when the liquid‐water mixing ratio derived from the vertical humidity profile is considered under moist‐adiabatic conditions.

Alternative methods to predict actual evapotranspiration illustrate the importance of accounting for phenology – Part 2: The event driven phenology model

Abstract. Evapotranspiration (ET) flux constitutes a major component of both the water and energy balances at the land surface. Among the many factors that control evapotranspiration, phenology poses a major source of uncertainty in attempts to predict ET. Contemporary approaches to ET modeling and monitoring frequently summarize the complexity of the seasonal development of vegetation cover into static phenological trajectories (or climatologies) that lack sensitivity to changing environmental conditions. The Event Driven Phenology Model (EDPM) offers an alternative, interactive approach to representing phenology. This study presents the results of an experiment designed to illustrate the differences in ET arising from various techniques used to mimic phenology in models of land surface processes. The experiment compares and contrasts two realizations of static phenologies derived from long-term satellite observations of the Normalized Difference Vegetation Index (NDVI) against canopy trajectories produced by the interactive EDPM trained on flux tower observations. The assessment was carried out through validation of predicted ET against records collected by flux tower instruments. The VegET model (Senay, 2008) was used as a framework to estimate daily actual evapotranspiration and supplied with seasonal canopy trajectories produced by the EDPM and traditional techniques. The interactive approach presented the following advantages over phenology modeled with static climatologies: (a) lower prediction bias in crops; (b) smaller root mean square error in daily ET – 0.5 mm per day on average; (c) stable level of errors throughout the season similar among different land cover types and locations; and (d) better estimation of season duration and total seasonal ET.

Dry deposition to forests—On the use of data from clearings

To quantify dry deposition rates to forest canopies normally requires the use of a tower erected so that measurements can be made above the treetops, as well as often within the canopy. However, wet deposition protocols call for deployment of instruments in forest clearings. The question arises as to whether forest clearing measurements can be used to estimate dry deposition rates to the surrounding forest canopy. A trial program was implemented in the 1980s, to explore this possibility. Two nearby sites were set up at Oak Ridge, Tennessee, as part of the Dry Deposition Inferential Method network, now a component of the Atmospheric Integrated Research Monitoring Network (AIRMoN). One site was a tower extending to about 35 m above the floor of a mixed deciduous forest, with trees about 27 m tall. The second site was in the forest clearing used for measurements under the protocols of the National Atmospheric Deposition Program (NADP). The data indicate that clearing concentrations of SO 2, SO 4 2−, and HNO 3 are consistently and significantly different from those above the forest, as measured at the nearby tower. In addition, turbulence data obtained in the clearing do not lead to estimates of deposition velocities like those derived from the tower instrumentation. The data indicate that the ratio F f/ F c (where F f is the dry deposition rate to the forest canopy and F c is that of the same chemical to the clearing) varies with the chemical species of interest, and with the time of year in a manner that suggests a direct linkage with the leaf area index of the canopy. On the average, F f/ F c appears to be slightly above 2.0 for SO 2 and HNO 3, and slightly below 2.0 for SO 4 2−.

Expressions of Sensible Heat Flux Based on a Dimensional Analysis

Abstract Based on a dimensional analysis, relationships between the temperature structure coefficient C2T, the variance of vertical wind speed σ2w or the velocity structure coefficient C2V, and the sensible heat flux in the boundary layer have been developed. Only the turbulent statistical parameters and the characteristic heights are included in the resulting equations. These equations show a possible dynamic balance relationship between the sensible heat flux, C2T and C2V or σ2w, and reveal the potential for measuring sensible heat flux with sodar. The primary verification of the equations is carried out using the data measured with tower instruments at a fixed level and the data from a Minnesota experiment. The comparisons show good agreement between calculations and direct measurements in the surface layer.

Design and performance of an acoustic antenna for a phased array Doppler sodar

An acoustic array antenna which has a narrow main lobe and suppressed sidelobes is studied. The directivity function formed by the multiple point sources is used to estimate the aperture of an array and the spacing between the transducer elements to make a desired beam. Then the aperture tapering based on a Chebyshev distribution and the effect of an acoustic shield are evaluated to suppress the sidelobe radiation. The total performance of the phased array system has been examined by comparison with tower instruments. The field test shows that the measured beam pattern agrees with the calculated one and the sodar-derived winds have a correlation of 0.96 and a standard deviation of differences of about 0.47–0.52 m s−1 compared with in situ measurements. These statistical values are as good as or even better than those from the previous experiment. Copyright © 1998 Royal Meteorological Society

Wind measurements using a Doppler sodar

Doppler sodars have been widely used to measure the wind profiles up to several hundred meters. The comparisons with tower instruments show that the Doppler sodars offer the reliable estimations of the mean wind speed (U) and direction (D). From the comparison experiments held at Meteorological Research Institute (MRI), the correlation coefficients are better than 0.9 for U and D, and the rms differences are 0.7-2 mis and 10-25 deg for U and D respectively. An evaluation of the standard deviation of the vertical wind (σDw) is somewhat difficult, but we have the correlation coefficients better than 0.8 and the rms differences of 0.15-0.2 m/s forσDw. Theheavy precipitation and/or the strong wind reduce the maximum detectable height and degrade the accuracy of the wind measurements.Measurement errors, which seem to be caused by the beam deflection due to refractive changes in the acoustic propagation, prevent the accurate observation of instantaneous horizontal wind velocities. Though thdeviation of the horizontal wind (σDu) can be deduced by the simplified method using the tilted beam axis components, the five-beam phased array Doppler sodar has the capability to measureσu andσv.

Comparison of a Doppler Sodar with Bivanes and Cup Anemometers

Abstract A commercial Doppler sodar was operated near a 300-m tower instrumented with bivanes and cup anemometers. Sodar measurements of wind speed and direction and a turbulence variable (σw) were compared with tower measurements at 90, 180, and 300 m. A total of 2084 15-min periods of data were compared during one year. The bias and precision of the sodar data were calculated with respect to the bivanes and anemometers. It was found that, with the exception of the azimuth bias, the sodar measurements exhibited a somewhat greater bias and higher (worse) precision than found in previous studies. The azimuth bias of the sodar with respect to the bivanes was much less than reported in previous studies. Reasons for the differences between the sodar and the tower instruments are discussed. Inspection of tower and sodar data indicated that with experience the sodar data could be quality assured reasonably well without the need for direct comparison with a tower. The study underscores the value of manual data i...

Test of a Phased Array Sodar by Intercomparison with Tower Data

Abstract The phased array sodar PA2 recently manufactured by the French REMTECH enterprise was tested using tower data for comparison. Wind speed, wind direction, and the standard deviation of the horizontal wind direction (σθ) were measured continuously using the sodar and the tower instruments. The 10-min mean values were recorded between 60 and 200 m above ground level for a period of two weeks in October 1990 and then compared with each other. Furthermore, the availability of sodar data was investigated as a function of height. The intercomparison comprises a time series and a linear recession and correlation analysis. It shows that the new sodar can estimate the wind data in a reliable manner. The correlation is as good as or even better than that of the previous long-term intercomparisons of the old A0 sodar used at our institute. The root-mean-square difference between the lower and the sodar data is smaller at least for the wind speed and σ θ. The latter is considered an important parameter for th...

Variances of the vertical and horizontal wind measured by tower instruments and SODAR

At the Karlsruhe Nuclear Research Center, a monostatic Doppler SODAR has been operated near a 200 m high meteorological tower. Vertical profiles of the components of the wind vector and of the variances of the vertical wind and of the horizontal wind direction have been sampled continuously from the SODAR and the tower instruments as 30-min mean values. During seven episodes lasting more than 2 h, the instantaneous vertical wind speed was measured simultaneously by a sonic anemometer and the SODAR, and spectra were calculated. An intercomparison of the spectra and wind data measured directly by the SODAR and the tower instruments has been performed by a linear regression and correlation analysis referring to different height levels of measurement and stability classes.

Large-eddy simulation of turbulent flow above and within a forest

A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.

Meteorological Monitoring Using Sodar for Electric Utility Air Quality Applications

Performance of a three-component monostatic Doppler sodar was compared with performance of collocated meteorological tower instruments and evaluated for electric utility applications. The sensors were components of an air pollution monitoring network that was upgraded to assess the impact of coal conversion on ambient air quality at Northeast Utilities' Mt. Tom Generating Station situated in rugged terrain in the Connecticut River valley of western Massachusetts. To determine how well sodar and tower measurements agree, and how reliable sodar is for this type of application, meteorological data and system performance statistics obtained from August 1982 through July 1983 were examined. A statistical comparison of wind speed and direction measurements at 50 m indicated good agreement between the two monitoring techniques. High correlations also were observed for data collected during various weather conditions. System reliability was demonstrated by data recovery rates and sodar vertical range capabilities...

Wind and Wave Measurements Taken from a Tower in Lake Michigan

In July 1977 the Great Lakes Environmental Research Laboratory installed a lightweight, solar powered research tower in Lake Michigan. The tower, located 2 km offshore from Muskegon, Michigan, in 16 m of water, provided a stable platform for two levels of anemometers and air temperature sensors, a surface water temperature sensor, and an array of four wave staffs to measure meteorological and directional wave variables. Solar power was successfully used to provide power for tower instrumentation. The measurements, while short-lived due to a guy wire failure in October 1977, comprise over 1300 well-documented hourly wind and wave data for further detailed studies. This report presents a detailed description of the instrumentation, data collection, and data reduction systems. The results show that the triangular array of wave staffs provides reasonable wave direction information, although nearshore waves appear to have a larger onshore component of momentum than would be indicated by prevailing winds. Further detailed studies of wind and wave processes are in progress.

Estimating vertical diffusion from routine meteorological tower measurements

Data from an instrumented meteorological tower are used to calculate vertical dispersion coefficients and vertical diffusivities using surface layer concepts. The tower instrumentation is similar to that at many power plant sites and only a temperature gradient and wind speed are required for the calculations. The calculated vertical turbulence compares well with bivane data collected on the tower. Calculations of vertical dispersion also compare well with dispersion data collected at other sites.