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Eddy covariance

From Wikipedia, the free encyclopedia

Eddy covariance system consisting of an ultrasonic anemometer and infrared gas analyser (IRGA).
Eddy covariance system consisting of an ultrasonic anemometer and infrared gas analyser (IRGA).

The eddy covariance (also known as eddy correlation and eddy flux) technique is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. The method analyzes high-frequency wind and scalar atmospheric data series, and yields values of fluxes of these properties. It is a statistical method used in meteorology and other applications (micrometeorology, oceanography, hydrology, agricultural sciences, industrial and regulatory applications, etc.) to determine exchange rates of trace gases over natural ecosystems and agricultural fields, and to quantify gas emissions rates from other land and water areas. It is frequently used to estimate momentum, heat, water vapour, carbon dioxide and methane fluxes.[1][2][3][4][5]

The technique is also used extensively for verification and tuning of global climate models, mesoscale and weather models, complex biogeochemical and ecological models, and remote sensing estimates from satellites and aircraft. The technique is mathematically complex, and requires significant care in setting up and processing data. To date, there is no uniform terminology or a single methodology for the Eddy Covariance technique, but much effort is being made by flux measurement networks (e.g., FluxNet, Ameriflux, ICOS, CarboEurope, Fluxnet Canada, OzFlux, NEON, and iLEAPS) to unify the various approaches.

An eddy correlation instrument measuring oxygen fluxes in benthic environments.
An eddy correlation instrument measuring oxygen fluxes in benthic environments.

The technique has additionally proven applicable under water to the benthic zone for measuring oxygen fluxes between seafloor and overlying water.[6] In these environments, the technique is generally known as the eddy correlation technique, or just eddy correlation. Oxygen fluxes are extracted from raw measurements largely following the same principles as used in the atmosphere, and they are typically used as a proxy for carbon exchange, which is important for local and global carbon budgets. For most benthic ecosystems, eddy correlation is the most accurate technique for measuring in-situ fluxes. The technique's development and its applications under water remains a fruitful area of research.[7][8][9][10][11]

YouTube Encyclopedic

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  • ✪ Eddy Covariance: Measuring an Ecosystem's Breath
  • ✪ Eddy Covariance: Theory and Implementation
  • ✪ Eddy Covariance: Theory and Data Processing

Transcription

(music) Have you ever wondered how scientists measure CO2 emissions from local ecosystems here on earth? (error beep) Well, one way is a well defined scientific method known as eddy covariance. (laugh) Not that Eddie! Eddy covariance is how we measure an ecosystem's breathing. Its the CO2 and other gasses that are exchanged between soil, vegetation, and the air in an ecosystem. The data it provides help scientists develop models that forecast long-term trends in CO2 and greenhouse gasses, much like the National Weather Service forecasts the short-term weather. This data will allow policymakers to draft legislation to address pressing issues and improve people's quality of life. Eddy covariance quantifies gas fluxes, or changes, by directly measuring the movement of gasses, like CO2, between an ecosystem and the atmosphere. To simplify it, let's look at both words. An eddy is a circular motion of air created by temperature fluctuations. At night without the sun the air is cool and relatively stable with the raising of the sun the earth and air temperature begins to climb. This constant night and day fluctuation creates the wind we feel and the eddies of air that scientists measure. Covariance has two parts: "co" meaning together and "variance" meaning change. This means simultaneously measuring the differences between the concentration of a gas and the direction of a swirling wind. Eddy Covariance is primarily measured by two pieces of high tech gadgetry, mounted on tripods and towers above areas of interest, like forests, lakes or agricultural areas. The anemometer measures wind speed and direction. The infrared gas analyzer measures gas concentrations in the air. The data are collected simultaneously, thousands of times per minute. The wind and gas concentration data go into a complex series of equations that, after several assumptions, yield estimates of the movement of gas into or out of an ecosystem. Two types of observations are primarily used to approximate the gas exchange. Imagine we place a box over the ecosystem, the gas moving in and out of the top of the box is calculated as fluctuations around the mean, or zero. The gas accumulated inside the box is calculated from the trend of gas concentration measurements in that area. When calculated over time, we can see trends that reveal how much CO2, and other gases, are being reused by earth’s organisms and how much is released into the atmosphere. To really simplify, the system works by measuring how many gas molecules pass through a defined volume over a specific time. For example, the system might capture a measurement of seven CO2 molecules being carried upward towards the atmosphere by an eddy, then, in the next moment, only five CO2 molecules are recorded traveling downward towards the ground. Then we know that the net flux, over this specific time period is equal to two molecules of CO2 being emitted into the atmosphere. It isn’t your everyday Eddie that compiles this stuff. Eddy Covariance data are collected by individual researchers and organizations around the globe, including the National Ecological Observatory Network. And what’s really cool is that all these institutions contribute to an international database, FluxNet, to facilitate the use of eddy covariance data to contribute to global models at many timescales. Eddy Covariance measurements are really important because they are the most conclusive method for measuring how much CO2, and other gases, are taken in by photosynthesis and also emitted into earth’s atmosphere from things like respiration, geologic emissions, thawing permafrost, and human activity. Documenting these gas exchanges is important to understand trends in atmospheric concentrations. They allow for the development of accurate models to forecast greenhouse gas concentrations and other factors of climate change on our planet. So next time you think about how scientists measure CO2 and greenhouse emissions at a local level just remember Eddy… Eddy Covariance! (Laugh) Eddy covariance and other data are freely available at neonscience.org.

Contents

General principles

Representation of the air flow in the atmospheric boundary layer

Air flow can be imagined as a horizontal flow of numerous rotating eddies, that is, turbulent vortices of various sizes, with each eddy having horizontal and vertical components. The situation looks chaotic, but vertical movement of the components can be measured from the tower.

[4]
[4]

Physical meaning of the eddy covariance method

At one physical point on the tower, at Time1, Eddy1 moves parcel of air c1 down at the speed w1. Then, at Time2, Eddy2 moves parcel c2 up at the speed w2. Each parcel has gas concentration, pressure, temperature, and humidity. If these factors, along with the speed are known, we can determine the flux. For example, if one knew how many molecules of water went down with eddies at Time 1, and how many molecules went up with eddies at Time2, at the same point, one could calculate the vertical flux of water at this point over this time. So, vertical flux can be presented as a covariance of the vertical wind velocity and the concentration of the entity of interest.

[4]
[4]

Mathematical foundation

In mathematical terms, "eddy flux" is computed as a covariance between instantaneous deviation in vertical wind speed (w') from the mean value (w-overbar) and instantaneous deviation in gas concentration, mixing ratio (s'), from its mean value (s-overbar), multiplied by mean air density (ρa). Several mathematical operations and assumptions, including Reynolds decomposition, are involved in getting from physically complete equations of the turbulent flow to practical equations for computing "eddy flux", as shown below.

[4]
[4]

Major assumptions

  • Measurements at a point can represent an upwind area
  • Measurements are done inside the boundary layer of interest
  • Fetch/flux footprint is adequate – fluxes are measured only at area of interest
  • Flux is fully turbulent – most of the net vertical transfer is done by eddies
  • Terrain is horizontal and uniformed: average of fluctuations is zero; density fluctuations negligible; flow convergence & divergence negligible
  • Instruments can detect very small changes at high frequency, ranging from minimum of 5 Hz and to 40 Hz for tower-based measurements

Eddy covariance processing software

Currently (2011) there are many software programs [12] to process eddy covariance data and derive quantities such as heat, momentum, and gas fluxes. The programs range significantly in complexity, flexibility, number of allowed instruments and variables, help system and user support. Some programs are open-source software, while others are closed-source or proprietary.

Examples include free fully supported and documented open-source software such as EddyPro; free unsupported open-source programs such as ECO2S and ECpack; free closed-source packages such as EdiRe, TK3, Alteddy, and EddySoft. Eddy covariance processing algorithms can also be implemented in commercial software such as Matlab or the free open-source software R.

Summary

In summation, the 3D wind and another variable (usually gas concentration, temperature or momentum) are decomposed into mean and fluctuating components. The covariance is calculated between the fluctuating component of the vertical wind and the fluctuating component of gas concentration. The measured flux is proportional to the covariance.

The area from which the detected eddies originate is described probabilistically and called a flux footprint. The flux footprint area is dynamic in size and shape, changing with wind direction, thermal stability and measurements height, and has a gradual border.

The effect of sensor separation, finite sampling length, sonic path averaging, as well as other instrumental limitations, affect frequency response of the measurement system and may need a co-spectral correction, especially noticeable with closed-path instruments and at low heights below 1 to 1.5 m.

Uses of eddy covariance method

Common uses:

Novel uses:

  • Precision irrigation, precision agriculture
  • Carbon Sequestration and Capture monitoring
  • Landfill gas emissions into the atmosphere
  • Emissions of gases displaced by hydraulic fracturing into the atmosphere
  • Gas leak detection and location
  • Methane emission from permafrost regions
  • Biogenic VOCs emission
  • Reactive trace gas exchange flux measurement

Related methods

Eddy accumulation

True eddy accumulation

The true eddy accumulation technique can be used to measure fluxes of trace gases for which there are no fast enough analysers available, thus where the eddy covariance technique is unsuitable. The basic idea is that upwards moving air parcels (updrafts) and downwards moving air parcels (downdrafts) are sampled proportionally to their velocity into separate reservoirs. A slow response gas analyser can then be used to quantify the average gas concentrations in both updraft and downdraft reservoirs.[13][14]

Relaxed eddy accumulation

The main difference between the true and the relaxed eddy accumulation technique is that the latter samples air with a constant flow rate that is not proportional to the vertical wind speed.[15][16][17]

See also

References

  1. ^ Baldocchi, D., B. Hicks, and T. Meyers. 1988. Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69, 1331-1340
  2. ^ Verma, S.B.: 1990, Micrometeorological methods for measuring surface fluxes of mass and energy, Remote Sensing Reviews 5(1): 99-115
  3. ^ Lee, X., W. Massman, and B. Law. 2004. Handbook of Micrometeorology. Kluwer Academic Publishers, The Netherlands, 250 pp.
  4. ^ Burba, G., 2013. Eddy Covariance Method for Scientific, Industrial, Agricultural and Regulatory Applications: a Field Book on Measuring Ecosystem Gas Exchange and Areal Emission Rates. LI-COR Biosciences, Lincoln, USA, 331 pp.
  5. ^ Aubinet, M., T. Vesala, D. Papale (Eds.), 2012. Eddy Covariance: A Practical Guide to Measurement and Data Analysis. Springer Atmospheric Sciences, Springer Verlag, 438 pp.
  6. ^ Berg, P., H. Røy, F. Janssen, V. Meyer, B. B. Jørgensen, M. Hüttel, and D. de Beer. 2003. Oxygen uptake by aquatic sediments measured with a novel non-invasive eddy correlation technique. Marine Ecology Progress Series. 261:75-83.
  7. ^ University of Virginia. Aquatic Eddy Covariance Research Lab. Retrieved: 22 June 2015.
  8. ^ The Florida State University. Eddy Correlation - Further Development and Studies of Flow and Light driven dynamics of Benthic Oxygen Exchange. Retrieved: 22 June 2015.
  9. ^ Leibniz-Institute of Freshwater Ecology and Inland Fisheries. Eddy Correlation in Natural Waters. Retrieved: 22 June 2015.
  10. ^ Max Planck Institute for Marine Microbiology. Eddy Correlation System (ECS). Retrieved: 22 June 2015.
  11. ^ Centre for Coastal Biogeochemistry Research. Eddy Correlation. Retrieved: 22 June 2015.
  12. ^ M. Mauder, T. Foken, R. Clement, J. A. Elbers, W. Eugster, T. Grunwald, B. Heusinkveld, and O. Kolle. 2007. Quality control of CarboEurope flux data – Part II: Inter-comparison of eddy-covariance software, Biogeosciences Discuss., 4, 4067–4099
  13. ^ R. E. Speer, K. A. Peterson, T. G. Ellestad, J. L. Durham (1985). "Test of a prototype eddy accumulator for measuring atmospheric vertical fluxes of water vapor and particulate sulfate". Journal of Geophysical Research. 90: 2119. doi:10.1029/JD090iD01p02119.CS1 maint: Multiple names: authors list (link)
  14. ^ Siebicke, Lukas (2017). "True eddy accumulation and eddy covariance methods and instruments intercomparison for fluxes of CO2, CH4 and H2O above the Hainich Forest". 19th EGU General Assembly, EGU2017. 19: 18076. Bibcode:2017EGUGA..1918076S.
  15. ^ Businger, Joost A.; Oncley, Steven P.; Businger, Joost A.; Oncley, Steven P. (1990-04-01). "Flux Measurement with Conditional Sampling". Journal of Atmospheric and Oceanic Technology. 7 (2): 349–352. doi:10.1175/1520-0426(1990)007<0349:fmwcs>2.0.co;2.
  16. ^ Osterwalder, S.; Fritsche, J.; Alewell, C.; Schmutz, M.; Nilsson, M. B.; Jocher, G.; Sommar, J.; Rinne, J.; Bishop, K. (2016-02-15). "A dual-inlet, single detector relaxed eddy accumulation system for long-term measurement of mercury flux". Atmos. Meas. Tech. 9 (2): 509–524. Bibcode:2016AMT.....9..509O. doi:10.5194/amt-9-509-2016. ISSN 1867-8548.
  17. ^ Jonas Sommar, Wei Zhu, Lihai Shang, Xinbin Feng, Che-Jin Lin (2013). "A whole-air relaxed eddy accumulation measurement system for sampling vertical vapour exchange of elemental mercury". Tellus B: Chemical and Physical Meteorology. 65: 19940. doi:10.3402/tellusb.v65i0.19940.CS1 maint: Multiple names: authors list (link)

Further reading

External links

This page was last edited on 20 May 2019, at 02:08
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