Eddy-correlation technique

Development in recent years of fast-response instruments able to measure rapid fluctuations of the wind velocity (V ) and of fhe tracer concentration (c ), has made it possible to calculate the turbulent flux directly from the correlation expression in Equation (41), without having to resort to uncertain assumptions about eddy diffusivities. For example, Grelle and Lindroth (1996) used this eddy-correlation technique to calculate the vertical flux of CO2 above a foresf canopy in Sweden. Since the mean vertical velocity w) has to vanish above such a flat surface, the only contribution to the vertical flux of CO2 comes from the eddy-correlation term c w ). In order to capture the contributions from all important eddies, both the anemometer and the CO2 instrument must be able to resolve fluctuations on time scales down to about 0.1 s. 

The concepts involved in the eddy correlation technique and the factors involved in the design of appropriate chemical sensors are briefly examined in this chapter. The discussion focuses on surface-layer measurements because it is in this layer that the atmosphere-biosphere interaction is most readily examined. 

Figure 1. Isopleths o/Rc, a measure of the chemical resolution of the instrument with the eddy correlation technique.

More information is needed about the surface emission and deposition of trace atmospheric species. These fluxes can often be best measured by the eddy correlation technique with fast chemical sensors in conjunction with micrometeorological instrumentation. As analytical techniques for trace species progress, fast and sensitive sensors are becoming available for field research. Consideration must be given to matching the chemical sensors to the eddy correlation technique. 

Figure 6- Inter-comparison of NO fluxes at 5 meter height using eddy-correlation technique with NO soil emissions using dynamic chamber technique.

Berg, P., Roy, H., Janssen, F., Meyer, V., Jorgensen, B.B., Hiittel, M. and De Beer, D., 2003. Oxygen uptake by aquatic sediments measured with a novel non-invasive EDDY-correlation technique. Marine Ecology Progress Series, 261 75-83. 

Eddy correlation measurements require fast-response instrumentation to resolve the turbulent fluctuations that contribute primarily to the vertical flux. These requirements are particularly severe under stable conditions where response times on the order of 0.2 s or less may be required. In practice, it is often possible to use somewhat slower instruments and apply various corrections to the computed fluxes as compensation. The eddy correlation technique has been used in aircraft (Pearson and Steadman 1980 Lenschow et al. 1982) as well as with tower-mounted instruments. 

McMillen, R. T. (1988) An eddy correlation technique with extended applicability to non-simple terrain, Boundary-Layer Meteorol. 43, 231-245. 

A key issue in the research of our ecosystem and in atmospheric studies today is the ability to quantify even small concentrations of trace gases, and follow their evolution from a source to their final destination. For biosphere-atmosphere or air-sea exchange, trace gas flux measurements based on the eddy correlation technique in addition to high temporal resolution (sometimes less than 1 s) is required. Of the many... 

The eddy-correlation technique directly determines the flux of an atmospheric trace constituent through a plane that is parallel to the ground. It represents an instantaneous upward or downward transport of the species under investigation, ultimately yielding a net flux. Ideally, the meteorological conditions... 

The flux Ja is typically measured by pan evaporation, lake evaporation, cooling pond heat transfer or an eddy correlation technique that relates turbulent diffusion of water vapor to mass flux. The concentration of water vapor at elevation z, Ca(z), is determined from simultaneous measurements of temperature and relative humidity and an application of Equations 9.56 and 9.57. At the water surface (z = 0), it is assumed that the air temperature is equal to water temperature and that relative humidity is 100%, so water temperature measurements and Equation 9.56 are sufficient to determine Ca(z = 0). If the analog of heat transfer is used, then... 

The eddy correlation technique (Jones and Smith, 1977) measures turbulent vapor fluxes away from the water surface with the rapid response to water vapor and velocity measurements that are possible with current instruments. Then,... 

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