Abstract This paper addresses the different nomenclatures used to describe the principle of eddy motion as applied to Micrometeorology in particular and Biogeosciences in general. Different authors refer this approach by different names: eddy covariance method, eddy correlation method, eddy covariance technique, or using both terms ‘eddy covariance method’ and ‘eddy covariance technique’ interchangeably. There is a reason for each author to use a specific terminology, yet it is essential to standardize the vocabulary. The goal of this paper is to initiate discussion on the topic and suggest a preferred terminology. Introduction The term „eddy‟ is commonly used description in turbulent atmospheric motion analysis. Differences arise in the choice of words between „covariance or correlation‟, and „technique or method,‟ which could create confusion. • „eddy covariance method‟ (Jones, 2008; Ruppert et al., 2006; Kanda et al., 2004); • „eddy correlation method „(Anderson et al., 1984; Desjardins, 1977); • „eddy covariance technique‟ ( Baldocchi, 2003; Goulden et al., 1996; Ohtaki, 1984) • eddy correlation technique (McMillen, 1988, Shuttleworth et al., 1984); • Interchangeable use of both terms „eddy covariance method‟ and „eddy covariance technique‟ (Aubinet et al., 2000). In this paper four key words are thoroughly analyzed: technique, method, covariance, and correlation. The basic principles behind the analysis of turbulent fluid motion in the atmospheric boundary layer (ABL) are the conservation laws for mass, momentum and energy. Three different interconnected classical approaches have been developed to analyze eddy motion in the ABL: Navier-Stokes equations (NSE), the dynamical systems approach and the conventional statistical theory (Fois et al., 2001). Navier-Stokes equations are extensions of Euler‟s Equations for fluid motion. A number of assumptions are made on the NSE to derive equations that describe fluxes of momentum, trace gas concentrations, and temperature (energy) in the ABL. Reynolds‟ averaging technique (Reynolds, 1895) is applied on the NSE to derive the eddy flux equations for trace gas concentrations, momentum, and temperature. The objective of this paper is to analyze the nomenclatures used to describe the analysis of eddy motion in micrometeorology and suggest a more acceptable terminology. Acknowledgments The preparation of this document was made possible through financial support provided by the Water of the West Program of the University of Idaho (WoW-UI). Funding for the participation of the first author at the Fall 2010 AGU Meeting was covered by the American Geophysical Union (AGU) and the Water of the West Program of the University of Idaho. Conclusion The direct way of measuring eddy fluxes of momentum, sensible and latent heat, concentrations of trace gases (CO 2 ,H 2 O, and others) is based on three conservation laws: conservation of mass, momentum and energy (Moncrieff et al., 1997; Anderson et al., 1984; Dyer and Pruitt, 1962; Swinbank, 1951). This approach is conceptualized by the three core assumptions, and three or more additional conditions(Marcolla, et al., 2005; Baldocchi, 2003). Based on the arguments made these set of assumptions and formulas define a „method‟ than a „technique.‟ However, the final set of equations taken individually are techniques. There is no strong point to support the argument that the method is based on the calculation of correlations, as all calculations of fluxes involve covariances. In a final word it is preferable to address the approach as ‘eddy covariance method,’ than the other nomenclatures. A13B-0191 Eddy Covariance Method or Technique? Yohannes G/Eyesus Getahun, Yohannes.getahun@vandals.uidaho.edu or lijyohannes@yahoo.com , Department of Biological & Agricultural Engineering / Water of the West Program, University of Idaho, Moscow, Idaho. Russell J. Qualls, rqualls@uidaho.edu, Department of Biological & Agricultural Engineering, University of Idaho, Moscow, Idaho. References Anderson, D. E., Verma, S. B., and Rosenberg, N. J., (1984). Boundary Layer Meteorology, 29, 263-272. Aubinet, M., et al., (2000). Advances in Ecol. Res., 30, 113-175. Baldocchi, D. D., (2003). Global Change Biology, 9, 479-492. Desjardins, R. L., (1977). J. Applied Meteorology, 16, 248-250. Foias, C., Manley, O., Rosa, R., and Temam, R., (2001). Navier- Stokes Equations and Turbulence. Foken, T., (2008). Micrometeorology. Springer-Verlag Berlin Heidelberg. Goulden M. L., et al., (1996). Global Change Biology, 2, 169-182. Jones, K. H., (2008). http://www.geos.ed.ac.uk/homes/s0197746/Jones_K_H_2008_MP hil_thesis_Measurement_of_advection_and_sur.pdf Kanda, M., et al., (2004). Boundary Layer Meteorology, 110, 381- 404. Leuning, R., (2007). Boundary Layer Meteorology, 123, 263-267. McMillen, R.T., (1988). Boundary Layer Meteorology, 43, 231-245. method, (2010). In Merriam-Webster Online Dictionary. Retrieved September 8, 2010, from http://www.merriam- webster.com/dictionary/method Ohtaki, E., (1984). Boundary Layer Meteorology, 29, 85-107. Reynolds, O., (1895). Philosophical Transactions of the Royal Society of London. A, 186, 123-164. Ruppert, J., et al., (2006). Agricultural & Forest Meteorology , 38, 5- 18. Shuttleworth, et al., (1984). Quarterly Journal of the Royal Meteorological Society, 110, 1143-1162. technique, (2010). In Merriam-Webster Online Dictionary. Retrieved September 8, 2010, from http://www.merriam- webster.com/dictionary/technique Webb, E. K., Pearman, G. I. and Leuning, R., (1980). Quarterly J. Royal Meteorological Society, 106, 85-100. Discussion … 1. Technique or Method ? … Reynolds‟ averaging techniques is applied on equations (1), (2), and (3). For any two variables X and Y and a constant a, perturbations from the mean (X ′ and Y ′ ) (Reynolds, 1895) and (5) (6) (7) (8) (9) (10) Final equations for calculating eddy fluxes; Trace gas concentration (c) flux (F c [gm -2 s -1 ]): (11) Sensible heat flux [Kms -1 ]: (12) Latent heat flux [gg -1 ms -1 ]: (13) Friction velocity (proxy-momentum flux) [m 2 s -2 ]: (14) Where c is concentration of the gas, c p is specific heat capacity at constant pressure, c vv specific heat capacity at constant volume of water vapor, and q is mixing ratio of water vapor; primes indicate perturbations from the mean. In a nutshell this framework has two sets of assumptions (core plus additional) and applies specific techniques, like the Reynolds postulates, to reach the final equations. Some conditions refer specific techniques (equations). So this approach (method) represents abstractions of how direct measurement of fluxes of trace gas concentration, momentum and energy are done in the ABL. Therefore referring this approach as a „method‟ than a „technique‟ is a reasonable argument. However, the set of formulas can be taken as a „technique‟ individually. 2. Eddy Covariance or Eddy Correlation? For two variables X and Y with standard deviations σ X and σ Y . Covariance : (15) Correlation : (16) Analysis of eddy motion in micrometeorology involves second order moments of two variables (see eqs. 11, 12, 13 and 14). Eddy „covariance‟ is the appropriate terminology to be used.     ' ' Y X Y Y X X XY          Y X Y X XY Y X Y Y X X     ' '      Discussion 1. Technique or Method? According to the Webster‟s Dictionary (2010) • method is ““1.a (i): a way, technique or process of or for doing something (ii): a body of skills or techniques. 2: a discipline that deals with the principles and techniques of scientific inquiry.” • technique is “a. as a body of technical methods (as in a craft or scientific literature), b. a method of accomplishing a desired aim.” There are some commonalities in descriptions of the two terms. However, a „method‟ is an abstraction of a set of techniques, while a „technique‟ is implementation of a method. NSE in vector for a unit volume of air in a control volume (CV) are the following: Conservation of mass: (1) Conservation of momentum: (2) Conservation of energy: (3) Where S c is storage of a specific trace gas in the CV, ρ is density, is velocity vector = V(u, v, w), P is atmospheric pressure, f is Coriolis parameter, g is acceleration due to gravity, υ kinematic viscosity, s is specific entropy, Q is specific heat energy, and T is temperature. The direct measurement of fluxes in the ABL is based on three core assumptions . • A non-accelerated turbulent atmosphere under steady state conditions (stationarity) (Marcolla, et al., 2005; Baldocchi, 2003). So with this • Horizontally homogenous underlying surface with unlimited flat terrain upwind of the sensors (Marcolla, et al., 2005; Baldocchi, 2003); • Negligible molecular diffusion (Marcolla, et al., 2005). Additional conditions (not exhaustive), • and f(v – u) = 0 for micro-scale process of ~ 1 km or less in the ABL, • Air as ideal gas in the ABL: P = ρRT , (4) where R is the universal gas constant. • Webb-Pearson-Leuning correction for density fluctuations of a sample of unsaturated air (Leuning, 2007; Webb, et al., 1980). X X X   ' Y Y Y   ' 0 ' '   Y X 0 , ,        t s t V t  0 ,      y v x u 0 2   V  P   1 V V t S c                   V g u v f P V V t V 2 1                 s V t s T Q       ' ' Y X Y X XY   Y X Y X  X a aX  Y X Y X    ' ' 2 * w u u   ' ' w q c Q vv E   ' ' w T c Q p H   ' ' w c F c    V