Eddy flux

The possibility of estimating water transport from the water surface into the atmosphere consists in assessing the water content of the lower part of the surface layer of the atmosphere, which forms spray and water vapor. The eddy flux of water... 

Figure 1. An example of a time sequence of eddy flux measurements of submicron sulfate particles, obtained during a recent study over a deciduous forest at Oak Ridge, TN. Note that negative values indicate fluxes directed towards the ground, i.e., deposition.

Lowlands. Sites in areas of discontinuous permafrost as well as other areas susceptible to permafrost melting should be part of the network. Static chamber flux measurements are legendary as labor-intensive, and they should be replaced where possible with automated chambers and supplemented with continuous eddy flux measurements (Hargreaves et al., 2001). Development of a held-portable, rapid-response CH4 sensor suitable for continuous eddy covariance measurements should be a high priority. 

The flux-profile relationships utilized above crop fields and grasslands (cf. Cellier and Brunet, 1992) may not be valid, and are therefore not generally accepted, above forests. In forests, the eddy correlation method offers a more direct approach to flux measurements. In this case, a fast responding sonic anemometer is used to indicate the velocity and direction of the vertical air movement component (up or down eddies). A fast responding analyzer records the concentration of the trace gas of interest in conjunction with anemometer measurements. Once data are collected over an appropriate time interval (e.g., 30min), the concentration measurements can be separated to those related to eddies moving up from the canopy or down into the canopy. The net eddy flux is then calculated according to... 

Direct measurement of turbulent fluctuations in canopies was not possible until relatively recently so their first recourse was to simple, first-order eddy diffusivity or X -theory models of turbulent fluxes. Typically the vertical eddy flux of a scalar, C, or of mean momentum, that is windspeed, U, was written as3... 

The mean scalar concentration c and its eddy flux / are divided into background and hill-induced perturbations as follows,... 

In the shear stress layer the divergence of the eddy flux of c becomes important so the linearised mass balance is,... 

Raupach et al 1992 [528] showed that in this region two other mechanisms become important in determining Ac x, c). The first is the changes induced by the hill in the eddy flux field Af(x, z) and hence in its divergence. The second is the change to the flux of c from the surface that occurs because the surface shear stress Ar(x, z) varies as the hill is traversed. The mechanism for this in the rough hill model of [528] is the... 

Upper canopy layer The dominant term in the upper canopy is the effect on the sink of the velocity perturbation through the last term of (5.27), r/2Lc [AuCB. Since Ail peaks at the hillcrest, this produces a minimum in Ac there. This effect can be interpreted as the canopy counterpart of the dominant role played by changes in surface stress on a rough hill in modulating the surface flux boundary condition, and thereby Ac, as discussed earlier. Hence, as a result of quite different dynamics, the dominant influences on Ac in the lower and upper canopy layers are in phase leading to a minimum in Ac at the hillcrest. This simple picture is modulated by other effects in the upper canopy layer, however. The eddy flux divergence couples the upper canopy to the shear stress layer above where advection is important at first order (5.26) and within which the contributions to Ac that are caused by canopy dynamics must decay to match the inviscid, streamline convergence effects around z = hi. 

The foregoing analysis together with (5.26) now allows us to non-dimensionalize the mass balance equations in a way that permits us to compare the expected magnitude of the advection terms with the eddy flux, FB + A/ = w c ... 

Horiz. Adv. Vert. Adv. Eddy Flux Divergence Scalar Source/S ink... 

Figure 5.5 a) Variation of the horizontal and vertical advection terms and the eddy flux over a sinusoidal hill covered with a tall canopy, b) Fractional error entailed in estimating the canopy sink S from the background eddy flux, F(z) alone. 

Louis, J.F. (1979) A parametric model of vertical eddies fluxes in the atmosphere, Boundary-Layer Meteorol. 17, 187-202. 

Here -C V is the turbulent (eddy) flux of the admixture. This vector can be determined using some closure hypothesis, based, as a rule, on empirical information. 

Boyd, J.P., The noninteraction of waves with the zonally-averaged flow on a spherical Earth, and the interrelationship of eddy fluxes of energy, heat and momentum. J Atmos Sci 33, 2285, 1976. 

Plumb, R.A., Eddy fluxes of conserved quantities by small amphtude waves. J Atmos... 

Flux-resolving, fluctuation-based methods In these methods the vertical turbulent eddy flux F is measured as pw c, where overbars and primes denote time means and fluctuations, respectively, and w is the vertical velocity component. They include eddy-covariance and eddy-accumulation methods for measuring fluxes from towers and aircraft. They are not normally regarded as atmospheric inverse methods, since that term usually refers to methods which relate a mean concentration field to a source density or surface flux distribution. 

Equation (3) takes a similar form for both time-averaged and instantaneous quantities. In the time-averaged case, F is a turbulent eddy flux pu c, where overbars and primes denote time means and fluctuations, respectively. In the instantaneous case, F is a molecular diffusive flux which in practice can be neglected in the open air (on Sq) relative to fluxes arising from fluid motion (the high Peclet number approximation). In this case the fluid-motion fluxes appear in the term pc(u — v), which is unaveraged and includes transport by turbulent fluctuations as well as by the mean flow. In contrast with the situation in the open air (on Sq), molecular fluxes can never be neglected at solid boundaries (S,), where they are responsible for all the scalar transport. 

To obtain field data to test and improve models of dry deposition of particles, improved techniques and/or new instrumentation appear to be critical (Nicholson, 1988). Particle-size-specific, eddy-flux measurements (see Section 19.5) of ambient particles appear promising (e.g., Neumann and den Hartog, 1985) but to date have suffered from poor statistics associated with few particles in specific size classes. Furthermore, similar studies at... 

The vector F is the quasi-geostrophic EP flux. Its components are the zonally averaged northward eddy flux of zonal momentum and the zonally averaged northward eddy flux of temperature (because d dp is proportional to temperature through the hydrostatic approximation). Equation (117) shows that the EP flux divergence, defined by Eq. (115), induces changes in the basic state. 

In summaiy, the EP flux is a vector in the meridional plane whose components, in quasi-geostrophic theory, measure the zonally averaged northward eddy fluxes of momentum and temperature. Maps of the EP flux and its divergence may be prepared from observations or the results of model calculations. Such maps may be used to infer where and to what extent the eddy motions are acting to change the mean flow and where transient, nonconservative eddy motions are present. 

Kormann R, Muller H, Werle P. 2001., Eddy flux measurements of methane over the fen Mumauer Moos , 11° 11 E, 47°39 N, using a fast tunable diode laser spectrometer . Atmos. Environ. 35(14) 2533-2544. 

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