Campbell Scientific Open Path Eddy Covariance (OPEC) - Manual (Page 40)

OPEC Open-Path Eddy-Covariance System
The surface layer (FIGURE 5-1) is comprised of approximately the lower 10%
of the atmospheric boundary layer (ABL). The fluxes of water vapor and heat
within this layer are nearly constant with height when the following criteria are
met: the surface has approximate horizontal homogeneity; and the relationship
z/h << 1 << z/z
om
is true, where z
sfc
is the height of the surface layer, h is the
height of the ABL, and z
om
is the roughness length of momentum. When the
above conditions are met, the flux of carbon dioxide, water vapor and heat,
within the surface layer, may be written as:
z c c
U F ′ ′ = ρ (1)
z v v
U L LE ′ ′ = ρ (2)
z p a
U T C H ′ ′ = ρ (3)
where F
c
is the carbon dioxide flux, ρ′
c
is the instantaneous deviation of the
carbon dioxide density from the mean,
z
U′ is the instantaneous deviation of
vertical wind speed from the mean, LE is the latent heat flux, L
v
is the latent
heat of vaporization, ρ′
v
is the instantaneous deviation of the water vapor
density from the mean, H is the sensible heat flux, ρ
a
is the density of air, C
p
is
the heat capacity of air at a constant pressure, T ′ is the instantaneous deviation
of air temperature from the mean (Stull, 1988; Massman et al., 2004).
The quantities
z c
U′ ′ ρ ,
z v
U′ ′ ρ , and,
z
U T ′ ′ are the covariances between vertical
wind speed and carbon dioxide density; vertical wind speed and vapor density;
and vertical wind speed and temperature. These quantities are computed online
by the CRBasic datalogger as are the cross products (second order moments)
required to apply a post processing coordinate rotation following Kaimal and
Finnigan (1994) and Tanner and Thurtell (1969).
The Open-Path Eddy-Covariance system directly measures latent and sensible
heat flux. If net radiation and soil heat flux are also measured, energy balance
closure may be examined using the surface energy balance equation:
LE H G R
n
+ = − (4)
where R
n
is the net radiation and G is the total soil heat flux. H, LE, and G are
defined as positive away from the surface and R
n
is positive toward the surface;
F
c
is defined, following the micrometeorological sign convention, as positive
away from the surface.
If a FW05 finewire thermocouple is not part of the system, the sensible heat
flux (H
c
) can be computed from the vertical wind and humidity corrected sonic
temperature.
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