Instruction manual

1-1
SECTION 1. SYSTEM OVERVIEW
1.1 REVIEW OF THEORY
The surface layer (Figure 1.1-1) 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
water vapor and heat, within the surface layer,
may be written as:
LE L w
v
v
=
′′
ρ
(1)
HCwT
ap
=
′′
ρ
(2)
where LE is the latent heat flux, L
v
is the latent
heat of vaporization, w
′
is the instantaneous
deviation of vertical wind speed from the mean,
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, and T
′
is
the instantaneous deviation of air temperature
from the mean (Stull, 1988).
The quantities
′′
w T and
′′
w
v
ρ
are the
covariances between vertical wind speed and
temperature, and vertical wind speed and vapor
density. These quantities can be readily
calculated on-line by the datalogger.
The 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:
RGHLE
n
−=+
(3)
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.
FIGURE 1.1-1. Ideal Vertical Profiles of
Virtual Potential Temperature and Specific
Humidity Depicting All the Layers of the
Atmospheric Boundary Layer.
1.2 SYSTEM DESCRIPTION
1.2.1 SURFACE FLUX SENSORS
The eddy covariance system consists of three
sensors that measure the fluctuations in vertical
wind speed, air temperature, and water vapor
density. The CA27 is a one dimensional sonic
anemometer with a fine wire thermocouple
(127). The CA27 has a path length of 10 cm
and frequency response of 40 Hz. The 127 is a
12.7
µ
m fine wire thermocouple with a
frequency response of greater than 30 Hz. The
small size and symmetric construction of the
127 thermocouple junction minimizes radiation
loading (Tanner, 1979), thus it is not necessary
to shield the 127.
The KH20 is an ultraviolet krypton hygrometer
(Campbell and Tanner, 1985) which is similar in
principle to the Lyman-alpha hygrometer (Buck,
1976), except that the source tube contains
krypton gas. The KH20 has a frequency
response of 100 Hz.
1.2.1.1 Additional Required Measurements
Ambient air temperature and humidity must be
measured. This information is used to make
corrections to the water vapor measurements
and calculate air density.