Instruction manual

SECTION 2. STATION INSTALLATION
2-2
2.1 SENSOR HEIGHT
The eddy covariance sensors must be mounted
at some height that ensures that the
measurements are being made within the local
surface layer. The local surface layer grows at
a rate of approximately 1 vertical meter per 100
horizontal meters. Thus, a height to fetch
(horizontal distance traveled) ratio of 1:100 can
be used as a rough rule of thumb for deter-
mining the measurement height.
The following references discuss fetch
requirements in detail: Brutsaert (1986); Dyer
and Pruitt (1962); Gash (1986); Schuepp, et al.
(1990); and Shuttleworth (1992). The fetch
should be homogenous and flat, and no abrupt
changes in vegetation height should exist
(Tanner, 1988). Consider two adjacent fields,
the first planted with 1 m tall corn and the
second with 0.5 m soybean. Eddy covariance
sensors mounted at 2 m above the corn field
should have a minimum of 200 m of fetch in all
the directions that the data is of interest,
particularly between the eddy covariance
sensors and the interface between the corn and
soybean field.
2.2 MOUNTING
The CA27 and KH20 are shipped with a 0.75
inch by 0.75 inch NU-Rail (P/N 1017). The NU-
Rail is used to attach the horizontal mounting
arm to a 0.75 inch pipe vertical mast. The top
section of a CM6 tripod (Figure 2-1) can be
used as a horizontal cross arm mount. It can
be replaced with a longer threaded pipe if
necessary.
The CM6 leg separation can be adjusted to give
a measurement height of 1.1 m to 2.0 m. If the
desired measurement height is outside this
range, a CM10 or other user-supplied mounting
hardware will be required.
The CA27 must be mounted perpendicular to
the surface. This is done so that no horizontal
component of the wind is measured. In most
applications the surface is perpendicular to
gravity, thus the bubble level on the top of the
CA27 can be used to level the sensor.
Mount the KH20 next to the CA27 with a
separation of 10 cm. The hygrometer should be
set back from the anemometer to minimize flow
distortions. Try to keep the hygrometer tubes
as far as possible from the fine wire
thermocouple. This is done to avoid measuring
temperature fluctuations caused by radiation
loading of the hygrometer tubes. Figure 2.2-1
depicts one possible mounting configuration.
Mount the KH20 so that the source tube (the
longer of the two tubes) is on top. Center the
path of the KH20 around the height of the fine
wire thermocouple on the CA27.
To mount the 127, place the 127 on the probe
arm located between the transducer arms.
Gently press and rotate the 127 until it slips into
place. To remove the 127, pull it horizontally
away from the CA27 base.
CAUTION: Do not twist the 127 once it is
seated on the mounting arm.
FIGURE 2.2-1. Top and Side View of the CA27 and KH20