Geoscience Reference
In-Depth Information
(a)
Plug
(b)
Reversing motor
housing
Thermopile
cables
Vane
Wet-bulb
Inner shield
Outer shield
Dry-bulb
PVC Tee
PVC Pipe
Radiation
shield
Stop
Water
reservoir
Plug
PVC Tee
Aspiration motor
(Viewed from the front
of the system)
Figure 7.7
(a) Schematic diagram of a Bowen ratio measuring system with interchanging temperature and humidity
sensors; (b) A Bowen ratio measuring system with interchanging sensors used over short vegetation. (From McCaughey,
1981, published with permission.)
Eddy correlation method
In the atmosphere a few meters or tens of meters above the ground, vertical
movement of atmospheric entities is almost entirely by turbulent transport. Near
a horizontal surface the mean wind,
u
, is parallel to the ground so at any point, P,
the average wind vector perpendicular to the ground,
w
, is near zero (Fig. 7.9).
However, the
turbulent eddies
in the air cause pseudo-random fluctuations in the
vertical wind,
w
′
, and in other atmospheric variables. In particular, there are
fluctuations
q
around the mean value of specific humidity,
q
, and fluctuations
q
v
′
′
around the mean value of virtual potential temperature,
q
v
.
Upward movement of water vapor in the turbulent field means that on average
there is a correlation between fluctuations of higher than average humidity and
movement of air away from the surface, i.e., a correlation with positive fluctuations
in vertical wind speed. Similarly lower than average fluctuations in average
humidity are on average correlated with negative fluctuations in vertical wind
speed. Integrating the product of the instantaneous value of
w
with the
instantaneous fluctuation in the volumetric latent heat content of the air, (
l
r
a
q
′
′
),
gives the time average outward flux of latent heat,
λ
E
, i.e.:
llr
E
=
a
q w
′
′
(7.7)
