Geoscience Reference
In-Depth Information
Figure 7.9
Mean
and vertical wind
speed components
parallel to and
perpendicular to
the surface. The
mean perpendicular
wind is zero, with
fluctuations in
vertical wind speed
caused by turbulent
eddies.
surface, atmospheric humidity, and temperature above the ground. Rapid response
sensors are required to capture high frequency fluctuations and the sensors must
have stable calibration over the period over which the time-average product is
calculated. To make the measurement, sometimes electronic hardware is used to
rapidly interrogate the sensors and to compute the fluctuating component of each
measured value and their instantaneous cross products, and then to average and
store the resulting fluxes, preferably over long periods. However, now that data
storage is inexpensive, simply storing the data for later analysis is an alternative.
In practice, the turbulent eddies responsible for the flux transfer occur over a
wide range of frequencies, with the spectrum of contributing frequencies being
determined by:
ambient horizontal wind speed - there is more transfer at higher frequencies
when wind speeds are greater;
●
sensor mounting height - there is more transfer at lower frequencies when
sensors are mounted farther from the surface;
●
aerodynamic roughness of the surface - there is more transfer at lower
frequencies over rougher surfaces such as forest than smoother surfaces such
as bare soil; and
●
atmospheric buoyancy - there is more transfer at lower frequencies in
unstable atmospheres.
●
In eddy correlation measuring systems, measurements are usually attempted using
sensors that can resolve frequencies from about ten cycles per second to about ten
cycles per hour.
Evaporation measurement from integrated water loss
Evaporation can be measured as the net water loss to the atmosphere from a
terrestrial surface over a given time period. If required, the time average latent heat
