Geoscience Reference
In-Depth Information
A
Wind direction
Figure 7.6 Upwind fetch of
micrometeorological sensors.
“Fetch” of instrument A
which they pass. The second way is, by measuring the water lost by evaporation
from the surface, to obtain the latent heat flux, and then to deduce the sensible
heat flux from the surface energy budget. These two different approaches are
discussed separately.
Micrometeorological measurement of surface energy fluxes
When energy fluxes are measured using micrometeorological techniques, the
measurement is of vertical flow made at a point with a sensor or sensors mounted
typically at a height of a few meters or tens of meters on a pole or tower in the
turbulent airstream comparatively close to the ground. Energy exchange is assumed
to be vertical and the energy fluxes are assumed independent of height and to be a
weighted average of the surface fluxes originating upwind of the instrumentation.
The upwind area sampled is called the fetch , see Fig. 7.6. The proportional
contributions from areas that lie within the fetch depend on the buoyancy of the
atmosphere, the height of the sensor(s) used, and on the aerodynamic roughness
of the upwind surface. Several different micrometeorological methods have been
used in the past but here attention is focused on the two still in common use.
Bowen ratio/energy budget method
One method that has been much used to measure latent and sensible heat fluxes is
based on the surface energy budget. The approach is theoretically simple. It relies
on the fact that it is always possible to provide an estimate of the sum of the latent
and sensible heat fluxes at any point from Equation (4.2) providing all the remain-
ing terms in the surface energy balance equation can be measured, thus:
(7.2)
HEA
+=
l
where A is the available energy given by Equation (4.3). To deduce the latent and
sensible heat fluxes separately, a second equation describing their interrelationship
is required and the ratio of the sensible to the latent heat fluxes, b , the Bowen ratio,
is used to provide this second relationship. Consequently, the approach is called
the Bowen ratio/energy budget method.
To estimate the ratio of the flux of sensible heat to the flux of latent heat it is
assumed that for height of the order of meters to tens of meters above the ground,
the transfer processes responsible for moving sensible heat vertically are the same
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