Geoscience Reference
In-Depth Information
fluctuating components of the wind (in comparison to the
mean value) in the horizontal (
u
) and vertical (
w
) direc-
tions. Such data can be measured at-a-point in the airflow
by a sonic anemometer (see Box 18.1) and so the require-
ment for a log-linear velocity profile over a significant
depth of boundary layer is dismissed. Measurements of
the Reynolds shear stress are becoming more common
in aeolian process research (van Boxel, Sterk and Arens,
2004; Weaver and Wiggs, 2010; Weaver, 2008), with fluc-
tuating components measured at 10 Hz and shear stress
averaged over 10 minutes showing a good correlation with
sediment transport rates (Weaver, 2008).
Box 18.1
From bagnold to sonics: measuring boundary layer airflow
Over the last 60 years one of the greatest contributions to improving our understanding of the processes of sediment
mobilisation by the wind has been the extraordinary technological development in the measurement of airflow.
From the 1940s to 1960s aeolian researchers often coped with bulky mechanical weather stations where data were
recorded on paper rolls controlled by clockwork and hourly average windspeeds were considered high resolution.
The bulk of these instruments meant that velocity could only reasonably be measured at a single height and often
several metres away from the surface. It is a sensation then that so much of our understanding of the physics of
blowing sand comes from this time period and that reference to the classic works of the time is still so prevalent.
In the 1970s to 1990s small, fast response cup anemometers were developed for use with electronic logging
systems. These allowed much higher resolution measurements in both space and time. The small diameter of the
anemometer bodies allowed several to be placed in vertical arrays without obstructing the airflow, thus effectively
measuring the vertical velocity profile and allowing a detailed investigation of shear velocity (
u
∗
). The small plastic
cups on these instruments responded quickly to accelerations and decelerations in airflow and these short response
and lag times, combined with electronic logging, enabled data to be recorded at minute intervals or better. However,
these instruments were prone to failure and required heavy maintenance if used in sandy environments for extended
periods. This was particularly the case as investigators attempted to measure the enigmatic relationships between
sand flux and wind velocity right at the top of the saltation layer. At this time it also became clear from wind
tunnel studies, where near-surface wind speeds could be measured over stabilised (nonsaltating) sand beds with
fast-response hot-wire anemometers, that measurement of turbulent frequencies in the airflow was going to be
required in the field.
As the twenty-first century began, aeolian geomorphologists had started using sonic anemometers in field
situations (Figure 18.4). These instruments measure wind velocity by calculating the delay in travel time of an
ultrasonic acoustic signal over a set path between paired sonic transducer heads. The difference in time delay
between both directions along the same path is proportional to the wind speed. Sonic anemometers offer huge
advantages in measurement techniques to desert geomorphologists. First, they calculate wind velocity within an
empty volume of air between the transducer heads, and hence there is no obstruction to the flow. Second, they
