Sonic anemometers in aeolian sediment transport research

Abstract

Fast response wind and turbulence instruments, including sonic anemometers are used more and more in the research of aeolian sediment transport. These instruments provide data on mean wind, but also on friction velocity, wind speed fluctuations and turbulence statistics, such as the U-W and W-T covariance (C UW and C WT ), which are a measure for the momentum flux and the sensible heat flux. This short paper will examine two problems that may arise when using sonic anemometers, namely the low/high frequency losses and the interpretation of the sonic anemometer measurements over sloping or non-homogeneous terrain.

Effects of precipitation on sonic anemometer measurements of turbulent fluxes in the atmospheric surface layer

“…The sound speed is averaged with three measurements along each axis, which depends on air density and is influenced by air temperature, therefore serving as an indicator of air temperature. Particularly, sonic anemometer-derived temperature is referred to as the sonic virtual temperature but not exactly the same as the virtual temperature, despite that the difference between them is substantially negligible (van Boxel et al, 2004). Given that the sound speed and sonic virtual temperature are related to each other (Kaimal and Businger, 1963), the sonic virtual temperature can be estimated as 2 (1 0.317 / ) (1 0.51 )…”

“…For example, the WindMaster Pro of Gill Instruments explicitly identifies in the product manual that it bears rain rate up to 300 mm h −1 . Nevertheless, the sensors of the sonic anemometer are sensitive to impurities, such as the raindrop (van Boxel et al, 2004). In previous studies, data collected during precipitation are generally removed to adequately guarantee the quality controls.…”

Sediment Mobilisation by the Wind

“…Sweeps and ejections contribute to a 'bursting process' in the flow where low-speed ejections of fluid away from the bed are followed by high-speed sweeps of flow towards the bed, and this process has been found to be important in accounting for sediment transport in rivers (Best, 1993). However, field investigations by Sterk, Jacobs and van Boxel (1998), Schönfeldt and von Löwis (2003), van Boxel, Sterk and Arens (2004), Leenders, van Boxel and Sterk (2005) and Weaver (2008) have found that the majority of instances of high aeolian saltation flux are rather associated with sweeps and outward interactions, i.e. flow events that have instantaneous horizontal velocities greater than the mean (Figure 18 Best, 1993).…”

“…Such data can be measured at-a-point in the airflow by a sonic anemometer (see Box 18.1) and so the requirement 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 fluctuating components measured at 10 Hz and shear stress averaged over 10 minutes showing a good correlation with sediment transport rates (Weaver, 2008).…”

“…These findings led Butterfield (1993) to conclude that with naturally fluctuating winds over a sand bed, the grain-laden boundary layer is in constant adjustment and may rarely achieve an equilibrium state. With the increasing application of sonic anemometers in the field (Walker, 2005;van Boxel, Sterk and Arens, 2004;Weaver, 2008) and the use of high-frequency grain impact sensors (Schönfeldt and von Löwis, 2003;Baas, 2004), investigations have now begun to observe the relationship between saltation dynamics and instantaneous turbulent peaks in wind velocities at much higher frequencies (1 to > 10 Hz). While the relationship between turbulent wind flow and sand transport is complex at such frequencies ( Figure 18.25) the association between the parameters has led many to question the efficacy of 'mean' measures of erosivity, such as mean wind velocity or shear velocity (u * ) to predict sediment fluxes (Stout, 1998;Baas and Sherman, 2005;Weaver, 2008).…”