Abstract
AbstractIn the atmospheric boundary layer, phenomena exist with challenging properties such as spatial heterogeneity, particularly during stable weak wind situations. Studying spatially heterogeneous features requires spatially distributed measurements on fine spatial and temporal scales. Fiber-Optic Distributed Sensing (FODS) can provide spatially distributed measurements, simultaneously offering a spatial resolution on the order of decimeters and a temporal resolution on the order of seconds. While FODS has already been deployed to study various variables, FODS wind direction sensing has only been demonstrated in idealized wind tunnel experiments. We present the first distributed observations of FODS wind directions from field data. The wind direction sensing is accomplished by using pairs of actively heated fiber optic cables with cone-shaped microstructures attached to them. Here we present three different methods of calculating wind directions from the FODS measurements, two based on using combined wind speed and direction information and one deriving wind direction independently from FODS wind speed. For each approach, the effective temporal and spatial resolution is quantified using spectral coherence. With each method of calculating wind directions, temporal resolutions on the order of tens of seconds can be achieved. The accuracy of FODS wind directions was evaluated against a sonic anemometer, showing deviations of less than 15° most of the time. The applicability of FODS for wind direction measurements in different environmental conditions is tested by analysing the dependence of FODS wind direction accuracy and observable scales on environmental factors. Finally, we demonstrate the potential of this technique by presenting a period that displays spatial and temporal structures in the wind direction.
Highlights
Exchange mechanisms of momentum, mass, and energy between the surface and the atmosphere in the stable weakwind boundary layer are crucial for our understanding of many processes, including, for example, fog formation (Oke 1987), pollutant dispersal (Hanna 1986, 1990), forest–atmosphere gas exchange (Oliveira et al 2013; Freundorfer et al 2019), or frost formation (Whiteman 2000)
With the two methods based on wind speed measurements, S15 and VR20 (Figs. 2b,e and 2c,f), a gap in the fiber-optic distributed sensing (FODS) wind directions can be identified along the axes of the FODS cross
The ability to measure spatially distributed horizontal wind directions is an important step toward the goal of employing distributed temperature sensing (DTS) as a fully threedimensional, spatially resolved flow sensor for the atmosphere and toward spatially distributed eddy covariance measurements
Summary
Mass, and energy between the surface and the atmosphere in the stable weakwind boundary layer are crucial for our understanding of many processes, including, for example, fog formation (Oke 1987), pollutant dispersal (Hanna 1986, 1990), forest–atmosphere gas exchange (Oliveira et al 2013; Freundorfer et al 2019), or frost formation (Whiteman 2000). These exchange processes typically are statistically parameterized using similarity theories (Holtslag and De Bruin 1988). Explicit measurements are useful in other conditions than during a stable weakwind boundary layer, since internal boundary layers and heterogeneous turbulence can occur during strong wind conditions
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