Suitability of fibre-optic distributed temperature sensing for revealing mixing processes and higher-order moments at the forest–air interface

Abstract

Abstract. The suitability of a fibre-optic distributed temperature sensing (DTS) technique for observing atmospheric mixing profiles within and above a forest was quantified, and these profiles were analysed. The spatially continuous observations were made at a 125 m tall mast in a boreal pine forest. Airflows near forest canopies diverge from typical boundary layer flows due to the influence of roughness elements (i.e. trees) on the flow. Ideally, these complex flows should be studied with spatially continuous measurements, yet such measurements are not feasible with conventional micrometeorological measurements with, for example, sonic anemometers. Hence, the suitability of DTS measurements for studying canopy flows was assessed. The DTS measurements were able to discern continuous profiles of turbulent fluctuations and mean values of air temperature along the mast, providing information about mixing processes (e.g. canopy eddies and evolution of inversion layers at night) and up to third-order turbulence statistics across the forest–atmosphere interface. Turbulence measurements with 3D sonic anemometers and Doppler lidar at the site were also utilised in this analysis. The continuous profiles for turbulence statistics were in line with prior studies made at wind tunnels and large eddy simulations for canopy flows. The DTS measurements contained a significant noise component which was, however, quantified, and its effect on turbulence statistics was accounted for. Underestimation of air temperature fluctuations at high frequencies caused 20 %–30 % underestimation of temperature variance at typical flow conditions. Despite these limitations, the DTS measurements should prove useful also in other studies concentrating on flows near roughness elements and/or non-stationary periods, since the measurements revealed spatio-temporal patterns of the flow which were not possible to be discerned from single point measurements fixed in space.

The NY-Ålesund TurbulencE Fiber Optic eXperiment (NYTEFOX): investigating the Arctic boundary layer, Svalbard

“…The accuracy of the fiber wind speed is computed as the standard deviation of the fractional absolute deviation of the fiber readings from those of the reference (CSAT3) instrument aggregated to 30 s. The bias systematically depends on the location along the fiber and ranges from an 8 % underestimation to a 13 % overestimation with an average overestimation of 4 %. In our setup, the fiber-optic cable was assumed to be in thermal equilibrium with the air and snow temperaturesa reasonable assumption for the low-solar-intensity environment of the polar night -which helps minimize the radiative error (Sigmund et al, 2017). The distance along the fiberoptic cable is resolved by range gating with known values for the speed of light and the length and geometry of the fiber-optic cable.…”

“…2), a high-resolution vertical profile consisting of a 2.5 m high column was used to sample snow and air temperature. The PVC fiber-optic cable was helically coiled around a support structure made from reinforcement fabric (Sigmund et al, 2017), resulting in a subcentimeter vertical resolution (see the data description in Sect. 4.1).…”

“…We deployed a large, horizontal, trapezoidal-shaped, 700 m long FODS array in combination with three vertical profiles at its corners that were about 7 m high to record air temperatures and wind speeds at high temporal (9 s) and spatial (0.127 m) resolution. Using a high-resolution coil-wrapped FODS column (Sigmund et al, 2017), air and snow temperatures were recorded along a 2.5 m vertical profile at subcentimeter resolution. To validate the results and place them in a broader context, FODS observations were complemented by measurements from three sonic anemometers at the corners of the FODS array to collect high-frequency wind measurements, as well as an acoustic wind profiler (minisodar, sodar is an acronym for "sound detection and ranging") yielding wind statistics between 10 and 300 m a.g.l.…”

Quantifying the coastal urban surface layer structure using distributed temperature sensing in Helsinki, Finland

“…DTS has already been used in a growing number of studies of the surface layer, including one-dimensional profile measurements of temperature (Keller et al, 2011;Higgins et al, 2018;Fritz et al, 2021;Peltola et al, 2021) and studies with two-or three-dimensional arrays (Thomas et al, 2011;Zeeman et al, 2015;Pfister et al, 2017Pfister et al, , 2019Mahrt et al, 2020). Furthermore, by usage of heated or wetted cables, DTS can be used to study wind speed (Sayde et al, 2015) or humidity (Schilperoort et al, 2018;Izett et al, 2019).…”

“…The performance of the DTS method for longer campaigns or continuous measurements has not been previously assessed. Various meteorological conditions can be challenging for DTS, as the method requires a relatively high amount of thermal turbulence to provide reliable estimates of thermal turbulence statistics (Peltola et al, 2021). Solar radiation can also induce an error in the measurements (De Jong et al, 2015), as does wetting of the cable due to precipitation or fog (Izett et al, 2019).…”