Fiber-Optic Distributed Temperature Sensing to quantify turbulence over space and time: A feasibility study

Abstract. Turbulence is essential for land atmosphere interactions; however, it is difficult to quantify due to its statistical and three dimensional nature. Typically, turbulence is determined using time series data to obtain turbulent data over a length scale (through Taylor's hypothesis). Ideally, turbulence is measured directly over multiple spatial dimensions as well as time, at high frequency. Currently turbulence measurements are limited to either time series (e.g., sonic anemometers) or integrated spatial measurements (e.g., scintillometers). However, direct spatiotemporal measurements of turbulence are lacking. In this study we aim to quantify turbulence over both time and space using fiber-optic distributed temperature sensing (DTS) using the structure parameter of temperature C_T². We conducted an experiment in the northeast of Spain in July 2021, where we measured temperature over a 70 m horizontal extent at a height of 2 m using a thin 0.5 mm fiber optic cable. The DTS experiment had a response time of 1.3 s and a response length of 0.35 m. We determined C_T² both spatially and temporally, through its definition as well as through the temperature turbulent spectrum, resulting in 4 results for C_T² from the DTS for each 30 minutes. Several (spectral) correction steps were applied to compensate for limited response rates, spatial averaging and noise. The C_T² values were compared with reference values from a sonic anemometer and a pair of scintillometers. For wind regimes with crosswind lower than 1.9 ms^-1 and mean absolute wind speed higher than 1.5 ms^-1, a correlation of R between 0.86 and 0.92 was found. The spectral DTS underestimates by a factor 3-4 in comparison to the sonic and by a factor 6 for the scintillometer. Similar results are found for the definition method and include a persistent offset of 0.04 for C_T². Hence, DTS correlates well with conventional reference instruments within the current limitations of DTS and it provides a direct and independent spatial measurement of turbulence. This opens the door to the use of DTS as a relative and spatial turbulence instrument to be used alongside an absolute reference instrument. The used setup and methodology can be scaled up to enable direct spatial measurements of turbulence at the km scale while keeping a spatial resolution in the order of meters.