Spatial temperature measurements at the land-air interface using Distributed Temperature Sensing

In this study we introduce four Distributed Temperature Sensing (DTS) set-ups deployed at an irrigated alfalfa field site (La Cendrosa) as part of the LIAISE field campaign during 15-30 July 2021 in the north-east of Spain.  The DTS technique relies on the temperature dependence of Raman backscattering of light in a fibre optic cable. DTS provides spatially distributed temperature data over time. The type of profile (horizontal vs vertical), spatial- and temporal resolution and distance covered depend on the measurement configuration. To observe land-atmosphere interactions, we installed four set-ups in the LIAISE campaign, each of which focus on a different aspect.

We deployed a 1.6 mm diameter fibre covering a total distance of 600 m, which measured temperature at 5 s and 25.4 cm resolutions. In this continuous cable we included three DTS set-ups: 1) a surface layer profile between 1.6 - 40 m with a 25.4 cm vertical resolution, 2) a canopy profile averaged horizontally over 2.5 m with cables at 8 vertical levels between 0 - 1 m height inside of the rapidly-growing alfalfa and 3) a soil profile between -0.5 - 0 m with a vertical resolution of 1.25 cm; the enhanced spatial resolution was obtained by winding the cable onto a coil that was installed in the soil. An overview of these measurements is given, with an emphasis on factors limiting accuracy and on potential use in further research.

A fourth set-up was a so-called “turbulence harp”, which used a thinner and faster optical cable (0.5 mm) over a horizontal path of 70 m, measuring at four heights between 0.40 m and 2.05 m. Measurements were made at 1 Hz and 12.7 cm resolutions and a turbulent temperature spectrum was resolved up to 0.15 Hz. From this set-up, turbulence intensity was expressed in terms of the structure parameter of temperature (C_T²). We estimated C_T² both from the temperature time series along the DTS cables, as well as, and this is a novel approach, from the spatial temperature series over the DTS cable. The spatially determined structure parameter correlated with a sonic anemometer C_T² estimate, with a correlation coefficient of 0.88.

This work outlines the potential for using DTS in land-atmosphere interaction campaigns and provides a first step towards using DTS in capturing turbulent information along a spatial temperature series.