Quantitative analysis of the radiation error for aerial coiled-fiber-optic distributed temperature sensing deployments using reinforcing fabric as support structure

Abstract

Abstract. In recent years, the spatial resolution of fiber-optic distributed temperature sensing (DTS) has been enhanced in various studies by helically coiling the fiber around a support structure. While solid polyvinyl chloride tubes are an appropriate support structure under water, they can produce considerable errors in aerial deployments due to the radiative heating or cooling. We used meshed reinforcing fabric as a novel support structure to measure high-resolution vertical temperature profiles with a height of several meters above a meadow and within and above a small lake. This study aimed at quantifying the radiation error for the coiled DTS system and the contribution caused by the novel support structure via heat conduction. A quantitative and comprehensive energy balance model is proposed and tested, which includes the shortwave radiative, longwave radiative, convective, and conductive heat transfers and allows for modeling fiber temperatures as well as quantifying the radiation error. The sensitivity of the energy balance model to the conduction error caused by the reinforcing fabric is discussed in terms of its albedo, emissivity, and thermal conductivity. Modeled radiation errors amounted to −1.0 and 1.3 K at 2 m height but ranged up to 2.8 K for very high incoming shortwave radiation (1000 J s −1 m −2 ) and very weak winds (0.1 m s −1 ). After correcting for the radiation error by means of the presented energy balance, the root mean square error between DTS and reference air temperatures from an aspirated resistance thermometer or an ultrasonic anemometer was 0.42 and 0.26 K above the meadow and the lake, respectively. Conduction between reinforcing fabric and fiber cable had a small effect on fiber temperatures (< 0.18 K). Only for locations where the plastic rings that supported the reinforcing fabric touched the fiber-optic cable were significant temperature artifacts of up to 2.5 K observed. Overall, the reinforcing fabric offers several advantages over conventional support structures published to date in the literature as it minimizes both radiation and conduction errors.

Suitability of fiber-optic distributed temperature sensing to reveal mixing processes and higher-order moments at the forest-air interface

“…The fibre-optic cable was not radiation-shielded and hence it was exposed to shortwave and longwave radiation transfer with its surroundings. 30-min mean temperature values were found to be biased by solar heating during daytime and longwave radiative cooling at night in accor-10 dance with prior studies (de Jong et al, 2015;Sigmund et al, 2017). Due to the forest canopy, these radiation-induced biases differ below and above the canopy, introducing biases into the temperature gradient.…”

“…Note, however, that in double-ended mode, each direction along the cable is measured sequentially, so that the temperature data was available at 0.5 Hz resolution. A white, thin (outer diameter 0.9 mm) aramid reinforced 50 µm multimode fibre-optic cable (AFL Telecommunications, Duncan, SC 29334, US) was utilised in this study in order to minimise the impact of solar heating on the measurements (de Jong et al, 2015;Sigmund et al, 2017) and to maximise the high frequency response of the setup.…”

“…Bulk of the studies have concentrated on nocturnal flows near the surface (Keller et al, 2011;Thomas et al, 2012;Zeeman et al, 2015;Pfister et al, 2017;Izett et al, 2019;Mahrt et al, 2019;Pfister et al, 2019) and a few have concentrated on transition periods (Higgins et al, 2018(Higgins et al, , 2019. By utilising different DTS measurement configurations, some studies have done distributed wind speed (heated cables) (Sayde et al, 2015;Pfister et al, 2017) or humidity (wetted cables) measurements (Euser et al, 2014;Schilperoort et al, 2018), whereas others have examined the radiation error of the cables (de Jong et al, 2015;Sigmund et al, 2017). However, thorough and critical analysis on the feasibility of DTS system to measure atmospheric scalar mixing has not been done since the pioneering study of Thomas et al (2012).…”

Missed Fog?

“…However, during weak-wind, clear-sky, nocturnal conditions, the error in DTS-observed temperature can be large due to radiation of the fibre itself (Sigmund et al. 2017), particularly for larger fibres, as was the case here. Indeed, while the errors in temperature are small at 1.5-m height (Fig.…”

“…2016; Sigmund et al. 2017). The performance of the DTS technique needs to be further tested in order to ensure its reliability under stable, foggy conditions, with the present study serving to encourage future research.…”