Near-surface wind speed is typically only measured by point observations. The actively heated fiber-optic (AHFO) technique, however, has the potential to provide high-resolution distributed observations of wind speeds, allowing for better spatial characterization of fine-scale processes. Before AHFO can be widely used, its performance needs to be tested in a range of settings. In this work, experimental results on this novel observational wind-probing technique are presented. We utilized a controlled wind tunnel setup to assess both the accuracy and the precision of AHFO under a range of operational conditions (wind speed, angles of attack and temperature difference). The technique allows for wind speed characterization with a spatial resolution of 0.3 m on a 1 s timescale. The flow in the wind tunnel was varied in a controlled manner such that the mean wind ranged between 1 and 17 m s^-1. The AHFO measurements are compared to sonic anemometer measurements and show a high coefficient of determination (0.92–0.96) for all individual angles, after correcting the AHFO measurements for the angle of attack. Both the precision and accuracy of the AHFO measurements were also greater than 95 % for all conditions. We conclude that AHFO has the potential to measure wind speed, and we present a method to help choose the heating settings of AHFO. AHFO allows for the characterization of spatially varying fields of mean wind. In the future, the technique could potentially be combined with conventional distributed temperature sensing (DTS) for sensible heat flux estimation in micrometeorological and hydrological applications. ... Read more

Distributed Temperature Sensing (DTS) with fibre optics is an emerging technique, which has been used for many environmental applications (lakes, glaciers, seasonal snow, streams, and soil) over the past decade. Recently DTS has been adapted to atmospheric studies to provide high temporal and spatial resolution observations: current technology allows for 1 Hz temporal resolution, and spatial to 30 cm with maximum range over 5 km. Temperature measurements can be leveraged to quantify other variables. Here wind speed is measured by including an actively heated fibre in addition to a non-heated (reference) fibre. In this ‘hot wire’ configuration, the temperature difference between the heated and reference cables can be related to the wind speed by way of an energy balance (Sayde et al., 2015). The technique has the potential to increase the data density of measurement in the atmosphere by one or two orders of magnitude, but has not yet ben rigorously tested in controlled conditions. Wind tunnel wind speed was measured with the DTS and a sonic anemometer. The measurements were performed with four angles of attack and with speeds from 1 to 17 m/s. The signal-to-noise ratio was computed across a range of heat settings (W/m). These results provide a design framework for the use of atmospheric DTS measurements, and establish a quantitative methodology with significant improvements in spatial and temporal resolution which we expect will give new insights into atmospheric processes. ... Read more