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Abstract. The paper presents a set of fiber-optic distributed temperature sensing (FODS) experiments to expand the existing microstructure approach for horizontal turbulent wind direction by adding measurements of turbulent vertical component, as well as turbulent sensible heat flux. We address the observational challenge to isolate and quantify the weaker vertical turbulent motions from the much stronger mean advective horizontal flow signals. In the first part of this study, we test the ability of a cylindrical shroud to reduce the horizontal wind speed while keeping the vertical wind speed unaltered. A white shroud with a rigid support structure and 0.6 m diameter was identified as the most promising setup in which the correlation of flow properties between shrouded and reference systems is maximized. The optimum shroud setup reduces the horizontal wind standard deviation by 35 %, has a coefficient of determination of 0.972 for vertical wind standard deviations, and a RMSE of less than 0.018 ms−1 when compared to the reference. Spectral analysis showed a fixed ratio of spectral energy reduction in the low frequencies, e.g., <0.5 Hz, for temperature and wind components, momentum, and sensible heat flux. Unlike low frequencies, the ratios decrease exponentially in the high frequencies, which means the shroud dampens the high-frequency eddies with a timescale <6 s, considering both spectra and cospectra together. In the second part, the optimum shroud configuration was installed around a heated fiber-optic cable with attached microstructures in a forest to validate our findings. While this setup failed to isolate the magnitude and sign of the vertical wind perturbations from FODS in the shrouded portion, concurrent observations from an unshrouded part of the FODS sensor in the weak-wind subcanopy of the forest (12–17 m above ground level) yielded physically meaningful measurements of the vertical motions associated with coherent structures. These organized turbulent motions have distinct sweep and ejection phases. These strong flow signals allow for detecting the turbulent vertical airflow at least 60 % of the time and 71 % when conditional sampling was applied. Comparison of the vertical wind perturbations against those from sonic anemometry yielded correlation coefficients of 0.35 and 0.36, which increased to 0.53 and 0.62 for conditional sampling. This setup enabled computation of eddy covariance-based direct sensible heat flux estimates solely from FODS, which are reported here as a methodological and computational novelty. Comparing them against those from eddy covariance using sonic anemometry yielded an encouraging agreement in both magnitude and temporal variability for selected periods.

&lt;p&gt;This study employed fiber-optic distributed temperature and wind speed data in the forest environment, together with a scalar gas sampling network and eddy covariance measurements, to address questions concerning scalar gas mixing and transport under weak-wind conditions. We also investigate advection term variability using distributed wind speed to determine how it influences the CO2 budget in the subcanopy. Preliminary results demonstrate that employing friction velocity and bulk shear filtering to define weak and strong wind conditions separates the vertical temperature and scalar gas profiles but not the horizontal. Furthermore, the biggest scalar gas spatial variations occur at night, when wind direction and TKE magnitudes are most influential. The inlets positioned in a route related to a clearing cut show the most variability among the scalar gas inlets, illustrating the influence of subcanopy architecture on mixing and transport along the subcanopy.&lt;/p&gt;

Observations from Raman backscatter-based Fiber-Optic Distributed Sensing (FODS) require reference sections of the fiber-optic cable sensor of known temperature to translate the primary measured intensities of Stokes and anti-Stokes photons to the secondary desired temperature signal, which also commonly forms the basis for other derived quantities. Here, we present the design and the results from laboratory and field evaluations of a novel Solid-Phase Bath (SoPhaB) using ultrafine copper instead of the traditional mechanically stirred liquid-phase water bath. This novel type is suitable for all FODS applications in geosciences and industry when high accuracy and precision are needed. The SoPhaB fully encloses the fiber-optic cable which is coiled around the inner core and surrounded by tightly interlocking parts with a total weight of 22 kg. The SoPhaB is thermoelectrically heated and/or cooled using Peltier elements to control the copper body temperature within ±0.04 K using commercially available electronic components. It features two built-in reference platinum wire thermometers which can be connected to the distributed temperature sensing instrument and/or external measurement and logging devices. The SoPhaB is enclosed in an insulated carrying case, which limits the heat loss to or gains from the outside environment and allows for mobile applications. For thermally stationary outside conditions the measured spatial temperature differences across SoPhaB parts touching the fiber-optic cable are <0.05 K even for stark contrasting temperatures of ΔT> 40 K between the SoPhaB’s setpoint and outside conditions. The uniform, stationary known temperature of the SoPhaB allows for substantially shorter sections of the fiber-optic cable sensors of less than <5 bins at spatial measurement resolution to achieve an even much reduced calibration bias and spatiotemporal uncertainty compared to traditional water baths. Field evaluations include deployments in contrasting environments including the Arctic polar night as well as peak summertime conditions to showcase the wide range of the SoPhaB’s applicability.

&lt;p&gt;This paper presents the findings of a series of experimental studies to investigate the variation of vertical flow characteristics after filtering horizontal flow using porous cylindrical shrouds. Exploring this research question implies improving the existing method of observing horizontal wind speed and direction using Distributed Temperature Sensing (DTS) to develop it for the vertical direction to capture continuous and distributed turbulence. The experiments were performed using two sonic anemometers and two pressure ports in the open experimental area; one of each sensor is located inside the cylindrical shroud. The flow statistics were compared between different shroud configurations with different shapes, colors, rigidity, and porosity. Based on the coefficient of determination and mean error between shrouded and unshrouded data, the white insect screen shroud with a rigid structure and 60 cm diameter and 145 cm height is determined as the most conducive setup. The optimum shroud setup reduces the horizontal wind standard deviation by 35 percent, having a coefficient of determination of 0.972 between vertical wind standard deviations and RMSE less than 0.018 m/s between shrouded and unshrouded set up. However, the comparisons confirm that the vertical flow remains unaltered while reducing the horizontal flow, but the spectral energy ratio between the shrouded and unshrouded setup shows different responses. This ratio decreases exponentially in the high frequencies, which means the shroud damps the high-frequency eddies with a temporal scale of fewer than 6 seconds. Despite high frequencies, the ratio remains constant in the low frequencies for all energy spectrums, including temperature, wind components, momentum, and sensible heat flux.&lt;/p&gt;