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1. Distributed observations of wind direction using microstructures attached to actively heated fiber-optic cables

   Lapo, K. ; Freundorfer, A. ; Pfister, L. ; Schneider, J. ; Selker, J. ; Thomas, C. ( November 2019 , AMT)

   The weak-wind boundary layer is characterized by turbulent and submeso-scale motions that break the assumptions necessary for using traditional eddy covariance observations such as horizontal homogeneity and stationarity, motivating the need for an observational system that allows spatially resolving measurements of atmospheric flows near the surface. Fiber­ Optic Distributed Sensing (FODS) potentially opens the door to observing a wide-range of atmospheric processes on a spatially 5 distributed basis and to date has been used to resolve the turbulent fields of air temperature and wind speed on scales of second and decimeters. Here we report on progress developing a FODS technique for observing spatially distributed wind direction. We affixed microstructures shaped as cones to actively-heated fiber-optic cables with opposing orientations to impose directionally-sensitive convective heat fluxes from the fiber-optic cable to the air, leading to a difference in sensed temperature that depends on the wind direction. We demonstrate the behavior of arange of microstructure parameters including aspect ratio, 10 spacing, and size and develop a simple deterministic model to explain the temperature differences as a function of wind speed. The mechanism behind the directionally-sensitive heat loss is explored using Computational Fluid Dynamics simulations and infrared images of the cone-fiber system. While the results presented here are only relevant for observing wind direction along one dimension it is an important step towards the ultimate goal of a full three-dimensional, distributed flow sensor. 

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2. Revisiting wind speed measurements using actively heated fiber optics: a wind tunnel study

   https://doi.org/10.5194/amt-13-5423-2020  

   van Ramshorst, Justus G. ; Coenders-Gerrits, Miriam ; Schilperoort, Bart ; van de Wiel, Bas J. ; Izett, Jonathan G. ; Selker, John S. ; Higgins, Chad W. ; Savenije, Hubert H. ; van de Giesen, Nick C. ( January 2020 , Atmospheric Measurement Techniques)

   null (Ed.)

   Abstract. Near-surface wind speed is typically only measured by point observations. The actively heated fiber-optic (AHFO) technique, however, has thepotential to provide high-resolution distributed observations of wind speeds, allowing for better spatial characterization of fine-scaleprocesses. Before AHFO can be widely used, its performance needs to be tested in a range of settings. In this work, experimental results on thisnovel observational wind-probing technique are presented. We utilized a controlled wind tunnel setup to assess both the accuracy and the precisionof AHFO under a range of operational conditions (wind speed, angles of attack and temperature difference). The technique allows for wind speedcharacterization with a spatial resolution of 0.3 m on a 1 s timescale. The flow in the wind tunnel was varied in a controlledmanner such that the mean wind ranged between 1 and 17 m s−1. The AHFO measurements are compared to sonic anemometer measurements andshow a high coefficient of determination (0.92–0.96) for all individual angles, after correcting the AHFO measurements for the angle ofattack. Both the precision and accuracy of the AHFO measurements were also greater than 95 % for all conditions. We conclude that AHFO has thepotential to measure wind speed, and we present a method to help choose the heating settings of AHFO. AHFO allows for the characterization ofspatially 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. 

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3. High‐Resolution Observations of Subsurface Fronts and Alongshore Bottom Temperature Variability Over the Inner Shelf

   https://doi.org/10.1029/2018JC014454  

   Connolly, Thomas P. ; Kirincich, Anthony R. ( January 2019 , Journal of Geophysical Research: Oceans)

   Circulation patterns over the inner continental shelf can be spatially complex and highly variable in time. However, few studies have examined alongshore variability over short scales of kilometers or less. To observe inner‐shelf bottom temperatures with high (5‐m) horizontal resolution, a fiber‐optic distributed temperature sensing system was deployed along a 5‐km‐long portion of the 15‐m isobath within a larger‐scale mooring array south of Martha's Vineyard, MA. Over the span of 4 months, variability at a range of scales was observed along the cable over time periods of less than a day. Notably, rapid cooling events propagated down the cable away from a tidal mixing front, showing that propagating fronts on the inner shelf can be generated locally near shallow bathymetric features in addition to remote offshore locations. Propagation velocities of observed fronts were influenced by background tidal currents in the alongshore component and show a weak correlation with theoretical gravity current speeds in the cross‐shore component. These events provide a source of cold, dense water into the inner shelf. However, differences in the magnitude and frequency of cooling events at sites separated by a few kilometers in the alongshore direction suggest that the characteristics of small‐scale variability can vary dramatically and can result in differential fluxes of water, heat, and other tracers. Thus, under stratified conditions, prolonged subsurface observations with high spatial and temporal resolution are needed to characterize the implications of three‐dimensional circulation patterns on exchange, especially in regions where the coastline and isobaths are not straight.

    

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4. Design and Evaluation of Sensor Housing for Boundary Layer Profiling Using Multirotors

   https://doi.org/10.3390/s19112481  

   Islam, Ashraful ; Houston, Adam L. ; Shankar, Ajay ; Detweiler, Carrick ( June 2019 , Sensors)

   null (Ed.)

   Traditional configurations for mounting Temperature–Humidity (TH) sensors on multirotor Unmanned Aerial Systems (UASs) often suffer from insufficient radiation shielding, exposure to mixed and turbulent air from propellers, and inconsistent aspiration while situated in the wake of the UAS. Descent profiles using traditional methods are unreliable (when compared to an ascent profile) due to the turbulent mixing of air by the UAS while descending into that flow field. Consequently, atmospheric boundary layer profiles that rely on such configurations are bias-prone and unreliable in certain flight patterns (such as descent). This article describes and evaluates a novel sensor housing designed to shield airborne sensors from artificial heat sources and artificial wet-bulbing while pulling air from outside the rotor wash influence. The housing is mounted above the propellers to exploit the rotor-induced pressure deficits that passively induce a high-speed laminar airflow to aspirate the sensor consistently. Our design is modular, accommodates a variety of other sensors, and would be compatible with a wide range of commercially available multirotors. Extensive flight tests conducted at altitudes up to 500 m Above Ground Level (AGL) show that the housing facilitates reliable measurements of the boundary layer phenomena and is invariant in orientation to the ambient wind, even at high vertical/horizontal speeds (up to 5 m/s) for the UAS. A low standard deviation of errors shows a good agreement between the ascent and descent profiles and proves our unique design is reliable for various UAS missions. 

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5. Atmospheric Turbulent Intermittency Over the Arctic Sea‐Ice Surface During the MOSAiC Expedition

   https://doi.org/10.1029/2023JD038639  

   Liu, Changwei ; Yang, Qinghua ; Shupe, Matthew D. ; Ren, Yan ; Peng, Shijie ; Han, Bo ; Chen, Dake ( July 2023 , Journal of Geophysical Research: Atmospheres)

   Turbulent motions in the Arctic stable boundary layer are characterized by intermittency, but they are rarely investigated due to limited observations, in particular over the sea‐ice surface. In the present study, we explore the characteristics of turbulent intermittency over the Arctic sea‐ice surface using data collected during the Multidisciplinary drifting Observation for the Study of Arctic Climate expedition from October 2019 to September 2020. We first develop a new algorithm, which performs well in identifying the spectral gap over the Arctic sea‐ice surface. Then the characteristics of intermittency are investigated. It is found that the strength of intermittency increases under the conditions of light surface wind speed, small surface wind speed gradient, and strong surface air temperature gradient. The momentum flux, sensible heat flux, and latent heat flux calculated by raw eddy‐covariance fluctuations are overestimated by 3%, 10%, and 24%, respectively, because submesoscale motions are included. Furthermore, the characteristics of the atmospheric boundary layer structure under various intermittency conditions reveal that strong low‐level jets are favorable to surface turbulent motions that result in weak intermittency, while strong temperature inversions above the surface layer suppress surface turbulent motions and lead to strong intermittency.

    

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