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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. Thermal Submesoscale Motions in the Nocturnal Stable Boundary Layer. Part 1: Detection and Mean Statistics

   https://doi.org/10.1007/s10546-021-00618-0  

   Pfister, Lena; Lapo, Karl; Mahrt, Larry; Thomas, Christoph K. (August 2021, Boundary-Layer Meteorology)

   null (Ed.)

   Abstract Submesoscale motions within the stable boundary layer were detected during the Shallow Cold Pool Experiment conducted in the Colorado plains, Colorado, U.S.A. in 2012. The submesoscale motion consisted of two air layers creating a well-defined front with a sharp temperature gradient, and further-on referred to as a thermal submesofront (TSF). The semi-stationary TSFs and their advective velocities are detected and determined by the fibre-optic distributed-sensing (FODS) technique. An objective detection algorithm utilizing FODS measurements is able to detect the TSF boundary, which enables a detailed investigation of its spatio–temporal statistics. The novel approach in data processing is to conditionally average any parameter depending on the distance between a TSF boundary and the measurement location. By doing this, a spatially-distributed feature like TSFs can be characterized by point observations and processes at the TSF boundary can be investigated. At the TSF boundary, the air layers converge, creating an updraft, strong static stability, and vigorous mixing. Further, the TSF advective velocity of TSFs is an order of magnitude lower than the mean wind speed. Despite being gentle, the topography plays an important role in TSF formation. Details on generating mechanisms and implications of TSFs on the stable boundary layer are discussed in Part 2. 

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3. Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

   https://doi.org/10.1007/s10546-021-00619-z  

   Pfister, Lena; Lapo, Karl; Mahrt, Larry; Thomas, Christoph K. (August 2021, Boundary-Layer Meteorology)

   null (Ed.)

   Abstract In the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena. 

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4. 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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5. A fiber-optic distributed temperature sensor for continuous in situ profiling up to 2 km beneath constant-altitude scientific balloons

   https://doi.org/10.5194/amt-16-791-2023  

   Goetz, J. Douglas; Kalnajs, Lars E.; Deshler, Terry; Davis, Sean M.; Bramberger, Martina; Alexander, M. Joan (January 2023, Atmospheric Measurement Techniques)

   Abstract. A novel fiber-optic distributed temperature sensing instrument, the Fiber-optic Laser Operated Atmospheric Temperature Sensor (FLOATS), was developed for continuous in situ profiling of the atmosphere up to 2 km below constant-altitude scientific balloons. The temperature-sensingsystem uses a suspended fiber-optic cable and temperature-dependent scattering of pulsed laser light in the Raman regime to retrieve continuous3 m vertical-resolution profiles at a minimum sampling period of 20 s.FLOATS was designed for operation aboard drifting super-pressure balloons inthe tropical tropopause layer at altitudes around 18 km as part of theStratéole 2 campaign. A short test flight of the system was conductedfrom Laramie, Wyoming, in January 2021 to check the optical, electrical, andmechanical systems at altitude and to validate a four-reference temperaturecalibration procedure with a fiber-optic deployment length of 1170 m. During the 4 h flight aboard a vented balloon, FLOATS retrieved temperatureprofiles during ascent and while at a float altitude of about 19 km. TheFLOATS retrievals provided differences of less than 1.0 ∘Ccompared to a commercial radiosonde aboard the flight payload during ascent.At float altitude, a comparison of optical length and GPS position at thebottom of the fiber-optic revealed little to no curvature in the fiber-opticcable, suggesting that the position of any distributed temperaturemeasurement can be effectively modeled. Comparisons of the distributed temperature retrievals to the reference temperature sensors show strongagreement with root-mean-square-error values less than 0.4 ∘C. Theinstrument also demonstrated good agreement with nearby meteorologicalobservations and COSMIC-2 satellite profiles. Observations of temperatureand wind perturbations compared to the nearby radiosounding profiles provide evidence of inertial gravity wave activity during the test flight. Spectral analysis of the observed temperature perturbations shows that FLOATS is an effective and pioneering tool for the investigation of small-scale gravity waves in the upper troposphere and lower stratosphere. 

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