Recently, a number of authors have used global particle tracking simulations to identify the effect that different surface currents have on marine litter accumulation, including the role of surface waves through the Stokes drift. However, in the upper-ocean boundary layer and in the presence of the Coriolis force, a wave-driven Eulerian flow forms that must be superimposed onto the Stokes drift in order to obtain the correct Lagrangian velocity. Taking into account both the Coriolis–Stokes force and the surface wave stress, Higgins et al. (2020), https://doi.org/10.1029/2020GL089189 derived an expression for this unsteady wave-driven Eulerian-mean flow in the form of a convolution between the Stokes drift and the so-called Ekman–Stokes kernel. In this paper, we apply this Ekman–Stokes kernel to generate a 12-year global hindcast of the wave-driven Eulerian current and show that its inclusion in particle tracking simulations has a significant effect on the distribution of small floating marine litter, such as microplastics. Using Lagrangian simulations, we find that the wave-driven Eulerian current is sensitive to the value of viscosity but generally opposes the dispersive behavior of the Stokes drift, reducing the amount of cross-Equator particle transport and transport to the polar regions, resulting in closer agreement between modeled and observed microplastic distributions.

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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.

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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. @en

Over the past ten years, Distributed Temperature Sensing (DTS) has been applied for monitoring many different environmental processes, from groundwater movement, to seepage into streams and canals, to soil moisture, and internal waves in lakes. DTS uses optical fibres, along which temperatures are determined by measuring Raman shifts in light that scatters back after a laser pulse has been sent into the fiber. Over the past decade, performance of DTS equipment has dramatically improved. It is now possible to determine fiber temperatures with 0.05 K accuracy, for each 25 cm along a fiber optic cable. With typical spatial resolutions of 1 m, cable lengths can run up to 5 km. Accuracy improves with integration over longer sampling intervals, but measurements over 60 s can give 0.1 K accuracy with proper in-field calibration. DTS can also be used for atmospheric properties such as air temperature, vapor pressure, and wind speed. This presentation provides a complete overview of recent advances in atmospheric DTS observations. Air temperature is the simplest, as one simply has to suspend a fiber optic cable along the profile of interest. This can be from a balloon or along poles. Care has to be taken to correct for radiative heating of the cable. Using a thin white cable minimalizes radiative effects and normally brings the measured temperature to within 1 K of actual air temperature, sufficient for studies on effects of shading in natural and urban landscapes. It is also possible to correct for radiative heating by modeling in some detail the cable’s thermal behavior or by using two cables of different diameters. Supporting structures may also have an effect on cable temperatures, which should be minimized or corrected for. Water vapor can be measured by comparing the temperatures of wet and dry cables. These wet and dry bulb temperatures allow derivation of humidity profiles, which, in turn, allows for Bowen-ratio type of calculations of latent and sensible heat fluxes. This has proven especially useful in otherwise difficult to measure profiles such as through forest canopies. Wind speed can be measured by including a conductive element in the fiber optic cable and heating the cable actively by sending a current through that element. In effect, the cable then acts as a hot wire anemometer but then over long lengths of cable and with high spatial resolutions. When carefully executed, experiments with heated cables give very detailed insight into turbulent processes in the lower boundary. It is even possible to resolve bigger individual turbulent and sub-meso-scale eddies for studying fast evolving fluid flows (orders of seconds). A comprehensive overview of atmospheric applications will be presented, together with pitfalls, common errors, and practical tips to avoid those in the field. @en