Abstract
Abstract. The weak-wind boundary layer is characterized by turbulent and submesoscale 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 distributed basis and to date has been used to resolve the turbulent fields of air temperature and wind speed on scales of seconds 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 a range of microstructure parameters including aspect ratio, 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.
Highlights
Laser pulses sent along a fiber-optic cable scatter back along the path of the fiber with a temperature-dependent shift in frequency, providing a powerful geophysical sensing technique called distributed temperature sensing (DTS) (Selker et al, 2006; Tyler et al, 2009)
Weak-wind boundary layers break many of the assumptions that underlie eddy covariance techniques (Thomas, 2011; Cheng et al, 2017; Pfister et al, 2019), which forms an obstacle for understanding the dynamics of turbulence during these conditions
We propose a similar approach combining the active heating with microstructures printed directly on the fiber-optic cable (FOC)
Summary
Laser pulses sent along a fiber-optic cable scatter back along the path of the fiber with a temperature-dependent shift in frequency, providing a powerful geophysical sensing technique called distributed temperature sensing (DTS) (Selker et al, 2006; Tyler et al, 2009). Previous work with atmospheric DTS has demonstrated the ability to observe atmospheric temperatures (Thomas et al, 2012), wet bulb temperature (Euser et al, 2014; Schilperoort et al, 2018), solar radiation (Sigmund et al, 2017; Petrides et al, 2011), and wind speed (Sayde et al, 2015) at a fine spatial and temporal resolution. We refer to this broader application of DTS technology as fiber-optic distributed sensing (FODS).
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