High-Resolution Fibre-Optic Temperature Sensing: A New Tool to Study the Two-Dimensional Structure of Atmospheric Surface-Layer Flow |
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Authors: | Christoph K Thomas Adam M Kennedy John S Selker Ayla Moretti Martin H Schroth Alexander R Smoot Nicholas B Tufillaro Matthias J Zeeman |
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Institution: | (1) Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA;(2) NOAA Earth System Research Laboratory, Boulder, CO, USA;(3) U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH, USA;(4) Present address: NorthWest Research Associates, Inc. (Bellevue Division), 25 Eagle Ridge, Lebanon, NH 03766-1900, USA;(5) Naval Postgraduate School, Monterey, CA, USA |
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Abstract: | We present a novel approach based on fibre-optic distributed temperature sensing (DTS) to measure the two-dimensional thermal
structure of the surface layer at high resolution (0.25 m, ≈0.5 Hz). Air temperature observations obtained from a vertically-oriented
fibre-optics array of approximate dimensions 8 m × 8 m and sonic anemometer data from two levels were collected over a short
grass field located in the flat bottom of a wide valley with moderate surface heterogeneity. The objectives of the study were
to evaluate the potential of the DTS technique to study small-scale processes in the surface layer over a wide range of atmospheric
stability, and to analyze the space–time dynamics of transient cold-air pools in the calm boundary layer. The time response
and precision of the fibre-based temperatures were adequate to resolve individual sub-metre sized turbulent and non-turbulent
structures, of time scales of seconds, in the convective, neutral, and stable surface layer. Meaningful sensible heat fluxes
were computed using the eddy-covariance technique when combined with vertical wind observations. We present a framework that
determines the optimal environmental conditions for applying the fibre-optics technique in the surface layer and identifies
areas for potentially significant improvements of the DTS performance. The top of the transient cold-air pool was highly non-stationary
indicating a superposition of perturbations of different time and length scales. Vertical eddy scales in the strongly stratified
transient cold-air pool derived from the DTS data agreed well with the buoyancy length scale computed using the vertical velocity
variance and the Brunt–Vaisala frequency, while scales for weak stratification disagreed. The high-resolution DTS technique
opens a new window into spatially sampling geophysical fluid flows including turbulent energy exchange. |
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