This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://link.springer.com/journal/10546.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 x 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 analyse the space-time dynamics of transient cold-air pools in the calm boundary layer. The time response and precision of the fibre 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 with a broad potential in environmental sciences including meteorology, hydrology, oceanography, and ecology
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