Abstract
Abstract. The suitability of a fibre-optic distributed temperature sensing (DTS) technique for observing atmospheric mixing profiles within and above a forest was quantified, and these profiles were analysed. The spatially continuous observations were made at a 125 m tall mast in a boreal pine forest. Airflows near forest canopies diverge from typical boundary layer flows due to the influence of roughness elements (i.e. trees) on the flow. Ideally, these complex flows should be studied with spatially continuous measurements, yet such measurements are not feasible with conventional micrometeorological measurements with, for example, sonic anemometers. Hence, the suitability of DTS measurements for studying canopy flows was assessed. The DTS measurements were able to discern continuous profiles of turbulent fluctuations and mean values of air temperature along the mast, providing information about mixing processes (e.g. canopy eddies and evolution of inversion layers at night) and up to third-order turbulence statistics across the forest–atmosphere interface. Turbulence measurements with 3D sonic anemometers and Doppler lidar at the site were also utilised in this analysis. The continuous profiles for turbulence statistics were in line with prior studies made at wind tunnels and large eddy simulations for canopy flows. The DTS measurements contained a significant noise component which was, however, quantified, and its effect on turbulence statistics was accounted for. Underestimation of air temperature fluctuations at high frequencies caused 20 %–30 % underestimation of temperature variance at typical flow conditions. Despite these limitations, the DTS measurements should prove useful also in other studies concentrating on flows near roughness elements and/or non-stationary periods, since the measurements revealed spatio-temporal patterns of the flow which were not possible to be discerned from single point measurements fixed in space.
Highlights
The majority of the interaction between the atmosphere and Earth’s surface takes place in a shallow air layer termed the atmospheric boundary layer (ABL)
We evaluate whether third-order statistics could be discerned from the distributed temperature sensing (DTS) data to reveal important transport process information of the sweep ejection cycles created by the coherent structures
The correlation between w and T is higher in the roughness sublayer than in the surface layer (Patton et al, 2010), and Monin– Obukhov (M–O) scaling is no longer valid. These results suggest that the height of the roughness sublayer at this site is about 50 m, which corresponds to approximately 3 times the height of the roughness elements
Summary
The majority of the interaction between the atmosphere and Earth’s surface takes place in a shallow air layer termed the atmospheric boundary layer (ABL). Insights on the atmospheric mixing processes in this layer are required in order to gain a better understanding on ecosystem–atmosphere feedbacks, air quality and weather-forecasting-related issues. Studies near the surface typically rely on time series analysis, since spatial details of the mixing close to the ground are difficult to measure with conventional instrumentation.
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