The Large eddy Observatory, Voitsumra Experiment 2019 (LOVE19) with high-resolution, spatially distributed observations of air temperature, wind speed, and wind direction from fiber-optic distributed sensing, towers, and ground-based remote sensing

Lapo, Karl; Freundorfer, Anita; Fritz, Antonia; Schneider, Johann; Olesch, Johannes; Babel, Wolfgang; Thomas, Christoph K.

doi:10.5194/essd-14-885-2022

Cited by 9 publications

(9 citation statements)

References 57 publications

(96 reference statements)

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“…7) hint at similar behaviour. However, new measurement strategies must be developed, including denser station spacing for different sites and use of integrated distributed temperature sensing (Pfister et al 2017;Lapo et al 2022). Multiple stations on the plateau would provide needed information for upwind conditions that feeds the downslope flow.…”

Section: Discussionmentioning

confidence: 99%

Horizontal Variations of Nocturnal Temperature and Turbulence Over Microtopography

Mahrt

2022

Boundary-Layer Meteorol

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Nocturnal spatial variation of temperature, wind, and turbulence over microtopography is generally poorly understood. Low amplitude microtopography covers much of the Earth’s surface and, with very stable conditions, can produce significant spatial variations of temperature and turbulence. We examine such variations over gentle terrain that include two shallow gullies that feed into a small valley. The gullies are covered by a sub-network of seven flux stations that is embedded within a larger network that covers the valley. The measurements indicate that gullies of only 2–5-m depth and 100-m width can often lead to spatial variations of temperature of several kelvin or more. Such variations depend on ambient wind speed and direction and the near-surface stratification. We investigate the surprising importance of microscale lee turbulence occurring over the gentle microtopography with slopes of only 5%. Near-surface stratification unexpectedly tends to increase with surface elevation on the slopes. We examine the potential causes of this puzzling behaviour of the near-surface stratification.

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Section: Discussionmentioning

confidence: 99%

Horizontal Variations of Nocturnal Temperature and Turbulence Over Microtopography

Mahrt

2022

Boundary-Layer Meteorol

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“…The whole eddy needs to be captured by the profile to derive meaningful estimates of the length scales with the method. For example, DTS cables affixed to a tethered balloon (Lapo et al, 2022) might provide extensive enough elevation for resolving daytime eddies with the method and/or capturing the whole surface layer in near-neutral conditions. The results here have implications for practical evaluation of the coupling between the forest-floor and above-canopy air layers.…”

Section: Discussionmentioning

confidence: 99%

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

Peltola

Aurela

Launiainen

et al. 2022

Quart J Royal Meteoro Soc

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Stably stratified roughness sublayer flows are ubiquitous yet remain difficult to represent in models and to interpret using field experiments. Here, continuous high‐frequency potential temperature profiles from the forest floor up to 6.5 times the canopy height observed with distributed temperature sensing (DTS) are used to link eddy topology to roughness sublayer stability correction functions and coupling between air layers within and above the canopy. The experiments are conducted at two forest stands classified as hydrodynamically sparse and dense. Near‐continuous profiles of eddy sizes (length scales) and effective mixing lengths for heat are derived from the observed profiles using a novel conditional sampling approach. The approach utilizes potential temperature isoline fluctuations from a statically stable background state. The transport of potential temperature by an observed eddy is assumed to be conserved (adiabatic movement) and we assume that irreversible heat exchange between the eddy and the surrounding background occurs along the (vertical) periphery of the eddy. This assumption is analogous to Prandtl's mixing‐length concept, where momentum is transported rapidly vertically and then equilibrated with the local mean velocity gradient. A distinct dependence of the derived length scales on background stratification, height above ground, and canopy characteristics emerges from the observed profiles. Implications of these findings for (1) the failure of Monin–Obukhov similarity in the roughness sublayer and (2) above‐canopy flow coupling to the forest floor are examined. The findings have practical applications in terms of analysing similar DTS data sets with the proposed approach, modelling roughness sublayer flows, and interpreting nocturnal eddy covariance measurements above tall forested canopies.

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“…The cones are made from polyethylene with a diameter and height of 12 mm, respectively, and 2 cm of spacing optimized by (Lapo et al, 2020b). Two warm and cold solid-phase calibration baths with a constant temperature developed by Thomas et al (2022) were deployed at the beginning and end of FO arrays to be used as a calibration reference (Fig. 2).…”

Section: Shroud Experiments At Waldstein Forestmentioning

confidence: 99%

“…Finally, the artifact-free calibrated FODS temperature was used to further analysis in this study. FODS data usually contains artifacts related to the heat exchange between the FO cable and heat sinks or sources other than air, such as precipitation, solar radiation, holders, and support structures (Lapo et al, 2022).…”

Section: Fods Calibrationmentioning

confidence: 99%

Toward quantifying turbulent vertical airflow and sensible heat flux in tall forest canopies using fiber-optic distributed temperature sensing

Abdoli

Lapo

Schneider

et al. 2022

Preprint

Self Cite

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Abstract. The paper presents a set of Fiber Optic Distributed Temperature Sensing (FODS) experiments to expand the existing microstructure approach for horizontal turbulent wind direction by adding measurements of turbulent vertical wind speed and direction, as well as turbulent sensible heat flux.We address the observational challenge to isolate and quantify the weaker vertical turbulent motions from the much stronger mean advective flow signals. In the first part of this study, we test the ability of a cylindrical shroud to reduce the horizontal wind speed while keeping the vertical wind speed unaltered. Shroud experiments were performed using two sonic anemometers and two pressure ports in an open experimental area over short grass, with one observing system located inside the cylindrical shroud, but without any fiber-optic (FO) cables. The flow statistics were compared across shroud configurations of different shapes, colors, rigidity, and porosity. The white insect screen shroud with the rigid structure and 0.6 m diameter was identified as the most promising setup in which the correlation of flow properties between shrouded and unshrouded systems is maximized, and the RMSE was significantly lower. The optimum shroud setup reduces the horizontal wind standard deviation by 35 %, has a coefficient of determination of 0.972 for vertical wind standard deviations, and a RMSE less than 0.018 ms-1 when comparing the shrouded to the unshrouded setup. Spectral analysis showed a fixed ratio of spectral energy reduction in the low frequencies, e.g., > 2 s, for temperature and wind components, momentum, and sensible heat flux. Unlike low frequencies, the ratios decrease exponentially in the high frequencies, which means the shroud dampens the high-frequency eddies with a time scale < 6 s, considering both spectra and cospectra together. In the second part, the optimum shroud configuration was installed around a heated fiber-optic cable with attached microstructures in a forest to validate our findings, but the analysis revealed a failure to isolate the magnitude and sign of the vertical wind perturbations from FODS. However, concurrent observations from an unshrouded part of the FODS sensor in the weak-wind subcanopy of the forest (12–17 m above ground level) yielded meaningful measurements of the vertical motions from coherent structures with distinct sweep and ejection phases. These signals allow for detecting the turbulent vertical airflow at least 60 % of the time, and 71 % when conditional sampling was applied. Comparison with vertical wind perturbations from sonic anemometry resulted in correlation coefficients of 0.35 and 0.36, which increased to 0.53 and 0.62 for conditional sampling. Evaluating the first direct sensible heat fluxes from FODS against those from the classic eddy covariance using sonic anemometry yielded an encouraging agreement in both magnitude and temporal variability for selected periods. This observational achievement is an important step toward developing a FODS-based flux sensor capable of resolving heat flux continuously across spatial and temporal scales.

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The Large eddy Observatory, Voitsumra Experiment 2019 (LOVE19) with high-resolution, spatially distributed observations of air temperature, wind speed, and wind direction from fiber-optic distributed sensing, towers, and ground-based remote sensing

Cited by 9 publications

References 57 publications

Horizontal Variations of Nocturnal Temperature and Turbulence Over Microtopography

Horizontal Variations of Nocturnal Temperature and Turbulence Over Microtopography

Probing eddy size and its effective mixing length in stably stratified roughness sublayer flows

Toward quantifying turbulent vertical airflow and sensible heat flux in tall forest canopies using fiber-optic distributed temperature sensing

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