Army Research Office, contracts W911NF-10-1-0361, W911NF-09-1-0271
Data from the sensor network will lead to a direct evaluation of Taylor’s hypothesis and an improved understanding of all motions contributing to the dispersion and diffusion in weak-wind stable conditions. The sensor network is a unique combination of observational techniques including optical fiber distributed temperature sensing (DTS), acoustic remote sensing (SODAR), networks of sonic anemometers, and laser-illuminated flow visualization from fog releases.
Weak-wind atmospheric transport near the surface remains one of the most poorly understood processes in the stable boundary layer. Cases of weak flows in combination with moderate surface heterogeneity represent a substantial part of our environment, yet these conditions render conceptual frameworks, such as commonly applied similarity theories for turbulent fluxes, inadequate. Under these conditions, ground concentrations of contaminants can remain extremely high and the spatio-temporal dynamics of the plume difficult to predict.
This is the selection of images taken during the preparation and installation of the field experiments, as well as during the actual measurement period. Enjoy!
Research: today was ground-breaking (ceremony) for this year's summer experiment DONUTSS-2011 at the Botany & Plant Pathology Lab when Matthias Zeeman dug the first trenches that will hold the wooden framework to suspend the optical fibers used in the heart of the experiment: a large quasi three-dimensional array of optical fibers sampled by the laser-based distributed temperature sensing (DTS) technique will allow sampling of the thermal structure of the near-surface flow in unprecedented spatial and temporal resolution. Temperature measurements from DTS will be complemented by wind, humidity, and turbulence observations from a network of sonic anemometers, paired ground-based acoustic remote sounders, and fog releases to study the weak wind, stable boundary layers. The acronym 'DONUTSS' stands for Direct Observation of Near-sUrface Turbulent and Submeso Structures. The experiment is funded by the Army Research office and the National Science Foundation.
Publication: 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). Observations were obtained from a vertically oriented fibre optics array during the DONUTSS campaign. 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 further 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 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. The full citation is:
Thomas, C.K., Kennedy, A.M., Selker, J.S., Moretti, A., Schroth, M.H., Smoot, A.R., Tufillaro, N.B. and Zeeman, M.J., 2012. High-resolution ﬁbre-optic temperature sensing: A new tool to study the two-dimensional structure of atmospheric surface layer ﬂow. Boundary-Layer Meteorol., 142: 177-192. DOI: 10.1007/s10546-011-9672-7.
October 19, 2010: We conducted a release of artificially created fog using two fog machines at different locations in close proximity to the fiber array. Individual fog pulses were 20 sec long and spaced 3 min apart throughout the experiment. The video information was recorded with two cameras: one located South-East of the array in about 50 m distance on the ground, and one attached to a pulley system on the tall tower located at about 20 m above ground.
The sped-up video clearly indicate the presence of meandering motions while the general flow is Northerly. A cold air 'lens' builds up in the shallow gulley, in which the base of the fiber array is located, and sloshes back and forth most likely driven by larger-scale pressure perturbations of unknown origin. The air is strongly stably stratified with a vertical gradient of about +1 K/m. Toward the end of the movie, the surface heating breaks up the stable stratification leading to enhanced turbulent mixing as indicated by the shear driven overturning of larger eddies.
September 22, 2010: After addressing some issues with keeping the top supporting boom for the pulleys from tilting, the first full-sized optical fiber array was deployed (see movie, sped up 8x). Its size is 8 x 8 m with 35 vertical runs of fiber spaced at a distance of 0.25m. Two separate fibers, one black and one white each with an outer diameter of 0.9 mm, are laced through the 35 top and bottom pulleys. Given a spatial resolution of 0.25 m of the high-resoution DTS laser system, a total of approximately 1120 independent temperature measurements are obtained once per second (1 Hz). The next step will be the installation of sonic anemometers and thermo-hygrometers at the base of the array and next to the fibers at various heights to evaluate sensor performance and uncertainty.
The full-size array featuring a 8 x 8 m domain at 0.25m resolution.
John and Christoph lace the optical fibers through the pulleys.
The field crew (from left to right):
John Selker, Evan Deblander, Matthias Zeeman, Alex Smoot, Adam Kennedy, Helen Kennedy, Mike Unsworth, Christoph Thomas
September 16, 2010: After months of designing, constructing, and installing the towers and pulley systems to support the optical fiber of the Distributed Temperature Sensing (DTS) array, two optical fibers were laced through the 36 top and bottom pulleys evenly spaced at 25 cm. The challenge is to avoid kinking the fiber which would lead to breaking the inner optical quartz fiber carrying the laser signal from the DTS system unit.
Outcome: The first deployment of the optical fiber in the pulley array was a success as the fiber did not break, kink, or jump out of the pulleys' grooves when the top boom was raised. Subsequent measurements of the DTS temperature indicated a good data quality. However, the alignment of the top boom to which the pulley assembly is mounted needs to be improved before the fiber can be deployed over the entire size of the array in this stage of the experiment (10 x 10 m).
A closeup image of the custommade pulleys.
The deployed 0.9 mm thick (OD) pair of fibers consisting of black and white strands.
Adam and Alex carefully check the fiber for integrity and alignment.
Research award: A new research grant has been received for the proposal entitled "Observing Spatial Structure of Near Surface Atmospheric Motions Using a Combination of Optical and Acoustic in-situ and Remote Sensing techniques". This project is funded by the Army Research Office, Environmental Sciences Division, Atmospheric Sciences for the period July 01, 2010 - June 30, 2013. Its a collaborative effort between the College of Oceanic and Atmospheric Sciences (COAS): Christoph Thomas (PI), Larry Mahrt (Co-PI), and the Department of Biological and Ecological Engineering (BEE): John Selker (Co-PI).
The primary goal is to deploy a novel optical fiber Distributed Temperature Sensing (DTS) system capable of very high spatial (0.25m) and temporal (1Hz) resolution in a vertical array (20 x 20m) over a real, moderately complex and structured surfaces with the objective to to directly measure the spatial structure of the near-surface flow under weak-wind conditions. Measurements by the DTS will be augmented with observations from sonic anemometers, acoustic remote sensing (SODAR), and laser-illuminated artificial fog releases. The DTS instrument will be provided by the Center for Transformative Environmental Monitoring Programs.