Analysis of the weak-wind stable boundary layer using spatially distributed wind and temperature observations from active fiber-optic distributed temperature sensing

We present a novel observational technique that was applied to study the dynamics of transient shallow cold-air drainages and pools in undulating terrain in weak-wind conditions. Wind speed and air temperature at thousands of locations were measured simultaneously at high spatial and temporal resolution using optical paired passive and actively heated fibers with a distributed temperature sensing system (DTS). The fibers were deployed in a transect across a shallow gully with a total length of 230 m at three levels (0.5, 1, and 2m above ground level) over a period of two months during the Shallow Cold Pool (SCP) Experiment in Northern Colorado in October through November 2012.

While we previously demonstrated that air temperature and the thermal structure of the near-surface turbulent flow can be measured using the DTS technique (Thomas et al., 2012), the novelty of the approach presented here consists of additionally mapping wind speed on scales of several hundred meters at a fine spatial (0.25m) and temporal (5s) resolution. Analogous to a hot-wire anemometer, our approach is based on the principal of velocity-dependent heat transfer from a heated surface: The temperature difference between the heated fiber and an unheated reference is proportional to the convective cooling of the air flowing past the heated surface when the fiber's radiation balance is known.

We develop and present the theoretical basis for the DTS wind and temperature measurements and validate it against point observations from commonly used instrumentation including sonic anemometers and thermo-hygrometers. We present a space-time analysis of the near-surface gully flow and temperature field using the orthogonal multi-resolution decomposition for selected cases. The novel approach, which allows studying the spatial structure of stable boundary layers on scales spanning four orders of magnitude (0.1 – 1,000m), opens up many important opportunities for testing fundamental assumptions and concepts in micrometeorology including, but not limited to turbulent length scales, the validity of Taylors hypothesis, terrain effects and slope flows, and internal boundary layers.

References: Thomas, C.K., Kennedy, A.M., Selker, J.S., Moretti, A., Schroth, M.H., Smoot, A.R., Tufillaro, N.B., 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