Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

Abstract

In the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

The NY-Ålesund TurbulencE Fiber Optic eXperiment (NYTEFOX): investigating the Arctic boundary layer, Svalbard

“…One goal of the NYTEFOX campaign was to provide a proof of concept for future applications of the horizontal fiber-optic distributed sensing technique in similarly chal-M.-L. Zeller et al: NYTEFOX -Ny-Ålesund Turbulence Fiber Optic Experiment lenging environments. While the feasibility of FODS has been proven for midlatitude boundary layers (Thomas et al, 2012;Sayde et al, 2015;Peltola et al, 2020;Schilperoort et al, 2020;Pfister et al, 2021a), the high-quality FODS data and their physical consistency with other more traditional near-surface meteorological observations underline the technical feasibility and the functionality of FODS deployments in extreme temperature and wind conditions of the polar regions. Note that temperatures during the measuring period dropped to −30 • C with an average of −17 • C, which is extraordinarily cold for NY-Ålesund and more representative of the higher Arctic.…”

“…2) fiber-optical cable to obtain wind speed measurements in addition to those of air temperature. The underlying principle of wind speed measurements is the changing temperature difference between both fibers due to convective cooling of the heated cable (see Sayde et al, 2015, andvan Ramshorst et al, 2020, for details). As this cooling is sensitive to the angle of attack, only winds orthogonal to the fiber are represented correctly.…”

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

“…This technique measures the temperature along a fiber-optic cable at a time resolution of a few seconds and spatial resolution of tens of centimeters up to 20 km in total length (Thomas and Selker, 2021). It is capable of observing atmospheric flows under physically poorly understood sub-meso motions Pfister et al, 2021). The distributed temperature sensing (DTS) technique's utility in atmospheric measurement is not limited to measuring the distributed temperature of the air, water, ice, snow, soil, and plant, but various applications such as measurement of evapotranspiration, soil moisture, humidity, shortwave radiation, and wind speed have been tested successfully (Predosa, 2016;Schilperoort et al, 2020;Thomas and Selker, 2021).…”