Revealing The Morning Transition In The Mountain Boundary Layer Using Fiber-Optic Distributed Temperature Sensing

In the morning, the nocturnal stable boundary layer, SBL, transitions into its daytime convective counterpart substantially impacting the distribution of temperature, humidity, and pollutants. Applying distributed temperature sensing (DTS) below a tethered balloon (2-200 m) and along a tower (0-11 m), for the first time we observed three morning transitions (MTs) in a mountain boundary layer with high temporal (<10 s) and spatial (<0.25 m) resolutions. We show that MTs are best derived from a change in static stability from synchronous DTS observations. Our findings confirm that the MT occurs at the SBL top and bottom simultaneously, and identify horizontal heat advection as a main driver aiding solar surface heating in this midrange mountain valley. We conclude that heterogenous land use and mountainous topography cause complex interactions between valley-scale and local airflows leading to thermal signatures characterized by strong, small-scale variability. Our study highlights DTS as a crucial tool for investigating complex thermodynamic processes.Plain Language Summary On calm nights, a nocturnal stable boundary layer forms near the Earth's surface characterized by increasing temperatures with height and very little mixing. Its counterpart is the daytime convective boundary layer, CBL, which has the opposite characteristics. Both the stable and convective boundary layers have received much attention; however, the in-between morning transition (MT) has received less attention, which can degrade weather and air quality forecasts. To better understand the morning transition, we measured air temperature below a tethered balloon and on a tower during the MT with a technique called distributed temperature sensing (DTS) using thin fiber-optic cables. Our unique design enabled an unprecedented view of the MT revealing its detailed and fast-changing structure. The morning transition is a highly variable process whose theoretical description and experimental investigation had previously been simplified due to observational limitations. We found that it starts simultaneously at the top and bottom of the stable layer, and can take an hour to complete. It is strongly influenced by the surrounding land. While more work using DTS is necessary to understand why the air takes so long to respond to the rising sun, we demonstrate how critical this technique is for future research.Key PointsA novel technique, distributed temperature sensing (DTS) below a tethered balloon, reveals unknown details of the morning transitionThe morning transition, despite its strong spatiotemporal variability, is robustly determined by changes in static stabilityDTS provides first evidence that the transition occurs simultaneously at the stable boundary bottom and top, and is subject to advection and counter-gradient flux