Abstract
AbstractIn 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.
Highlights
The midlatitude atmospheric boundary layer, ABL, undergoes a diurnal cycle between a daytime convective boundary layer, CBL, and a nocturnal stable boundary layer, SBL when strong synoptic forcing is absent (Stull, 1988)
With MT phase (MTP) onset, the submesoscale thermal structures disappeared, giving way to a larger-scale horizontal temperature gradient across the field, with colder air near the Flying Fiber-Optics Experiment (FlyFox) launch site and warmer air to the north, especially on sunnier days (MT2, MT3) likely caused by gentle, but persistent cold-air valley-slope flows acting in concert with the residual cold-air pool
When MTP ended, the larger-scale horizontal thermal structures were replaced by their smaller-scale CBL counterparts
Summary
The midlatitude atmospheric boundary layer, ABL, undergoes a diurnal cycle between a daytime convective boundary layer, CBL, and a nocturnal stable boundary layer, SBL when strong synoptic forcing is absent (Stull, 1988). It is difficult to investigate and understand since important drivers including advection, shear, and entrainment are typically simplified or neglected (Angevine et al, 2020; Bange et al, 2007; Wildmann et al, 2015). Angevine et al (2001) and Svensson et al (2011) defined the MT by the times between sunrise, the first sign change of sensible heat flux, H, and first statically unstable stratification across 200-m height. Angevine et al (2001) found that the surface heat flux alone was too weak to drive the transition at the observed speed emphasizing the importance of an entrainment heat flux from above the SBL. Lapworth (2006) found a close relationship between sign changes in H and static stability.
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