Observing vertical coupling near the surface in a shallow mid-range mountain valley using a suite of ground-based remote sensing and tower observations

Abstract

<p>Mid-latitude atmospheric boundary layers (ABL) in complex, mountainous terrain are often complicated because the large-scale radiative and dynamic forcings are modulated by local-scale forcings which may dominate the near-surface transport. The large-scale forcings of interest in our study are geostrophic winds and cloudiness which are known to cause variations in ABL depth and vertical coupling. The local-scale forcings we investigate are the slope, aspect, and land cover of valley shoulders and bottom which can create high-density cold airflows and pools often associated with submeso-scale motions. These topography-related phenomena may lead to vertical decoupling between the surface, the surface layer and the ABL in absence of strong large-scale synoptic forcing. Understanding the mechanisms by which the large-scale synoptic and local-scale topographic forcings interact has remained poorly understood despite many observational and modeling studies, but is crucial to understanding and quantifying mass and heat exchange in locations to weak winds.</p><p>We present results from the Large eddy Observation Voitsumra Experiment (LOVE) in summer 2019 conducted in a mid-range mountain valley in the Fichtelgebirge mountains, Germany over a two-month period as part of the ERC DarkMix project. Observations consist of fine to medium-scale (1s to 10 min) measurements from ground-based remote sensing including a ceilometer (150 to 8000 m above ground), wind Lidar (80 to 800 m above ground), and Sodar-Rass (15 to 300 m above ground) in combination with sonic anemometry and fiber-optic distributed temperature and wind sensing. The objective is to identify the mechanisms by which the land surface gets coupled or decoupled from the near-surface air aloft eventually forming the ABL, stable boundary layer, or residual layer. Particular attention is given to the stable weak-wind flow regime often persisting from sunset to sunrise. </p><p>We test the following two hypotheses: (1) The observed meandering of the near-surface nocturnal flow in the lowest tens of meters is the result of three competing flow modes generated by cold-slope flows from a closely co-located valley slope by net-radiative cooling, an along-valley flow supported by a weak synoptic pressure gradient, and a colder-air pool collecting at the valley bottom. Differences in the relative temperature of the three modes cause quasi-oscillatory variations in static stability and thus vertical coupling. (2) Erosion of the near-surface inversion starts well before arrival of the direct shortwave radiation at the valley bottom caused by radiative warming of the surrounding mountain slopes and enhanced mixing from aloft. As a result, coupling the land-surface to the evolving ABL may be achieved earlier than anticipated from the local surface energy balance in the valley bottom.</p>