title: Improving our Understanding of the Atmospheric Weak-wind Boundary Layer using Spatially Explicit Observations near the Ground Surface
Creators: name: Pfister, Lena
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Fiber-optics
subject: Atmosphere
subject: Stable Boundary Layer
subject: Submeso-scale motions
subject: 500
subject: 530
subject: 550

description: Within the atmospheric boundary layer energy and matter are most effectively exchanged with the earth’s surface by turbulence. Turbulence is the irregular almost random fluctuation in velocity, temperature, and scalars. Research accordingly focuses on turbulent exchange processes. While those processes are mostly understood during the day, we need to improve our understanding of the nocturnal boundary layer especially during calm winds. Correspondingly, this doctoral thesis investigated turbulence within the nocturnal boundary layer using spatially explicit observations near the ground surface. The observations were taken during the Shallow Cold Pool experiment (SCP) in Colorado, USA, in 2012. The data set had a unique combination of different techniques also featuring fiber-optic distributed sensing~(FODS) with spatial continuous measurements. The gentle terrain of the field site was chosen as it commonly is assumed to have a rather small impact on the nocturnal boundary layer and represents most of the earth's surface. For investigating turbulence, we developed a nighttime classification scheme based on a surface energy balance which determined static stability, wind regime, and longwave radiative forcing as the three forcing parameter. Not only each forcing parameter had a specific impact on turbulence but also the three selected night classes determined by the combination of them, hence, they were further investigated. The first night class represented conditions with strong dynamic forcing elevating near-surface temperature by topographically induced mixing at the North shoulder of the valley. The second night class was a concurrence of topographically induced mixing and cold air at the bottom of the valley due to strong radiative cooling. The third night class was characteristic of weak winds eroding the impact of mechanical mixing but enhancing the impact of cold air within the valley. Consequently, the proposed classification scheme is successful in sorting the experimental data into physically meaningful temperature and flow regimes representing turbulence within the boundary layer. The classification scheme, however, was not successful in detecting submeso-scales motions which also impact turbulence within the weak-wind boundary layer significantly. A follow up study showed that at three different field site including SCP the variability of temperature is significantly increased during the submeso scale and usually is larger than the nocturnal temperature trend. Accordingly, a case study of the SCP data featuring a submeso-scale motion was investigated in detail. During weak winds a transient cold-air pool developed within the valley which was displaced uphill towards the North shoulder by a South-Westerly flow. At the North shoulder temperatures were usually elevated due to turbulent mixing. Consequently, the two air masses created a sharp boundary which we refer to as thermal submeso-front (TSF) in the following studies. We anticipate that these interactions are globally common. Further investigations are necessary to fully understand the relation between temperature variability, wind speed and direction, the topography, and TSFs. Correspondingly, for the last two studies a detection algorithm was developed which accurately determined the TSF location. This was the first study being able to continuously track a submeso-scale motion. TSFs were frequently occurring within the nocturnal boundary emphasizing their relevance. TSFs consist of two layers which push against each other forcing the TSF up and down the valley side wall in a wave like motion. The warm-air layer is mechanically generated by topographically induced mixing at the plateau-edge, while the cold-air layer is thermo-dynamically driven by topographically induced cold-air drainage. TSFs vanish during strong wind speed and spatially homogeneous wind direction which most likely erodes any cold air. The key to these insights was FODS as we could conditionally average parameters depending on the occurrence and location of TSFs. TSFs impact the boundary layer significantly. During TSFs ergodicity assumptions are invalid as their advective velocity is an order of magnitude lower than the mean wind speed. The mean difference of the sensible heat flux between the air layers of TSFs is 30~Wm$^{-2}$, hence, the impact on turbulence is strong. At the valley bottom the air layers of TSFs are stacked which increases static stability beyond the capability of radiative forcing. Here, the decoupled cold-air layer also invalidates flux-gradient similarity theory. Unfortunately, no distinct forcing for TSFs nor a relation to a wind or thermal regime could be determined. FODS outperformed point observations as even the dense network of the SCP experiment missed TSFs most of the time. So far many submeso scale motions are detected, but their relation, interaction, and needed forcing is not well understood. We need to change from classification schemes using vertical forcing mechanism and focus on the relation between motions on multiple scales. Further, classification schemes and modeling studies need to incorporate the impacts of topography as well as horizontal advection to improve our understanding of the nocturnal boundary layer.