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Abstract The vertical structure of the observed stable boundary layer often deviates substantially from textbook profiles. Even over flat homogeneous surfaces, the turbulence may not be completely related to the surface conditions and instead generated by elevated sources of turbulence such as low-level jets and transient modes. In stable conditions, even modest surface heterogeneity can alter the vertical structure of the stable boundary layer. With clear skies and low wind speeds, cold-air drainage is sometimes generated by very weak slopes and induces a variety of different vertical structures. Our study examines the vertical structure of the boundary layer at three contrasting tower sites. We emphasize low wind speeds with strong stratification. At a given site, the vertical structure may be sensitive to the surface wind direction. Classification of vertical structures is posed primarily in terms of the profile of the heat flux. The nocturnal boundary layer assumes a variety of vertical structures, which can often be roughly viewed as layering of the heat-flux divergence (convergence). The correlation coefficient between the temperature and vertical velocity fluctuations provides valuable additional information for classification of the vertical structure. 

Abstract This study examines nocturnal temperature changes on time‐scales of 5–60 min over gentle terrain. Such temperature variations are often important after the early evening period of rapid cooling and can lead to large temporary warming or enhanced cooling. The time–space structure of temperature changes is examined statistically with a network of flux stations over gentle topography. Large temperature changes are often associated with coherent propagating modes and associated temporary reduction or elimination of the valley cold pool, local drainage flows, and lee turbulence. The largest variations of temperature with time occur for intermediate wind speeds. Low wind speeds correspond to greater spatial variability of temperature but less time dependence. Two nondimensional ratios are developed to represent the relative importance of temporal and spatial variability. 

Abstract Nocturnal spatial variation of temperature, wind, and turbulence over microtopography is generally poorly understood. Low amplitude microtopography covers much of the Earth’s surface and, with very stable conditions, can produce significant spatial variations of temperature and turbulence. We examine such variations over gentle terrain that include two shallow gullies that feed into a small valley. The gullies are covered by a sub-network of seven flux stations that is embedded within a larger network that covers the valley. The measurements indicate that gullies of only 2–5-m depth and 100-m width can often lead to spatial variations of temperature of several kelvin or more. Such variations depend on ambient wind speed and direction and the near-surface stratification. We investigate the surprising importance of microscale lee turbulence occurring over the gentle microtopography with slopes of only 5%. Near-surface stratification unexpectedly tends to increase with surface elevation on the slopes. We examine the potential causes of this puzzling behaviour of the near-surface stratification. 

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. 

Abstract Submesoscale motions within the stable boundary layer were detected during the Shallow Cold Pool Experiment conducted in the Colorado plains, Colorado, U.S.A. in 2012. The submesoscale motion consisted of two air layers creating a well-defined front with a sharp temperature gradient, and further-on referred to as a thermal submesofront (TSF). The semi-stationary TSFs and their advective velocities are detected and determined by the fibre-optic distributed-sensing (FODS) technique. An objective detection algorithm utilizing FODS measurements is able to detect the TSF boundary, which enables a detailed investigation of its spatio–temporal statistics. The novel approach in data processing is to conditionally average any parameter depending on the distance between a TSF boundary and the measurement location. By doing this, a spatially-distributed feature like TSFs can be characterized by point observations and processes at the TSF boundary can be investigated. At the TSF boundary, the air layers converge, creating an updraft, strong static stability, and vigorous mixing. Further, the TSF advective velocity of TSFs is an order of magnitude lower than the mean wind speed. Despite being gentle, the topography plays an important role in TSF formation. Details on generating mechanisms and implications of TSFs on the stable boundary layer are discussed in Part 2.