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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 Points: A 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 [ABSTRACT FROM AUTHOR]

Air flows may be decoupled from the underlying surface either due to strong stratification of air or due to canopy drag suppressing cross‐canopy mixing. During decoupling, turbulent fluxes vary with height and hence identification of decoupled periods is crucial for the estimation of surface fluxes with the eddy covariance (EC) technique and computation of ecosystem‐scale carbon, heat, and water budgets. A new indicator for identifying the decoupled periods is derived using forces (buoyancy and canopy drag) hindering movement of a downward propagating air parcel. This approach improves over the existing methods since (1) changes in forces hindering the coupling are accounted for, and (2) it is based on first principles and not on ad hoc empirical correlations. The applicability of the method is demonstrated at two contrasting EC sites (flat open terrain, boreal forest) and should be applicable also at other EC sites above diverse ecosystems (from grasslands to dense forests). Plain Language Summary: Air flows may be disconnected (i.e., decoupled) from the surface below, meaning that the properties of the flow (e.g., wind speed, temperature, concentrations of gases, pollutants, or particles) do not react to changes at the surface. During these periods, air temperatures near the ground decrease and concentrations of gases, pollutants and particles increase significantly since they are not transported upwards, but rather stay close to the ground. These decoupling periods can take place when the air is strongly stratified (e.g., clear sky, weak wind nights) or thick forest canopies inhibit air mixing. Controls on flow decoupling are poorly understood, yet the phenomenon has significance for scientific monitoring networks and also for the general public due to its connection for example, to air quality and frost formation. In this study, we derive a new indicator for flow decoupling, demonstrate its applicability at two measurement sites and discuss variables controlling decoupling in the light of this new indicator. Key Points: A universal indicator for air flow vertical decoupling is derivedThe indicator enables analytical estimation of flow decoupling dependency on height, stratification, and leaf area indexThe indicator should be applicable at most flux measurement sites, since it relies only on basic micrometeorological measurements [ABSTRACT FROM AUTHOR]

The article discusses the ice attenuation and relative basal reflectivity at the major catchments funneling ice from East Antarctica to the Ross Ice Shelf. It evaluates the two independent remotely sensed geophysical data set and tackles the significance of basal lubrication on modulating the ice dynamics. The surface ice velocities and the general association of higher flow speeds with high radar reflection strength are also presented.