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How to interpret meteorological measurements made at a given level over a surface with regard to characteristic properties such as roughness, albedo, heat, moisture, carbon dioxide, and other gases is an old question which goes back to the very beginnings of modern micrometeorology. It is made even more challenging when it is unclear whether these measurements are only valid for this point/region and precisely describe the conditions there, or if they are also influenced by surrounding areas. After 50 years of field experiments, it has become both apparent and problematic that meteorological measurements are influenced from surfaces on the windward side. As such, extending these measurements for inhomogeneous experimental sites requires a quantitative understanding of these influences. When combined with atmospheric transport models similar to air pollution models, the {u2018}footprint{u2019} concept {u2013} a fundamental approach introduced roughly 20 years ago {u2013} provides us with information on whether or not the condition of upwind site homogeneity is fulfilled. Since these first models, the development of more scientifically based versions, validation experiments and applications has advanced rapidly. The aim of this book is to provide an overview of these developments, to analyze present deficits, to describe applications and to advance this topic at the forefront of micrometeorological research.

A new scaling is derived that yields a Reynolds-number-independent profile for all components of the Reynolds stress in the near-wall region of wall-bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behaviour.

We compute fully local boundary layer scales in three-dimensional turbulent Rayleigh–Bénard convection. These scales are directly connected to the highly intermittent fluctuations of the fluxes of momentum and heat at the isothermal top and bottom walls and are statistically distributed around the corresponding mean thickness scales. The local boundary layer scales also reflect the strong spatial inhomogeneities of both boundary layers due to the large-scale, but complex and intermittent, circulation that builds up in closed convection cells. Similar to turbulent boundary layers, we define inner scales based on local shear stress that can be consistently extended to the classical viscous scales in bulk turbulence, e.g. the Kolmogorov scale, and outer scales based on slopes at the wall. We discuss the consequences of our generalization, in particular the scaling of our inner and outer boundary layer thicknesses and the resulting shear Reynolds number with respect to the Rayleigh number. The mean outer thickness scale for the temperature field is close to the standard definition of a thermal boundary layer thickness. In the case of the velocity field, under certain conditions the outer scale follows a scaling similar to that of the Prandtl–Blasius type definition with respect to the Rayleigh number, but differs quantitatively. The friction coefficient $\\def \\xmlpi #1{}\\def \\mathsfbi #1{\\boldsymbol {\\mathsf {#1}}}\\let \\le =\\leqslant \\let \\leq =\\leqslant \\let \\ge =\\geqslant \\let \\geq =\\geqslant \\def \\Pr {\\mathit {Pr}}\\def \\Fr {\\mathit {Fr}}\\def \\Rey {\\mathit {Re}}c_{\\epsilon }$ scaling is found to fall right between the laminar and turbulent limits, which indicates that the boundary layer exhibits transitional behaviour. Additionally, we conduct an analysis of the recently suggested dissipation layer thickness scales versus the Rayleigh number and find a transition in the scaling. All our investigations are based on highly accurate spectral element simulations that reproduce gradients and their fluctuations reliably. The study is done for a Prandtl number of $\\mathit{Pr}=0.7$ and for Rayleigh numbers that extend over almost five orders of magnitude, $3\\times 10^5\\le \\mathit{Ra} \\le 10^{10}$ , in cells with an aspect ratio of one. We also performed one study with an aspect ratio equal to three in the case of $\\mathit{Ra}=10^8$ . For both aspect ratios, we find that the scale distributions depend on the position at the plates where the analysis is conducted.

Recent progress in understanding subcritical transition to turbulence is based on the concept of the edge, the manifold separating the basins of attraction of the laminar and the turbulent state. Originally developed in numerical studies of parallel shear flows with a linearly stable base flow, this concept is adapted here to the case of a spatially developing Blasius boundary layer. Longer time horizons fundamentally change the nature of the problem due to the loss of stability of the base flow due to Tollmien–Schlichting (TS) waves. We demonstrate, using a moving box technique, that efficient long-time tracking of edge trajectories is possible for the parameter range relevant to bypass transition, even if the asymptotic state itself remains out of reach. The flow along the edge trajectory features streak switching observed for the first time in the Blasius boundary layer. At long enough times, TS waves co-exist with the coherent structure characteristic of edge trajectories. In this situation we suggest a reinterpretation of the edge as a manifold dividing the state space between the two main types of boundary layer transition, i.e. bypass transition and classical transition.

The atmospheric boundary layer undergoes transitions between stable and convective states. Over land, in undisturbed conditions, these transitions occur daily in the morning and late afternoon or early evening. Though less well studied and presenting more challenges than the fully stable and fully convective states, such transitions have been the subject of growing interest over the last few decades. During transitions, all forcings are weak, and few simplifications are possible. Factors such as terrain, radiation, advection, and subsidence can seldom be safely neglected. Here, we review research on transitions over recent decades, with an emphasis on work published in Boundary - Layer Meteorology . The review is brief and inevitably reflects the interests and views of the authors.

Over the last 50 years the large-eddy simulation (LES) technique has developed into one of the most prominent numerical tools used to study transport processes in the atmospheric boundary layer (ABL). This review examines development of the technique as a tool for ABL research, integration with state-of-the-art scientific computing resources, and some key application areas. Analysis of the published literature indicates that LES research across a broad range of applications accelerated starting around 1990. From that point in time, robust research using LES developed in several different application areas and based on a review of the papers published in this journal, we identify seven major areas of intensive ABL–LES research: convective boundary layers, stable boundary layers, transitional boundary layers, plant canopy flows, urban meteorology and dispersion, surface heterogeneity, and the testing and development of subgrid-scale (SGS) models. We begin with a general overview of LES and then proceed to examine the SGS models developed for use in ABL–LES. After this overview of the technique itself, we review the specific model developments tailored to the identified application areas and the scientific advancements realized using the LES technique in each area. We conclude by examining the computational trends in published ABL–LES research and identify some resource underutilization. Future directions and research needs are identified from a synthesis of the reviewed literature.

Free-stream turbulence (FST) and its effect on boundary-layer transition is an intricate problem. Elongated unsteady streamwise streaks of low and high speed are created inside the boundary layer and their amplitude and spanwise wavelength are believed to be important for the onset of transition. The transitional Reynolds number is often simply correlated with the turbulence intensity (${Tu}$), and the characteristic length scales of the FST are often considered to have a small to negligible influence on the transition location. Here, we present new results from a large experimental measurement campaign, where both the ${Tu}$ and the integral length scale ($\\Lambda _x$) are varied ($1.8\\,\\% < {Tu}< 6.2\\,\\%$; $16\\ \\textrm {mm}< \\Lambda _x < 26\\ \\textrm {mm}$). In the current experiments it has been noted that on the one hand, for small $Tu$, an increase in $\\Lambda _x$ advances transition, which is in agreement with established results. On the other hand, for large $Tu$, an increase in $\\Lambda _x$ postpones transition. This trend can be explained by the fact that an optimal ratio between FST length scale and boundary-layer thickness at transition onset exists. Furthermore, our results strengthen the fact that the streaks play a key role in the transition process by showing a clear dependence of the FST characteristics on their spanwise scale. Our measurements show that the aspect ratio of the streaky structures correlates with an FST Reynolds number and that the aspect ratio can change by a factor of two at the location of transition. Finally, we derive a semi-empirical transition prediction model, which is able to predict the influence of $\\Lambda _x$ for both small and high values of ${Tu}$.