Large-Eddy Simulations of the Steady Wintertime Antarctic Boundary Layer Abstract Observations of two typical contrasting weakly stable and very stable boundary layers from the winter at Dome C station, Antarctica, are used as a benchmark for two centimetre-scale-resolution large-eddy simulations. By taking the Antarctic winter, the effects of the diurnal cycle are eliminated, enabling the study of the long-lived steady stable boundary layer. With its homogeneous, flat snow surface, and extreme stabilities, the location is a natural laboratory for studies on the long-lived stable boundary layer. The two simulations differ only in the imposed geostrophic wind speed, which is identified as the main deciding factor for the resulting regime. In general, a good correspondence is found between the observed and simulated profiles of mean wind speed and temperature. Discrepancies in the temperature profiles are likely due to the exclusion of radiative transfer in the current simulations. The extreme stabilities result in a considerable contrast between the stable boundary layer at the Dome C site and that found at typical mid-latitudes. The boundary-layer height is found to range from approximately \(50\,\mathrm {m}\) to just \(5\,\mathrm {m}\) in the most extreme case. Remarkably, heating of the boundary layer by subsidence may result in thermal equilibrium of the boundary layer in which the associated heating is balanced by the turbulent cooling towards the surface. Using centimetre-scale resolutions, accurate large-eddy simulations of the extreme stabilities encountered in Antarctica appear to be possible. However, future simulations should aim to include radiative transfer and sub-surface heat transport to increase the degree of realism of these types of simulations.

Flow Separation in the Lee of a Crater Rim Abstract The nearly circular Meteor Crater, Arizona, is located on an extensive, slightly sloping plain, above which a south-westerly katabatic flow forms during undisturbed, clear-sky nights. For the katabatic flow over the upstream crater rim, the resulting flow regime in the lee depends on the upstream wind speed. For a shallow katabatic flow with a wind-speed maximum of about 5 m s \(^{-1}\) or less at a height of about 20 m above the ground, the flow decelerates as it approaches the crater. Cold-air intrusions form, that is, cold air spills over the crater rim and runs down the inner south-west sidewall. For a deep katabatic flow with a wind-speed maximum located above the 50-m high measurement tower and comparatively higher wind speeds, the flow accelerates towards the crater. The flow separates in the immediate lee of the crater rim, forming a wake over the south-west crater sidewall. The wake can either be small, affecting only the upper part of the sidewall, or large, affecting the entire crater sidewall or even the crater floor. When flow separation occurs, the wake region over the crater sidewall is characterized by low wind speeds and potentially a return circulation near the surface. Particularly for large wakes, stability in the crater atmosphere is reduced and relatively high wind speeds occur at the crater floor, which is otherwise submerged in a strong surface-based inversion. Turbulent kinetic energy at the crater sidewall is typically higher during cold-air intrusions than is the case during flow separation, but high values can occur at the floor when a large wake forms.

Statistical Investigation of Flow Structures in Different Regimes of the Stable Boundary Layer Abstract A combination of methods originating from non-stationary time-series analysis is applied to two datasets of near-surface turbulence in order to gain insights on the non-stationary enhancement mechanism of intermittent turbulence in the stable atmospheric boundary layer (SBL). We identify regimes of SBL turbulence for which the range of time scales of turbulence and submeso motions, and hence their scale separation (or lack of separation), differs. Ubiquitous flow structures, or events, are extracted from the turbulence data in each flow regime. We relate flow regimes characterized by very stable stratification, but differing in the dynamical interactions and in the transport properties of different scales of motion, to a signature of flow structures thought to be submeso motions.

Missed Fog? Abstract Conventional in situ observations of meteorological variables are restricted to a limited number of levels near the surface, with the lowest observation often made around 1-m height. This can result in missed observations of both shallow fog, and the initial growth stage of thicker fog layers. At the same time, numerical experiments have demonstrated the need for high vertical grid resolution in the near-surface layer to accurately simulate the onset of fog; this requires correspondingly high-resolution observational data for validation. A two-week field campaign was conducted in November 2017 at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. The aim was to observe the growth of shallow fog layers and assess the possibility of obtaining very high-resolution observations near the surface during fog events. Temperature and relative humidity were measured at centimetre resolution in the lowest 7 m using distributed temperature sensing. Further, a novel approach was employed to estimate visibility in the lowest 2.5 m using a camera and an extended light source. These observations were supplemented by the existing conventional sensors at the site, including those along a 200-m tall tower. Comparison between the increased-resolution observations and their conventional counterparts show the errors to be small, giving confidence in the reliability of the techniques. The increased resolution of the observations subsequently allows for detailed investigations of fog growth and evolution. This includes the observation of large temperature inversions in the lowest metre (up to 5 K) and corresponding regions of (super)saturation where the fog formed. Throughout the two-week observation period, fog was observed twice at the conventional sensor height of 2.0 m. Two additional low-visibility events were observed in the lowest 0–0.5 m using the camera-based observations, but were missed by the conventional sensors. The camera observations also showed the growth of shallow radiation fog, forming in the lowest 0.5 m as early as two hours before it was observed at the conventional height of 2 m.

Large-Eddy Simulation of the Effects of Wind-Direction Fluctuations on Turbulent Flow and Gas Dispersion Within a Cubical Canopy Abstract Large-eddy simulation of turbulent flow and gas dispersion in a cubical canopy is used to investigate the effect of wind-direction fluctuations on gas dispersion. Square blocks are set at regular intervals on the bottom surface, with line sources placed within the first, second, third, fifth and seventh rows. Large-eddy simulation without wind-direction fluctuations produces a good prediction of the mean streamwise velocity component, and the standard deviations of the fluctuations in the streamwise and spanwise velocity components, obtained from a wind-tunnel experiment. Wind-direction fluctuations marginally affect the mean streamwise velocity component above the canopy in the first row, and do not significantly affect the component beyond the third row. The standard deviations of the fluctuations in the streamwise and spanwise velocity components above the canopy are also affected by wind-direction fluctuations, but within the canopy the components are less sensitive to the fluctuations beyond the third row. The spatially-averaged concentrations within the canyon with wind-direction fluctuations before the third row are marginally greater than concentrations without the fluctuations, but they are essentially identical beyond the fifth row. The low-frequency turbulent flow that passes through the canyon is generated with and without wind-direction fluctuations.

Spatial Variation of Statistical and Spectral Properties of the Stream Wise and Wall-Normal Velocity Fluctuations in the Near-Neutral Atmospheric Surface Layer Abstract Based on high-quality near-neutral atmospheric-surface-layer (ASL) data obtained from an observational site located in flat desert and existing experimental results for a friction Reynolds number Reτ < 106, the spatial variation of the streamwise and wall-normal velocity statistics and the Reτ dependence of the scaling parameters are investigated. The near-neutral ASL results show that, for Reτ > 106, the change of the second-order statistics of the streamwise velocity component with height is consistent with the experimental results for Reτ < 106, while the second-order statistics of the wall-normal velocity component increase linearly with height. In combination with the experimental results for Reτ < 106, the variation of the slope of streamwise turbulence intensity with height and the variation of the slope and intercept of the wall-normal turbulence intensity with height are quantified to reveal that both the streamwise and wall-normal velocity variances follow a generalized logarithmic law, with the distribution of the streamwise and wall-normal velocity components satisfying sub- and super-Gaussian distributions. Spectral analysis shows that the variation of the wavenumber corresponding to the pre-multiplied streamwise and wall-normal spectra peaks with the ratio z/δ obeys the form \( k_{x} \delta = a(\delta /z)^{b} \) , where z is the height, kx is the streamwise wavenumber, and δ is the ASL thickness. For the pre-multiplied streamwise spectra, \( b = 0.5 \pm 0.1 \) and the value of a may have a Reynolds-number dependence, while for the pre-multiplied wall-normal spectra, \( a = 2.2 \pm 0.09 \) and \( b = 1.0 \pm 0.1 \) . The von Kármán constant, the Townsend–Perry constant and other parameters also display a weak Reynolds-number dependence.

Large-Eddy Simulation of Erosion and Deposition over Multiple Two-Dimensional Gaussian Hills in a Turbulent Boundary Layer Abstract We investigate the effects of one or more hills on the solid-particle saltation layer, and focus on the effect of the recirculation zone that plays an important role in solid-particle erosion or entrapment. The aerodynamic features of the flow have been presented previously (Huang et al. in Environ Fluid Mech 18:581–609, 2018) and the influence of hill separation was discussed in light of the classification deduced from the urban canopy scheme of Oke (Energy Build 11:103–113, 1988). Here, large-eddy simulations (LES) coupled with Lagrangian tracking of solid particles over multiple two-dimensional Gaussian hills in a turbulent boundary layer are performed using the atmospheric Advanced Regional Prediction System. Models for the interaction of particles with the soil are used, especially for take-off and rebound, and the boundary layer at different external velocities is first simulated. Numerical results are compared with experiments performed in our laboratory (Simoëns et al. in Procedia IUTAM 17:110–118, 2015) to collect particle concentration and velocity profiles, with the different forces acting on the particles at the wall analyzed. Accumulation and erosion zones are investigated regarding the shear velocity, and different fluxes as function of the Shields number are defined and discussed. Lower momentum transfer and exchange between the recirculation region and the mixing zone in the wake-flow regime result in an increase in the number of trapped particles compared with the skimming-flow regime.

Characteristics of Turbulent Coherent Structures in Atmospheric Flow Under Different Shear–Buoyancy Conditions Abstract Turbulent coherent structures in the atmospheric boundary layer exert unsteady loads on mechanical and civil structures and greatly contribute to pollutant dispersion and heat dissipation. Much has been deduced about the characteristics of these structures at the laboratory scale in pure shear-driven flows. We examine the influence of atmospheric stability (shear–buoyancy variation) on the newly discovered properties of these turbulence features using observations obtained from a test facility at an onshore site on the east coast of Malaysia. Three ultrasonic anemometers placed at 1.7, 3 and 12 m above ground collected 124 30-min samples of the undisturbed flow. Contrary to expectations, the decline in shear stress in stable stratification reduced the time delay of the peak cross-correlation, implying an increase in the inclination angle of these structures. A wavelet analysis shows that, although the time scale of the vortex packets decreases as the atmosphere becomes increasingly stable, the super-streak time scale increases. The monotonic increase in the energy content in the convective direction results in an enhanced modulating effect for the large super-streaks on the small vortex packets. Analyzing the structure coherence defined as the temporal extension of the streamwise velocity depression reveals two stages of the life cycle of convective rolls. In the first stage, a super-streak couple consisting of a warm updraft and a cold downdraft appears simultaneously at a Monin–Obukhov stability parameter \(\zeta = -\,3.5\) . In the second stage, the warm updraft strengthens and the cold downdraft weakens.

Flexible Treatment of Radiative Transfer in Complex Urban Canopies for Use in Weather and Climate Models Abstract We describe a new approach for modelling the interaction of solar and thermal-infrared radiation with complex multi-layer urban canopies. It uses the discrete-ordinate method for describing the behaviour of the radiation field in terms of a set of coupled ordinary differential equations that are solved exactly. The rate at which radiation intercepts building walls and is exchanged laterally between clear-air and vegetated parts of the urban canopy is described statistically. Key features include the ability to represent realistic urban geometry (both horizontal and vertical), atmospheric effects (absorption, emission, and scattering), and spectral coupling to an atmospheric radiation scheme. In the simple case of a single urban layer in a vacuum, the new scheme matches the established matrix-inversion method very closely when eight or more streams are used, but with the four-stream configuration being of adequate accuracy in an operational context. Explicitly representing gaseous absorption and emission in the urban canopy is found to have a significant effect on net fluxes in the thermal infrared. Indeed, we calculate that for the mid-latitude summer standard atmosphere at mean sea level, 37% of thermal-infrared energy is associated with a mean free path of less than 50 m, which is the typical mean line-of-sight distance between walls in an urban area. The interaction of solar radiation with trees has been validated by comparison to Monte Carlo benchmark calculations for an open forest canopy over both bare soil and snow.

Effects of Path Averaging in a Sonic Anemometer on the Estimation of Turbulence-Kinetic-Energy Dissipation Rates Abstract The turbulence-kinetic-energy dissipation rate is a fundamental property in turbulent flows, but its direct measurement in the atmospheric surface layer is still a challenge. Indirect estimates are often obtained from inertial-subrange laws using turbulence statistics of the longitudinal velocity component. In this study, synthetic turbulence data are used to investigate the impact of path-averaging effects present in sonic anemometer data on the inertial subrange of the second-order structure function. Path averaging reduces the energy levels in the second-order structure function, creating a negative bias in the estimates of the dissipation rate. The effect is dependent on the path-averaging transfer function, mean wind speed and path length. A simple correction for the bias on the basis of existing transfer functions is applied and tested with data obtained from two separate sonic anemometers. Compared to the spectrum, the second-order structure function after the correction becomes the optimum statistical measure for indirect estimation of the dissipation rate, due to its lower random error and insensitivity to aliasing effects.