Acoustic tomography inside the atmospheric boundary layer

Acoustic travel time tomography will be applied on the atmosphere to directly provide spatially averaged values of meteorological quantities (temperature and wind field) which are needed for the evaluation of numerical models as well as to complete in situ point measurements and conventional studies of the atmospheric surface layer over natural surfaces. Sound waves propagate through the atmosphere with different sound velocities according to the distribution of air temperature and wind vector and become, therefore, an information carrier about the medium properties. In the presented study acoustic travel time data were measured between sources and receivers placed around an array of 200×260 m2 as initial values for the tomographic reconstruction. Thereby, a frequently applied procedure of ray tomography, the simultaneous iterative reconstruction technique, characterized by simple handling and small computational requirements, was used. Additionally, a special system of equations for the isolation of the temperature and wind influence on the sound speed was solved. To estimate the sound ray propagation and to simulate acoustic travel times during various meteorological conditions a ray tracing model based on a modified version of Snell’s law corresponding to the influence of wind vector will be introduced.

Acoustic travel-time tomography of the atmosphere is a nonlinear inverse problem which attempts to reconstruct temperature and wind velocity fields in the atmospheric surface layer using the dependence of sound speed on temperature and wind velocity fields along the propagation path. This paper presents a statistical-based acoustic travel-time tomography algorithm based on dual state-parameter unscented Kalman filter (UKF) which is capable of reconstructing and tracking, in time, temperature, and wind velocity fields (state variables) as well as the dynamic model parameters within a specified investigation area. An adaptive 3-D spatial-temporal autoregressive model is used to capture the state evolution in the UKF. The observations used in the dual state-parameter UKF process consist of the acoustic time of arrivals measured for every pair of transmitter/receiver nodes deployed in the investigation area. The proposed method is then applied to the data set collected at the Meteorological Observatory Lindenberg, Germany, as part of the STINHO experiment, and the reconstruction results are presented.

The low-frequency electric field associated with the propagation of an aerial shock wave in the atmospheric surface layer was studied experimentally and theoretically. The experiment involved the recording of variations of the vertical component E2 of the electric field strength in the atmosphere by means of electrostatic flux meters and rod antennas set up on the surface of the ground. Analysis of the experimental data shows that electric field perturbations are caused by changes in the shock wave density of the volume electric charge formed by heavy ions and aerosol particles. An approximate proportionality is observed between the amplitude of electric field perturbations, shock wave parameters, and concentration of charged particles in the atmospheric electrode layer.

Understanding the organization and dynamics of turbulence structures in the atmospheric surface layer (ASL) is important for fundamental and applied research in different fields, including weather prediction, snow settling, particle and pollutant transport, and wind energy. The main challenges associated with probing and modeling turbulence in the ASL are (i) the broad range of turbulent scales associated with the different eddies present in high Reynolds number boundary layers ranging from the viscous scale (on the order of millimeters) up to large energy-containing structures (on the order of kilometers); (ii) the nonstationarity of the wind conditions and the variability associated with the daily cycle of the atmospheric stability; and (iii) the interactions among eddies of different sizes populating different layers of the ASL, which contribute to momentum, energy, and scalar turbulent fluxes. Creative and innovative measurement techniques are required to probe near-surface turbulence by generating spatiotemporally resolved data in the proximity of the ground and, at the same time, covering the entire ASL height with large enough streamwise extent to characterize the dynamics of larger eddies evolving aloft. To this aim, the U.S. National Science Foundation sponsored the development of the Grand-scale Atmospheric Imaging Apparatus (GAIA) enabling super-large snow particle image velocimetry (SLPIV) in the near-surface region of the ASL. This inaugural version of GAIA provides a comprehensive measuring system by coupling SLPIV and two scanning Doppler lidars to probe the ASL at an unprecedented resolution. A field campaign performed in 2021–22 and its preliminary results are presented herein elucidating new research opportunities enabled by the GAIA measuring system.

A large-scale spanwise and wall-normal array of sonic anemometers in the atmospheric surface layer is used to acquire all three components of instantaneous fluctuating velocity as well as temperature in a range of stability conditions. These data permit investigation of the three-dimensional statistical structure of turbulence structures. Based on a similar dataset, Krug et al. (Boundary-Layer Meteorol., vol. 172, 2019, pp. 199–214) reported a self-similar range of wall-attached turbulence structures under both unstable and near-neutral stability conditions. They considered only a wall-normal array and thus assessed statistical structure in the wall-normal direction, in relation to the streamwise wavelength. The present work extends the view of a self-similar range of turbulence structures, by including the statistical structure in the spanwise direction. Moreover, by analysing the phase shift between synchronized measurements in the spectral domain, it is inferred how a scale-dependent inclination angle in the streamwise/wall-normal plane varies with stability. Results suggest that the self-similar wall-attached structures have similar aspect ratios between streamwise/wall-normal scales and streamwise/spanwise scales such that $\lambda _x/\Delta z : \lambda _x/\Delta y \approx 1$ for both near-neutral and unstable conditions. Under the most unstable conditions, coherent structures with $\lambda _x/\delta = 1$ are inclined at angles as high as $65^{\circ }$ relative to the solid boundary, while larger scales $\lambda _x/\delta = 6$ exhibit inclination angles of approximately $35^{\circ }$ . For near-neutral stability conditions, the angle tends towards $12^{\circ }$ for all scales. It is noted that in the near-neutral condition, the structure inclination angle and the aspect ratio – and thus the statistical modelling of coherent structures in the atmospheric surface layer – are highly sensitive to the value of the stability parameter.

The numerical time-dependent three-dimensional model [Kovalets, I.V. and Maderich, V.S.: 2001, Int. J. Fluid Mech. Res. 30, 410–429] of the heavy gas dispersion in the atmospheric boundary layer has been improved by parameterizing momentum and heat fluxes on the surface of Earth using Monin–Obukhov similarity theory. Three parameterizations of heat exchange with the surface of Earth were considered: (A) formula of Yaglom A.M. and Kader B.A. [1974, J. Fluid Mech. 62, 601–623] for forced convection, (B) interpolation formula for mixed convection and (C) similarity relationship for mixed convection [Kader, B.A. and Yaglom, A.M.: 1990, J. Fluid Mech. 212, 637–662]. Two case studies were considered. In the first study based on experiment of Zhu et al., J. Hazard Mater 62, 161–186], the interaction of an isothermal heavy gas plume with an atmospheric surface layer was simulated. It was found that stable stratification in the cloud essentially suppresses the turbulence in the plume, reducing the turbulent momentum flux by a factor of down to 1/5 in comparison with the undisturbed value. This reduction essentially influences velocities in the atmospheric boundary layer above the cloud, increasing the mean velocity by a factor of up to 1.3 in comparison with the undisturbed value. A simulation of cold heavy gas dispersion was carried out in the second case based on field experiment BURRO 8. It was shown that both forced and free convections under moderate wind speeds significantly influence the plume. The relative rms and bias errors of prediction the plume’s height were σH ≈ 30% and ɛH = − 10%, respectively, for parameterization B, while for A and C the errors were σH ≈ 80% and ɛH ≈ − 65%. It is therefore advised to use the simple parameterization B in dense gas dispersion models.

Transport of momentum and scalars in turbulent boundary-layer flows over complex terrain has been of great interest in the atmospheric sciences and wind engineering communities. Applications include but are not limited to weather forecasting, air pollution dispersal, aviation safety control, and wind energy project planning. While linear models have been well accepted to predict boundary-layer flows over topography with gentle slope, modeling flow separation and recirculation induced by topography of sufficiently steep slope has to be achieved through non-linear models, such as Reynolds-averaged Navier-Stokes (RANS) solvers and Large-Eddy Simulations (LES). High-quality measurements in the field and laboratory settings are in high demand for development and validation of such numerical models. Dynamics of the separated boundary-layer flows over steep topography is affected by the shape and size of the topography, surface characteristics (e.g., roughness and temperature distribution) and atmospheric thermal stability. Majories of wind-tunnel experiments of boundary-layer flows over representative and idealized topography features (e.g. 2-D or 3-D hills, axisymmetric bumps) do not take thermal stability effects into account due to challenges in physical simulation. We conducted experimental investigation of stablystratified boundary layers over a steep 2-D hill in the thermally-controlled boundary-layer wind tunnel. The 2-D model hill has a steepest slope of 0.73 and its shape follows a cosine square function. High-resolution Particle Image Velocimetry (PIV) provides dynamic information of the separated shear layer, the recirculation zone and flow reattachment. Mean surface shear stress and surface heat flux were directly measured in the wake. Results indicate that suppressed turbulence in the stable boundary layer noticeably alters the topology of the circulation zone. Surface shear stress and surface heat flux downwind of the 2D hill slowly approach the equilibrim values of the non-disburbed boundary layers. This work can improve our understanding of the effects of thermal stability on steep topography, and provide reliable datasets for development and validation of numerical models.

During the total solar eclipse on March 29, 2006 [1], researchers in Kislovodsk measured variations in the flux of the total shortwave radiation, meteorological elements (wind speed and direction and relative air humidity), and turbulence parameters in the surface atmospheric layer, vertical profiles of air temperature in the layer 0‐600 m, functions of particle size distribution (0.20‐1.50 µ m), and concentrations of light ions. It was found that the decrease in the maximum air temperature caused by the eclipse reached 3.5°C in the surface atmospheric layer and approximately 2°C at an altitude of 600 m a.s.l. After the full phase of the eclipse, turbulent kinetic energy decreased by a factor of 2.5, dispersion of the vertical component of the wind speed decreased by a factor of 2.3, turbulent heat flux decreased by a factor of 3.5, and dispersion of turbulent pulsations of air temperature decreased by a factor of 10. Despite the long history of investigations of eclipses [2], the influence of the solar eclipses on processes in the boundary atmospheric layer has not yet been studied sufficiently enough [3‐5]. It is clear that a solar eclipse should lead to variations in the thermal regime of the boundary atmospheric layer, increase in the relative air humidity, and consequent increase in the size of aerosol particles. Variations in the parameters of aerosol along with other factors can change notably the electric characteristics of the surface atmosphere [6]. Decrease in the near-surface air temperature can also lead to variations in the turbulence regime. In order to study the quantitative variations in the above-mentioned parameters of the lower atmosphere during the total solar eclipse in the last decade of March in Kislovodsk (altitude ~900 m a.s.l.), the following measurements were carried out at the urban meteorological station: flux F of total shortwave radiation (CNR1 net radiometer (Kipp and Zonen, Netherlands); accuracy ± 10 W/m 2 ), meteorological parameters (accuracy of measurements: air temperature ± 0.3°C , relative humidity ± 5% , components of wind speed ± 0.15 m/s), turbulent pulsations (digitization step 0.1 s) of three components of wind speed and air temperature using a Meteo-2M acoustic meteorological station (Institute of Atmospheric Optics, Tomsk), vertical profiles of air temperature (time averaging 5 min, spatial averaging 50 m, accuracy of measurements ± 0.5°C ) in layer 0‐ 600 m (MTP-5 UHF microwave profiler of the Central Aerological Observatory [8]), concentrations of negative light ions using a SIGMA-1 counter of aeroions (accuracy of each measurement ± 30% , time constant 1.5 s, mobility of ions >0.4 cm 2 / W · s), and distribution function of the dry base of aerosol particles in the size range 0.20‐1.50 µ m (LAS-P laser spectrometer of aerosols, averaging time of differential countable concentrations of particles 15 s, and random error of measurements of countable concentrations 20%). Calibration of the laser spectrometer was carried out using nuclear filters, which allowed us to increase the accuracy of determination of the boundaries of the particle size ranges determined by the spectrometer.

Characterization of turbulence scales in the atmospheric surface layer with the continuous wavelet transform

Acoustic travel-time tomography of the atmosphere is a nonlinear inverse problem which attemps to reconstruct temperature and wind velocity fields in the atmospheric surface layer (ASL) using the dependence of sound speed on temperature and wind velocity fields along the propagation path. A new statistical-based algorithm is introduced in this paper based on fixed-point unscented Kalman filter (UKF) which is capable of reconstructing and tracking temperature and wind velocity fields within a specified investigation area.

Aerodynamic surface stress imposed by the atmospheric surface layer (ASL) drives aeolian sediment transport processes. When the imposed stress exceeds fluid threshold, splashing sand grains release fine (aerosol) particles. This process stops when the imposed stress falls below an impact threshold. Turbulence in the ASL is composed of elongated streaks of relatively high and low streamwise momentum (high‐momentum and low‐momentum regions, HMR and LMR, respectively), and these streaks are aligned with the prevailing winds. Streamwise‐wall‐normal turbulent stress is the concurrent product of fluctuations in streamwise and vertical velocity. These production mechanisms are categorized into quadrants: sweeps, inner interactions, ejections, and outer interactions, where the first two and last two occur within HMRs and LMRs, respectively. Under typical ASL conditions, the time‐averaged shear velocity is bound between 0.3 and 0.5 m/s, demonstrating the importance of fluctuations in mobilizing sand grains via saltation: time‐averaged shear velocity exceeding threshold does not correspond with typical ASL conditions. Given the aforementioned contributions to streamwise‐wall‐normal turbulent stresses from sweeps and ejections, it is self‐evident that under typical conditions only sweeps possess the momentum required to exceed threshold. But, any dust released is trapped by downwelling from aloft; ejections rarely exceed threshold, but they contain the positive vertical velocity needed to entrain. These attributes represent an entrainment paradox. Complementary field data are used to demonstrate the existence of an entrainment paradox. Large eddy simulation has been used to capture space‐time evolution of an idealized ASL and identify mechanistic flow physics central to entrainment.

A number of studies already suggested that entrainment of warm/dry air from aloft degrades the similarity between air temperature (T) and specific humidity (q) in the atmospheric surface layer (ASL). Less is known about entrainment of cool/dry air on the ASL T‐q similarity. Using eddy covariance measurements over a large inland water surface, enhanced T‐q similarity with increased friction velocity (u*) was measured during daytime unstable conditions. Explaining this enhancement is the main goal of this work. Cool/dry air masses originating and advecting from land set the upper boundary condition on a quasi‐equilibrated internal boundary layer embedding the warm/moist ASL over water. With increased u*, the entrainment velocity (dhI/dt ∝ u*) increases, thereby enhancing entrainment of cool/dry air originating from land, where t is time and hI is the internal boundary layer depth. By analyzing the scale‐dependent (spectral) correlation coefficients, the enhanced T‐q similarity with increased u* was shown to originate from spectral correlation coefficients in the middle‐ to low‐frequency ranges (large eddies ≫ z, the distance from the surface). With further increases in u*, similarity between T and q within large eddies was further enhanced and phase differences between T and q was reduced. Quadrant analysis showed large increases in flux and time fractions of downward large cool/dry eddies with increasing u*. These results provide evidence that enhanced entrainment of synchronized, large cool/dry eddies was a primary cause for the increased T‐q similarity with increasing u*.

The turbulent structures in atmospheric surface layer (ASL) are investigated in wind tunnel with hot-wire anemometers in present study. The results show that there exist two typical turbulent structures characterised by their streamwise length scales, i.e. large-scale motions (LSMs) and very large scale motions (VLSMs) as reported recently in pipe flow, channel flow, zero-pressure-gradient turbulent boundary layers and near-neutral ASLs. Moreover, a transition region containing both LSMs and VLSMs is first reported in present investigation, and this region depends on the Reynolds numbers. In the transition region, as the wall-normal distance increases, the turbulent energy ratio of LSMs is gradually weakened but VSLMs strengthened.

Previous measurements of spray, conducted with various techniques, are reviewed; the results are reanalyzed and compared to provide parametric descriptions of spray in the atmospheric surface layer. The number of droplets is shown to decrease in a power law form with elevation above the water surface. The size distribution of droplets peaks around 200 μm; on the side of larger droplets the frequency of occurrence of droplets decreases with increasing size and appears to approach asymptotically an equilibrium shape at high winds. These results are used along with calculations of droplet motions to determine the mean velocity and the mean distance of traveling of droplets and, subsequently, the concentration of droplets and the effects of spray on momentum, heat, and mass transfers across the air‐water interface. Finally, the results appear to indicate that the spray in laboratory tanks is produced by bubble bursting.

All our many-year studies of the gas-exchange processes were carried out at one observation site (BALO) in the littoral of Southern Baikal. Thus, the question about applicability of conclusions and estimates based on these data to the entire littoral zone inevitably arises. In 2014, we succeeded for the first time in organizing the combined research campaign for detailed mapping of the spatial variability of water characteristics in general for the littoral of southern Baikal. The spatial partial distribution of biogenic elements and the pressure of carbon dioxide and methane in the atmospheric surface layer and the surface water layer were studied in August 5-11 of 2014 from onboard of I.D. Papanin Research Vessel (RV). These studies covered, in section, the entire water area of the southern Baikal. The fluorescent characteristics, concentration of biogenic elements, partial pressure of carbon dioxide and methane in the surface water layer and the atmospheric surface layer were determined there. The continuous measurements of carbon dioxide and methane in the surface water and in the atmosphere in the course of the vessel were conducted by a mobile shipborne system, which was complemented with the flow-through fluorimeter. It has been found that the main features of fluxes and the value of dРg, as well as fluorescent characteristics are in a good agreement with the results of many-year measurements obtained in the BALO Observatory for the summer period.