Surface Layer Scaling Research Articles

Uniform momentum and temperature zones in unstably stratified turbulent flows

Wall-bounded turbulent flows exhibit a zonal arrangement, in which streamwise velocity organizes into uniform momentum zones (UMZs), separated by thin layers of elevated interfacial shear. While significant research efforts have focused on these structural features in neutrally stratified flows, the effects of unstable thermal stratification on UMZs and on analogous uniform temperature zones (UTZs) have not been considered previously. In this article, statistical properties of UMZs and UTZs are investigated using a suite of large eddy simulations of unstably stratified turbulent channel flow spanning weakly to highly convective conditions. When normalized by the friction velocity and stability-dependent mixing length, the mean velocity gradient based on UMZ interfacial velocity jumps and the vorticity thickness exhibits good collapse for all stabilities, establishing a link between UMZ properties and scaling predictions from Monin–Obukhov similarity theory. A similar relationship is found between UTZ properties and surface-layer scaling of the mean temperature gradient. In the mixed layer, mean UMZ depth is quasi-constant with wall-normal distance, while the deepest UTZs are found in the centre of the boundary layer. These instantaneous structures are found to be linked to the well-mixed velocity and temperature profiles in the convective mixed layer. Conditional averaging indicates that both UMZ and UTZ interfaces are associated with ejections of momentum and warm updrafts below the interface and sweeps of momentum and cool downdrafts above the interface. These results demonstrate a tangible connection between instantaneous structural features, mean properties and scaling laws in unstably stratified flows.

Turbulence in a coastal environment: the case of Vindeby

Abstract. The one-point and two-point power spectral densities of the wind velocity fluctuations are studied using the observations from an offshore mast at Vindeby Offshore Wind Farm, for a wide range of thermal stratifications of the atmosphere. A comparison with estimates from the FINO1 platform (North Sea) is made to identify shared spectral characteristics of turbulence between different offshore sites. The sonic anemometer measurement data at 6, 18, and 45 m a.m.s.l. (above mean sea level) are considered. These heights are lower than at the FINO1 platform, where the measurements were collected at heights between 40 and 80 m. Although the sonic anemometers are affected by transducer-flow distortion, the spectra of the along-wind velocity component are consistent with those from FINO1 when surface-layer scaling is used, for near-neutral and moderately diabatic conditions. The co-coherence of the along-wind component, estimated for vertical separations under near-neutral conditions, matches remarkably well with the results from the dataset at the FINO1 platform. These findings mark an important step toward more comprehensive coherence models for wind load calculation. The turbulence characteristics estimated from the present dataset are valuable for better understanding the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, the datasets recorded at Vindeby and FINO1 cover only the lower part of the rotor of state-of-the-art offshore wind turbines. Further improvements in the characterisation of atmospheric turbulence for wind turbine design will require measurements at heights above 100 m a.m.s.l.

The role of scale factor in the formation of prod-uct properties under the action of surface modifi-cation

Based on theoretical and experimental research in the fields of solid-state physics and physical materials science, it is proved that the surface layer of a solid body, which is deformed, is an independent functional subsystem and radically affects large-scale levels of plastic flow and destruction of the product as a whole. As is known, the most effective method of improving the performance of products is the grinding of grain, because it is the grain boundary (substructural) mechanism of strengthening which provides an increase in the structural strength of the product. In this regard, special attention is paid to submicro- and nano-structuring of the surface. Goal. The aim of the work is to study the process of structure formation of the surface layer under the action of ion bombardment (IB) and its effect on the properties of products taking into account the scale factor. To achieve this goal, the following tasks were set: to evaluate the characteristics of the surface microstructure after IB and to study its tensile behavior in cylindrical and flat samples of low-carbon steel in order to take into account the scale factor in changing their properties. The submicro-structuring of the surface by ion bombardment is carried out in the work and its influence on the behavior of products during tensile deformation is investigated. It is established that the presence of a thin modified layer (with a constant core) significantly changes the properties of the product under force. The decisive role belongs to the contribution of the surface layer (scale factor) – the ratio of the area of the modified layer to the volume of the product: if it is <1 the effect of hardening is better realized while maintaining plasticity, and if ˃ 1, it is a significant effect of plasticization which maintains (or even increases) hardening.

The COTUR project: remote sensing of offshore turbulence for wind energy application

Abstract. The paper presents the measurement strategy and data set collected during the COTUR (COherence of TURbulence with lidars) campaign. This field experiment took place from February 2019 to April 2020 on the southwestern coast of Norway. The coherence quantifies the spatial correlation of eddies and is little known in the marine atmospheric boundary layer. The study was motivated by the need to better characterize the lateral coherence, which partly governs the dynamic wind load on multi-megawatt offshore wind turbines. During the COTUR campaign, the coherence was studied using land-based remote sensing technology. The instrument setup consisted of three long-range scanning Doppler wind lidars, one Doppler wind lidar profiler and one passive microwave radiometer. Both the WindScanner software and LidarPlanner software were used jointly to simultaneously orient the three scanner heads into the mean wind direction, which was provided by the lidar wind profiler. The radiometer instrument complemented these measurements by providing temperature and humidity profiles in the atmospheric boundary layer. The scanning beams were pointed slightly upwards to record turbulence characteristics both within and above the surface layer, providing further insight on the applicability of surface-layer scaling to model the turbulent wind load on offshore wind turbines. The preliminary results show limited variations of the lateral coherence with the scanning distance. A slight increase in the identified Davenport decay coefficient with the height is partly due to the limited pointing accuracy of the instruments. These results underline the importance of achieving pointing errors under 0.1∘ to study properly the lateral coherence of turbulence at scanning distances of several kilometres.

Potential and challenges of wind measurements using met-masts in complex topography for bridge design: Part II – Spectral flow characteristics

This study quantifies the influence of the local topography on sonic anemometer measurements mounted on tall met-masts deployed in Norwegian fjords. This assessment is done by analysing the second-order structure of turbulence in the frequency space. The objective is to evaluate how the data collected by these masts can help with the design of future fjord-crossing bridges. Using measurements from eight masts deployed in three different fjords, spectral characteristics common to the different locations were identified. Among the characteristics investigated, the one-point velocity spectra and the vertical coherence of turbulence are of special interest for bridge design. Using only situations with a mean wind speed of 12 ​m ​s−1 and above and rather stationary flow conditions, the remaining wind direction sectors could be classified into two main groups: those with a long fetch over smooth topography and those with a rough heterogeneous terrain upstream of the masts. It was found that if the velocity spectra are normalized by an appropriate estimate of the friction velocity, surface-layer scaling may be applicable in Norwegian fjords, at least under near-neutral conditions. The velocity spectra, estimated with long upstream fjord fetches have characteristics consistent with those collected in coastal and offshore environments. The Davenport model showed limited capabilities in describing the co-coherence of turbulence on vertical separations. The dependency of the Davenport decay coefficient with the height and spatial separation is highlighted and may be substantial in mountainous terrain. Although the spectral flow characteristics estimated on the shore of fjords are relevant for the design of fjord-crossing bridge towers, they may not easily be extrapolated for the modelling of the turbulent flow in the middle of the fjord. Nevertheless, they are still valuable to complement computational fluid dynamic simulations, wind tunnel tests, or remote sensing of wind across the fjord.

Stability to oxidation resistance of heat-resistant nickel alloy with γ′-phase structure

Heightened interest to multicomponent alloying of nickel is connected with the search of new compositions of oxidation- and heat-resistant alloys on the basis of nickel solid solution or its intermetallics. In the present work, the author has investigated the resistance to high-temperature oxidation of an alloy of Ni – Al – Mo – W – Nb system which can be used as a basis for creation of dispersion-strengthened inert particles of carbides and nitrides of two-phase thermally stable superalloys with a γ′-phase matrix. Samples of the alloy were subjected to oxidation on air at 900 – 1300 °C during 1 – 125 hours. Weight reduction ( ΔМ , gr) was measured which after that was recalculated into indicators of change of samples weight for a time unit, rationing for the area of initial samples surface ( Δm , gr/(m 2 ·hour)) and “burn” rate of surface layer (scaling loss h, micron/hour). It is shown that at oxidation of Ni – Al – Mo – W – Nb alloy at all temperatures there is a reduction of samples weight because of formation of fragile and friable superficial scale. Dependences of this indicator on oxidation time are close to the linear. With growth of temperature, processes of weight reduction are intensified. It is offered to raise oxidation resistance of Ni – Al – Mo – W – Nb alloy by short-term preliminary oxidation at temperature of 1300 °С on air. The observable effect of increase of oxidation resistance is caused by formation in scale of NiAl 2 O 4 layer, more effectively protecting an alloy from interaction with oxygen. Experiences on oxidation with the use of inert platinum marks have shown that it is necessary to consider oxygen diffusion through oxide film into metal as a mechanism, supervising oxidation of Ni – Al – Mo – W – Nb alloy at high temperatures in case of presence of NiAl 2 O 4 on the surface layer. Activation energy of oxidation of Ni – Al – Mo – W – Nb alloy was calculated at 900 – 1300 °С and without preliminary oxidation. This value is equal to 220,000 J/mol that is characteristic for activation energy of nickel self-diffusion.

Atmospheric Science with InSight

In November 2018, for the first time a dedicated geophysical station, the InSight lander, will be deployed on the surface of Mars. Along with the two main geophysical packages, the Seismic Experiment for Interior Structure (SEIS) and the Heat-Flow and Physical Properties Package (HP3), the InSight lander holds a highly sensitive pressure sensor (PS) and the Temperature and Winds for InSight (TWINS) instrument, both of which (along with the InSight FluxGate (IFG) Magnetometer) form the Auxiliary Sensor Payload Suite (APSS). Associated with the RADiometer (RAD) instrument which will measure the surface brightness temperature, and the Instrument Deployment Camera (IDC) which will be used to quantify atmospheric opacity, this will make InSight capable to act as a meteorological station at the surface of Mars. While probing the internal structure of Mars is the primary scientific goal of the mission, atmospheric science remains a key science objective for InSight. InSight has the potential to provide a more continuous and higher-frequency record of pressure, air temperature and winds at the surface of Mars than previous in situ missions. In the paper, key results from multiscale meteorological modeling, from Global Climate Models to Large-Eddy Simulations, are described as a reference for future studies based on the InSight measurements during operations. We summarize the capabilities of InSight for atmospheric observations, from profiling during Entry, Descent and Landing to surface measurements (pressure, temperature, winds, angular momentum), and the plans for how InSight’s sensors will be used during operations, as well as possible synergies with orbital observations. In a dedicated section, we describe the seismic impact of atmospheric phenomena (from the point of view of both “noise” to be decorrelated from the seismic signal and “signal” to provide information on atmospheric processes). We discuss in this framework Planetary Boundary Layer turbulence, with a focus on convective vortices and dust devils, gravity waves (with idealized modeling), and large-scale circulations. Our paper also presents possible new, exploratory, studies with the InSight instrumentation: surface layer scaling and exploration of the Monin-Obukhov model, aeolian surface changes and saltation / lifing studies, and monitoring of secular pressure changes. The InSight mission will be instrumental in broadening the knowledge of the Martian atmosphere, with a unique set of measurements from the surface of Mars.

Acoustic inversion for Monin-Obukhov similarity parameters from wind noise in a convective boundary layer.

The prediction accuracy of outdoor sound is in large part limited by uncertainties in the state of the atmosphere. These uncertainties can potentially be reduced by inferring scaling parameters of the atmospheric surface layer from wind noise. Screened microphones sense wind noise as a result of mean atmospheric flow, turbulent eddy interaction with the windscreen, and pressure fluctuations within the turbulent flow. Under conditions of terrain homogeneity and atmospheric quasi-steadiness, the Monin-Obukhov similarity theory (MOST) states that only a handful of parameters governs the dynamics of the atmospheric surface layer. This study explores the relationships of atmospheric similarity parameters to the acoustic spectrum of wind noise in a convective boundary layer. Ambient noise data collected in a high desert during a 2007 long-range sound propagation experiment are analyzed for the purposes of establishing a nondimensional empirical relationship between the acoustic power spectrum and MOST parameters. Furthermore, this paper examines the consequences of inferring surface-layer scaling parameters with different parameter priors. This study shows that, for minimizing the variance in the inversion, the most important parameter to constrain is the Obukhov length.

Investigation of the pressure–strain-rate correlation and pressure fluctuations in convective and near neutral atmospheric surface layers

The pressure–strain-rate correlation and pressure fluctuations in convective and near neutral atmospheric surface layers are investigated. Their scaling properties, spectral characteristics, the contributions from the different source terms in the pressure Poisson equation and the effects of the wall are investigated using high-resolution (up to $2048^{3}$) large-eddy simulation fields and through spectral predictions. The pressure–strain-rate correlation was found to have the mixed-layer and surface-layer scaling in the strongly convective and near neutral atmospheric surface layers, respectively. Its apparent surface-layer scaling in the moderately convective surface layer is due to the slow variations of the mixed-layer contribution, and is an inherent problem for single-point statistics in a multi-scale surface layer. In the strongly convective surface layer the pressure spectrum has an approximate $k^{-5/3}$ scaling range for small wavenumbers ($kz\ll 1$) due to the turbulent–turbulent contribution, and does not follow the surface-layer scaling, where $k$ and $z$ are the horizontal wavenumber and the distance from the surface respectively. The pressure–strain-rate cospectrum components have a $k^{-1}$ scaling range, consistent with our prediction using the surface layer parameters. It is dominated by the buoyancy contribution. Thus the anisotropy in the surface layer is due to the energy redistribution caused by the density fluctuations of the large eddies, rather than the turbulent–turbulent (inertial) effects. In the near neutral surface layer, the turbulent–turbulent and rapid contributions are primarily responsible for redistribution of energy from the streamwise velocity component to the vertical and spanwise components, respectively. The pressure–strain-rate cospectra peak near $kz\sim 1$, and have some similarities to those in the strongly convective surface layer for $kz\ll 1$. For the moderately convective surface layer, the pressure–strain-rate cospectra change signs at scales of the order of the Obukhov length, thereby imposing it as a horizontal length scale in the surface layer. This result provides strong support to the multipoint Monin–Obukhov similarity recently proposed by Tong & Nguyen (J. Atmos. Sci., vol. 72, 2015, pp. 4337–4348). We further decompose the pressure into the free-space (infinite domain), the wall reflection and the harmonic contributions. In the strongly convective surface layer, the free-space contribution to the pressure–strain-rate correlation is dominated by the buoyancy part, and is the main cause of the surface-layer anisotropy. The wall reflection enhances the anisotropy for most of the surface layer, suggesting that the pressure source has a large coherence length. In the near neutral surface layer, the wall reflection is small, suggesting a much smaller source coherence length. The present study also clarifies the understanding of the role of the turbulent–turbulent pressure, and has implications for understanding the dynamics and structure as well as modelling the atmospheric surface layer.

Air–Sea/Land Interaction in the Coastal Zone

Atmospheric turbulence measurements made at the U.S. Army Corps of Engineers Field Research Facility (FRF) located on the Atlantic coast near the town of Duck, North Carolina during the CASPER-East Program (October–November 2015) are used to study air–sea/land coupling in the FRF coastal zone. Turbulence and mean meteorological data were collected at multiple levels (up to four) on three towers deployed at different landward distances from the shoreline, with a fourth tower located at the end of a 560-m-long FRF pier. The data enable comparison of turbulent fluxes and other statistics, as well as investigations of surface-layer scaling for different footprints, including relatively smooth sea-surface conditions and aerodynamically rough dry inland areas. Both stable and unstable stratifications were observed. The drag coefficient and diurnal variation of the sensible heat flux are found to be indicators for disparate surface footprints. The drag coefficient over the land footprint is significantly greater, by as much as an order of magnitude, compared with that over the smooth sea-surface footprint. For onshore flow, the internal boundary layer in the coastal zone was either stable or (mostly) unstable, and varied dramatically at the land-surface discontinuity. The offshore flow of generally warm air over the cooler sea surface produced a stable internal boundary layer over the ocean surface downstream from the coast. While the coastal inhomogeneities violate the assumptions underlying Monin–Obukhov similarity theory (MOST), any deviations from MOST are less profound for the scaled standard deviations and the dissipation rate over both water and land, as well as for stable and unstable conditions. Observations, however, show a poor correspondence with MOST for the flux-profile relationships. Suitably-averaged, non-dimensional profiles of wind speed and temperature vary significantly among the different flux towers and observation levels, with high data scatter. Overall, the statistical dependence of the vertical gradients of scaled wind speed and temperature on the Monin–Obukhov stability parameter in the coastal area is weak, if not non-existent.

The function of the turbulent energy dissipation in the atmospheric surface layer

Introduction. The problem of energetics of atmospheric processes is one of the important problem in the modern meteorology. Distribution of the sources and sinks of energy, especially near the surface, also turbulent transport and transformation of energy reflect all features of the processes generated in the atmospheric boundary layer, their behavior and intensity.
 Purpose. A purpose of the work is a description of processes of production of turbulent kinetic energy (TKE) and dissipation rate of TKE in the surface layer, because transition of heat energy into TKE and vice versa, occurs continuously in the presence of wind shift. These changes are reflected in the actual state of the surface layer, which is registered with the meteorological observations.
 Investigation methods. On the basis of the assumption that the TKE dissipation rate is directly proportional to its production, an atmospheric surface layer model, including the surface layer parameterization based on the Monin-Obukhov similarity theory and the modified Businger relationships, is proposed. The model provides sufficiently accurate assessment of the energy conservation law both on the surface and in the air flux. Statistical features of atmospheric turbulence, such as the vertical gradients of substances, dispersion and covariance are universal functions of hydrostatic stability of the surface layer, therefore non-dimensional functions of TKE dissipation and smoothing of temperature inhomogeneity are determined from the TKE budget equation, which are given to a non-dimensional form.
 Results. The dependence of dissipation from stratification and wind velocity over surfaces with different roughness is defined. Under unstable stratification the function of dissipation is greater than 1 (je>1) and in-creases with its growth, under stable stratification the function decreases to zero. If the conditions are close to neutral, the function form becomes an S-shape one. Under weak wind velocities (u £ 2m/s), the dissipation rate is small and does not exceed 5 cm2/c3. With increasing wind velocities, the stratification of layer tends to the neutral condition and the dissipation rate increases to 100-150 cm2/c3 depending on the surface roughness. The results are good consistent with the measurements, carried out for different stratifications and wind velocities.
 A similar test of “closure” of the TKE budget equation is executed for the forces, representing the buoyancy effect. Ratio of the non-dimensional function of rate of smoothing of temperature inhomogeneity, jq, and temperature gradient, jT, is equal to 1 ((jq/jT)»1,0), that confirms satisfiability of all the energy budget equations for the surface layer, used in the proposed model.
 Conclusion. The results are obtained not by observations, but by the atmospheric surface layer model, including all the energy budget equations, which provides execution of energy conservation law both on the surface and in the air flux. Standard meteorological observations contain information not only about meteorological parameters, representing the actual weather conditions, but also information about the surface layer scaling parameters, which allow to determine both the internal and sometimes external parameters of the surface layer and the atmospheric boundary layer.

The influence of early degenerative changes on the vulnerability of articular cartilage to impact-induced injury

BackgroundRecently, the structural changes in a bovine model of early degeneration were validated by our research group to be analogous to that in early human osteoarthritis. The hypothesis of this study was that the structural changes associated with increasing levels of degeneration would lead to higher levels of tissue damage in response to impact induced injury. MethodsA total of forty bovine patellae were obtained for this study. Cartilage-on-bone samples were extracted from the distal lateral quarter, a region known to be affected by varying levels of degeneration. A single impact drop test was applied to these samples delivering 2.3J of energy. A dynamic load cell and image capture at 2000fps allowed for the calculation of the reaction stress and coefficient of restitution. The extent of tissue damage was examined from the micro to ultrastructural levels using differential interference contrast optical microscopy and scanning electron microscopy respectively. FindingsThe impact mechanical properties of mildly degenerate articular cartilage were not significantly different but showed a significantly larger amount of structural damage. From comparing the mechanical and structural response of intact and mildly degenerate cartilage, to tissue showing increased macro-scale tissue degeneration, the significance of the surface layer and fibrillar scale transverse interconnectivity in effectively attenuating impact loads is demonstrated in this study. InterpretationThis study shows that even though articular cartilage can appear visibly normal under macroscopic observation, the micro-scale structural changes associated with very early stage osteoarthritis can have a significant effect on its vulnerability to impact damage.

Simulation of atmospheric dispersion of NOX over complex terrain region of Ranchi with FLEXPART-WRF by incorporation of improved turbulence intensity relationships

Abstract Accurate representation of air pollutant dispersion is essential for environmental management and planning purposes. In this study, semi-empirical relationships of turbulence intensity (σ u /u * , σ v /u * and σ w /u * ) as a function of surface layer scaling and local stability are developed following boundary layer similarity concepts at Ranchi, a complex terrain in Jharkhand, Eastern India for various seasons. The impact of the new turbulence parameterization for air pollution dispersion simulation is studied by incorporating the same in the Hanna scheme of FLEXPART-WRF Lagrangian Particle dispersion model over study region. The model is used to estimate the ground level concentrations of nitrogen oxides (NO x ) due to industrial and vehicular sources in study region. The meteorological parameters needed in air-quality simulation are simulated using the Advanced Research WRF (ARW) mesoscale model at high resolution (3 km). Three turbulence schemes (YSU, MYNN2 and ACM2) in ARW are alternatively tested in dispersion simulation and comparisons are made with available air quality data for eight days in different seasons (winter, pre-monsoon, monsoon and post-monsoon). Simulations with FLEXPART revealed distinct seasonal variation of dispersion patterns. It has been found that the new turbulence intensity relationships in FLEXPART improved the NO x concentration estimates by reducing the negative bias seen with default Hanna scheme. Further, the ARW simulated meteorological parameters using ACM2 and MYNN2 significantly reduced the bias in modeled pollutant concentrations. The study demonstrates the utility of high quality seasonal turbulence measurements in pollution dispersion model for better diffusion parameterization needed in air quality modeling.

Formulation of stability-dependent empirical relations for turbulent intensities from surface layer turbulence measurements for dispersion parameterization in a lagrangian particle dispersion model

Season- and stability-dependent turbulence intensity (σu/u*, σv/u*, σw/u*) relationships are derived from experimental turbulence measurements following surface layer scaling and local stability at the tropical coastal site Kalpakkam, India for atmospheric dispersion parameterization. Turbulence wind components (u′, v′, w′) measured with fast response UltraSonic Anemometers during an intense observation campaign for wind field modeling called Round Robin Exercise are used to formulate the flux–profile relationships using surface layer similarity theory and Fast Fourier Transform technique. The new relationships (modified Hanna scheme) are incorporated in a Lagrangian Particle Dispersion model FLEXPART-WRF and tested by conducting simulations for a field tracer dispersion experiment at Kalpakkam. Plume dispersion analysis of a ground level hypothetical release indicated that the new turbulent intensity formulations provide slightly higher diffusivity across the plume relative to the original Hanna scheme. The new formulations for σu, σv, σw are found to give better agreement with observed turbulent intensities during both stable and unstable conditions under various seasonal meteorological conditions. The simulated concentrations using the two methods are compared with those obtained from a classical Gaussian model and the observed SF6 concentration. It has been found that the new relationships provide comparatively higher diffusion across the plume relative to the model default Hanna scheme and provide downwind concentration results in better agreement with observations.

Canopy-wake dynamics and wind sheltering effects on Earth surface fluxes

The atmospheric boundary layer adjustment at the abrupt transition from a canopy (forest) to a flat surface (land or water) is investigated in a wind tunnel experiment. Detailed measurements examining the effect of canopy turbulence on flow separation, reduced surface shear stress and wake recovery are compared to data for the classical case of a solid backward-facing step. Results provide new insights into the interpretation for flux estimation by eddy-covariance and flux gradient methods and for the assessment of surface boundary conditions in turbulence models of the atmospheric boundary layer in complex landscapes and over water bodies affected by canopy wakes. The wind tunnel results indicate that the wake of a forest canopy strongly affects surface momentum flux within a distance of 35–100 times the step or canopy height, and mean turbulence quantities require distances of at least 100 times the canopy height to adjust to the new surface. The near-surface mixing length in the wake exhibits characteristic length scales of canopy flows at the canopy edge, of the flow separation in the near wake and adjusts to surface layer scaling in the far wake. Components of the momentum budget are examined individually to determine the impact of the canopy wake. The results demonstrate why a constant flux layer does not form until far downwind in the wake. An empirical model for surface shear stress distribution from a forest canopy to a clearing or lake is proposed.

High nocturnal ozone levels at a surface site in Kolkata, India: Trade-off between meteorology and specific nocturnal chemistry

The objective of our study is to evaluate the magnitude, frequency and time of nocturnal ozone maxima in a tropical urban city (Kolkata, India). Measurements are made for a continuous period of 6months from mid-October, 2010 to April, 2011. The maximum ozone observed is 48ppbv. On an average the occurrences of maxima are predominant during the early morning hours, 0300–0500h (IST), though there are incidences that maxima also appeared early at night (1900h). The frequency of occurrence is found to be maximum for the range of 10–16ppbv. The nocturnal ozone concentrations are further analyzed with respect to the meteorological parameters like pressure, temperature, relative humidity, wind speed, wind direction and Bulk Richardson Number (Rb), a surface layer scaling parameter. The night time chemistry of nitrogen oxides (NOx) and ozone (O3) are also explored in presence of the NO2–NO3–N2O5 cycle. There is an interesting trade-off between chemistry, vertical and horizontal transport as the night progresses.

The Apsley and Castro Limited-Length-Scale $${{k-\varepsilon}}$$ Model Revisited for Improved Performance in the Atmospheric Surface Layer

The limited-length-scale \({k-\varepsilon}\) model proposed by Apsley and Castro for the atmospheric boundary layer (Boundary-Layer Meteorol 83(1):75–98, 1997) is revisited with special attention given to its predictions in the constant-stress surface layer. The original model proposes a modification to the length-scale-governing \({\varepsilon}\) equation that ensures consistency with surface-layer scaling in the limit of small lm/lmax (where lm is the mixing length and lmax its maximum) and yet imposes a limit on lm as lm/lmax approaches one. However, within the equilibrium surface layer and for moderate values of z/lmax, the predicted profiles of velocity, mixing length, and dissipation rate using the Apsley and Castro model do not coincide with analytical solutions. In view of this, a general \({\varepsilon}\) transport equation is derived herein in terms of an arbitrary desired mixing-length expression that ensures exact agreement with corresponding analytical solutions for both neutral and stable stability. From this result, a new expression for \({C_{\varepsilon3}}\) can be inferred that shows this coefficient tends to a constant only for limiting values of z/L; and, furthermore, that the values of \({C_{\varepsilon3}}\) for z/L → 0 and z/L →∞ differ by a factor of exactly two.

The influence of Langmuir turbulence on the scaling for the dissipation rate in the oceanic boundary layer

A model for estimating the turbulent kinetic energy dissipation rate in the oceanic boundary layer, based on insights from rapid‐distortion theory, is presented and tested. This model provides a possible explanation for the very high dissipation levels found by numerous authors near the surface. It is conceived that turbulence, injected into the water by breaking waves, is subsequently amplified due to its distortion by the mean shear of the wind‐induced current and straining by the Stokes drift of surface waves. The partition of the turbulent shear stress into a shear‐induced part and a wave‐induced part is taken into account. In this picture, dissipation enhancement results from the same mechanism responsible for Langmuir circulations. Apart from a dimensionless depth and an eddy turn‐over time, the dimensionless dissipation rate depends on the wave slope and wave age, which may be encapsulated in the turbulent Langmuir numberLat. For large Lat, or any Lat but large depth, the dissipation rate tends to the usual surface layer scaling, whereas when Lat is small, it is strongly enhanced near the surface, growing asymptotically as ε ∝ Lat−2 when Lat → 0. Results from this model are compared with observations from the WAVES and SWADE data sets, assuming that this is the dominant dissipation mechanism acting in the ocean surface layer and statistical measures of the corresponding fit indicate a substantial improvement over previous theoretical models. Comparisons are also carried out against more recent measurements, showing good order‐of‐magnitude agreement, even when shallow‐water effects are important.

Source areas for a passively diffusing methane gas from South Asian paddy fields using the Flux Footprint Model

The annual rate of methane emissions from paddy fields is uncertain due to the differences in climatic conditions, soil characteristics, duration and patterns of flooding, and other parameters. Thus it is important to identify and estimate the source patches of methane emissions from the paddy fields, and also the magnitude of the flux.The main aim of this work is to identify and quantify the specific source areas for passively diffusing methane gas from a typical South Asian Paddy Field. The SAM2 model is used to characterize each of the source patches corresponding to each of the methane fluxes computed by an indirect method, based on the analytical solution of the Eulerian advection–diffusion equation for vertical diffusion. The results of the model are presented as characteristic dimensions of the source patches: the maximum source location (i.e. the upwind distance of the surface element with the maximum-weight influence), the near and far ends of the source area, and its maximal lateral extension.Micrometeorological parameters like wind direction, wind speed, temperature, relative humidity and concentration of methane gas are measured at certain time intervals. These data are fed into the meteorological pre-processor to generate time dependent atmospheric scaling factors (Richardson Number, Monin–Obukhov Length etc.). These are used directly in SAM2 to generate the elliptic patches with P (the fraction of the total integrated source weight function) varying from 40% (stable) to 99% (unstable). The simulations of SAM2 are related directly to the non-dimensional surface-layer scaling variables by the well-known Levenburg–Marquardt Non-Linear Least Square Method in a parameterized model, which provides a user-friendly estimate of the surface area responsible for the measured methane concentration. The application of SAM2 allows us to draw conclusions about the appropriateness of the spatial representation and localness of methane concentration measurements.

On the Connection between Dissipation Enhancement in the Ocean Surface Layer and Langmuir Circulations

Abstract A mechanism for the enhancement of the viscous dissipation rate of turbulent kinetic energy (TKE) in the oceanic boundary layer (OBL) is proposed, based on insights gained from rapid-distortion theory (RDT). In this mechanism, which complements mechanisms purely based on wave breaking, preexisting TKE is amplified and subsequently dissipated by the joint action of a mean Eulerian wind-induced shear current and the Stokes drift of surface waves, the same elements thought to be responsible for the generation of Langmuir circulations. Assuming that the TKE dissipation rate ɛ saturates to its equilibrium value over a time of the order one eddy turnover time of the turbulence, a new scaling expression, dependent on the turbulent Langmuir number, is derived for ɛ. For reasonable values of the input parameters, the new expression predicts an increase of the dissipation rate near the surface by orders of magnitude compared with usual surface-layer scaling estimates, consistent with available OBL data. These results establish on firmer grounds a suspected connection between two central OBL phenomena: dissipation enhancement and Langmuir circulations.