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The present study introduces "WindAware", a wind and turbulence prediction system that provides nowcasts of wind and turbulence parameters every 5 min up to 6 h over a predetermined airway over Chicago, Illinois, USA, based on 100 m high-resolution simulations (HRSs). This system is a long short-term memory-based recurrent neural network (LSTM-RNN) that uses existing ground-based wind data to provide nowcasts (forecasts up to 6 h every 5 min) of wind speed, wind direction, wind gust, and eddy dissipation rate to support the Uncrewed Aircraft Systems (UASs) safe integration into the National Airspace System (NAS). These HRSs are validated using both ground-based measurements over airports and upper-air radiosonde observations and their skill is illustrated during lake-breeze events. A reasonable agreement is found between measured and simulated winds especially when the boundary layer is convective, but the timing and inland penetration of lake-breeze events are overall slightly misrepresented. The WindAware model is compared with the classic multilayer perceptron (MLP) and the eXtreme Gradient Boosting (XGBoost) models. It is demonstrated by comparison to high-resolution simulations that WindAware provides more accurate predictions than the MLP over the 6 h lead times and has almost similar performance as the XGBoost model although the XGBoost's training is the fastest using its parallelized implementation. WindAware also has higher prediction errors when validated against lake-breeze events data due to their under-representation in the training dataset. [ABSTRACT FROM AUTHOR]

An approximation of the turbulent moments of the atmospheric convective layer is based on a variant of the local similarity theory using the concepts of the semiempirical theory of Prandtl turbulence. In the proposed variant of the local similarity theory, the second moment of vertical velocity and the "spectral" Prandtl mixing length are used as basic parameters. This approach allows us to extend Prandtl's theory to turbulent moments of vertical velocity and buoyancy and additionally offer more than ten new approximations. A comparison of the proposed approximation with other variants of the theory of local similarity is considered. It is shown that the selected basic parameters significantly improve the agreement between the local similarity approximations and experimental data. The approximations are consistent with observations in the turbulent convective layer of the atmosphere, the upper boundary of which nearly corresponds to the lower boundary of the temperature inversion. Analytical approximations of local similarity can find applications in the construction of high-order moment closures in the vortex of resolving numerical turbulence models, as well as in the construction of "mass-flux" parametrization. [ABSTRACT FROM AUTHOR]

Mesoscale‐to‐microscale coupling is an important tool for conducting turbulence‐resolving multiscale simulations of realistic atmospheric flows, which are crucial for applications ranging from wind energy to wildfire spread studies. Different techniques are used to facilitate the development of realistic turbulence in the large‐eddy simulation (LES) domain while minimizing computational cost. Here, we explore the impact of a simple and computationally efficient Stochastic Cell Perturbation method using momentum perturbation (SCPM‐M) to accelerate turbulence generation in boundary‐coupled LES simulations using the Weather Research and Forecasting model. We simulate a convective boundary layer (CBL) to characterize the production and dissipation of turbulent kinetic energy (TKE) and the variation of TKE budget terms. Furthermore, we evaluate the impact of applying momentum perturbations of three magnitudes below, up to, and above the CBL on the TKE budget terms. Momentum perturbations greatly reduce the fetch associated with turbulence generation. When applied to half the vertical extent of the boundary layer, momentum perturbations produce an adequate amount of turbulence. However, when applied above the CBL, additional structures are generated at the top of the CBL, near the inversion layer. The magnitudes of the TKE budgets produced by SCPM‐M when applied at varying heights and with different perturbation amplitudes are always higher near the surface and inversion layer than those produced by No‐SCPM, as are their contributions to the TKE. This study provides a better understanding of how SCPM‐M reduces computational costs and how different budget terms contribute to TKE in a boundary‐coupled LES simulation. Plain Language Summary: Grid nesting is a technique in atmospheric modeling where smaller model domains with finer resolutions are embedded in larger domains with coarser resolutions, allowing us to simulate processes for which a range of resolutions is important. When nesting a micro‐scale domain, which resolves atmospheric turbulence, within a mesoscale domain, where turbulence is parameterized instead, the micro‐scale domain needs to generate turbulence. This can take a long distance, reducing the area within the micro‐scale domain where turbulence is fully developed. Turbulence generation methods can speed up this process, leading to more efficient multi‐scale simulations. Here, we take a close look at one such turbulence generation method, which applies random perturbations to the momentum field along the boundaries of the micro‐scale domain. We test several different configurations of momentum perturbations and evaluate their impacts on the turbulent kinetic energy budget within the micro‐scale domain. Our results can be used as guidance on how to apply momentum perturbations most efficiently in grid‐nested simulations. Key Points: The fetch is reduced after applying momentum perturbation and the reduction is directly proportional to the amplitude of the perturbationsApplying momentum perturbation up to half the boundary layer height produces an adequate amount of turbulence without additional entrainmentMomentum perturbations result in turbulent kinetic energy budget terms of higher magnitude than simulations without perturbations [ABSTRACT FROM AUTHOR]

The atmospheric convective boundary layer (CBL) consists of three basic parts: (1) the surface layer unstably stratified and dominated by small-scale turbulence of very complex nature; (2) the CBL core dominated by the energy-, momentum-, and mass-transport of semi-organized structures (large-scale circulations), with a small contribution from small-scale turbulence produced by local structural shears; and (3) turbulent entrainment layer at the upper boundary, characterized by essentially stable stratification with negative (downward) turbulent flux of potential temperature. The energy- and flux budget theory developed previously for atmospheric stably-stratified turbulence and the surface layer in atmospheric convective turbulence is extended to the CBL core using budget equations for turbulent energies and turbulent fluxes of buoyancy and momentum. For the CBL core, we determine global turbulent characteristics (averaged over the entire volume of the semi-organized structure) as well as kinetic and thermal energies of the semi-organized structures as the functions of the aspect ratio of the semi-organized structure, the scale separation parameter between the vertical size of the structures and the integral scale of turbulence and the degree of thermal anisotropy characterized the form of plumes. The obtained theoretical relationships are potentially useful in modeling applications in the atmospheric convective boundary-layer and analysis of laboratory and field experiments, direct numerical simulations, and large-eddy simulations of convective turbulence with large-scale semi-organized structures. [ABSTRACT FROM AUTHOR]

Finite difference modelling of turbulent heat and mass flow visualization finds numerous applications in atmospheric flows/oceanic currents, wind turbines, thermal transfer in nuclear reactors, drag in oil pipelines, cooling of industrial machineries, and to investigate the complexity, dynamic and chaotic nature of the physical system. A turbulent phenomenon is effectively implemented in engineering, physics, earth sciences, bio‐engineering and medicine. Hence, motivated by the advantages of turbulence in various engineering fields, in the current article, a finite difference analysis is performed to demonstrate the k‐ε turbulence model‐based heat and mass lines visualization in boundary layer regime under turbulent buoyancy‐driven convective conditions along a cylinder. Turbulent flow characteristics are accurately explored by deploying the classical Newtonian flow model. Further, to accomplish a more sophisticated finite difference simulation, the effects of extra kinetic energy and its dissipation rate equations are considered. The produced Navier‐Stokes equations for time‐dependent turbulent heat and mass transmission are rendered to non‐dimensional by deploying suitable dimensionless numbers. The advanced coupled nonlinear turbulent unsteady buoyancy‐motivated vertical convection problem is then solved with a well‐sophisticated finite difference scheme such as Crank‐Nicolson technique using computational software. Authentication of current results with former solutions over a range of buoyancy number, Schmidt, and Prandtl parameters are presented. An extensive tabular and graphical discussion along with contours, heat and masslines visualization is included to enumerate the hydro‐dynamic, thermal and mass diffusion behaviour for the impact of emerged regulating numbers in the Prandtl regime. It is confirmed that, the accelerating turbulent buoyancy‐ratio number, maximizes the velocity, kinetic energy and dissipation rate at X=1.0$X = 1.0$. Further, the numerical values of laminar thermal and mass diffusion rates are monotonically enhanced when compared to the turbulent values. Also, to verify the current findings, the authors compared the LRN k‐ε model turbulent results with the existing solutions and found good agreement. Further, the uniqueness and novelty of the current investigation is the exploration of heat and masslines in unsteady buoyancy‐driven convection regime under the influence of k‐ε turbulence model which extends the former studies and offers a more precise appraisal of the thermal and mass diffusion lines via the Crank‐Nicolson analysis. [ABSTRACT FROM AUTHOR]

This study focuses on the application of two standard inflow turbulence generation methods for growing convective boundary layer (CBL) simulations: the recycle-rescale (R-R) and the digital filter-based (DF) methods, which are used in computational fluid dynamics. The primary objective of this study is to expand the applicability of the R-R method to simulations of thermally driven CBLs. This method is called the extended R-R method. However, in previous studies, the DF method has been extended to generate potential temperature perturbations. This study investigated whether the extended DF method can be applied to simulations of growing thermally driven CBLs. In this study, idealized simulations of growing thermally driven CBLs using the extended R-R and DF methods were performed. The results showed that both extended methods could capture the characteristics of thermally driven CBLs. The extended R-R method reproduced turbulence in thermally driven CBLs better than the extended DF method in the spectrum and histogram of vertical wind speed. However, the height of the thermally driven CBL was underestimated in about 100 m compared with the extended DF method. Sensitivity experiments were conducted on the parameters used in the extended DF and R-R methods. The results showed that underestimation of the length scale in the extended DF method causes a shortage of large-scale turbulence components. The other point suggested by the results of the sensitivity experiments is that the length of the driver region in the extended R-R method should be sufficient to reproduce the spanwise movement of the roll vortices. SIGNIFICANCE STATEMENT: Inflow turbulence generation methods for large-eddy simulation (LES) models are crucial for the better downscaling of meteorological mesoscale models (RANS models) to microscale models (LES models). Various CFD methods have been developed, but few have been applied to simulations of thermally driven convective boundary layers (CBLs). To address this problem, we focused on a method that recycles turbulence [the recycle-rescale (R-R) method] and another method that synthetically generates turbulence [the digital filter-based (DF) method]. This study extends the R-R method to manage turbulence in thermally driven CBLs. In addition, this study investigated the applicability of the DF method to thermally driven CBL simulations. Both extended methods are effective for downscaling experiments and capture the characteristics of thermally driven CBLs. [ABSTRACT FROM AUTHOR]

A physically based stochastic perturbation (PSP) scheme has been implemented in the convection-permitting ICON-D2 ensemble prediction system at Deutscher Wetterdienst (DWD) and run for a three-month trial experiment in summer 2021. The scheme mimics the impact of boundary-layer turbulence on the smallest resolved scales and impacts convective precipitation in particular. A weather-regime-dependent systematic evaluation shows that PSP efficiently increases ensemble spread of precipitation in weak synoptic forcing, while producing realistic convective structures. During strong forcing, the effect of the scheme is negligible, as expected by design. A probabilistic verification shows improvements in the forecast skill of other variables as well, especially the spread-to-skill ratio, but identifies starting points for further improvements of the method. [ABSTRACT FROM AUTHOR]

We investigate experimentally the phenomenon of turbulent thermal diffusion of micrometer-size solid particles in an inhomogeneous convective turbulence forced by one vertically oriented oscillating grid in an air flow. This effect causes the formation of large-scale inhomogeneities in particle spatial distributions in a temperature-stratified turbulence. We perform detailed comparisons of the experimental results with those obtained in our previous experiments with an inhomogeneous and anisotropic stably stratified turbulence produced by a one oscillating grid in the air flow. Since the buoyancy increases the turbulent kinetic energy for convective turbulence and decreases it for stably stratified turbulence, the measured turbulent velocities for convective turbulence are larger than those for stably stratified turbulence. This tendency is also seen in the measured vertical integral turbulent length scales. Measurements of temperature and particle number density spatial distributions show that particles are accumulated in the vicinity of the minimum of the mean temperature due to the phenomenon of turbulent thermal diffusion. This effect is observed in both convective and stably stratified turbulence, where we find the effective turbulent thermal diffusion coefficient for micrometer-size particles. The obtained experimental results are in agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

Turbulent dissipation rate (ɛ) is a crucial parameter in turbulence theory, and an essential component of higher‐order planetary boundary layer schemes for numerical weather prediction and climate models. It is most often modeled diagnostically based on the dissipation scaling ɛ ∝ e3/2/L, where e and L are the turbulence kinetic energy (TKE) and the size of the largest turbulent eddies, respectively. Utilizing three‐month‐long vertically‐extended observations accompanied by high resolution large‐eddy simulations, scaling‐based ɛ‐models are evaluated, focusing on their stability dependence under daytime convective conditions. The analysis uncovers biases in the parameterized ɛ profiles that cannot be corrected through tuning of model constants. The biases are attributed to the limited and even opposing stability dependence of the modeled dissipation length. Close examination reveals violation of the dissipation scaling by the inclusion of TKE associated with organized convection. A self‐similar dissipation length is obtained when only the isotropic component of TKE is considered. Plain Language Summary: Turbulent dissipation refers to the conversion of turbulence kinetic energy into heat through molecular viscosity. It determines the statistical behavior of turbulence and is of fundamental importance. Turbulent dissipation is a crucial process in the atmospheric boundary layer (ABL), which is immediately adjacent to the earth surface, and is characterized by vigorous turbulent motions. In numerical weather prediction and climate models, turbulent dissipation rate ɛ is most often modeled based on a scaling relation utilizing the characteristics of the largest turbulent eddies. Such ɛ‐models have seldom been evaluated due to the spatially‐confined and temporally‐limited observations. This study synthesizes a three‐month‐long observation data set obtained from a 300‐m tower in a recent field campaign with high resolution turbulence‐resolving numerical simulations, to investigate the turbulence dissipation in the daytime ABL and to evaluate two scaling‐based ɛ‐models. Analyses reveal model biases that are related to bulk ABL stability, and uncover a fundamental issue that leads to the apparent biases. Both theoretical and practical improvements on ɛ‐models are suggested. Key Points: Observational and numerical evidences uncover biases in parameterized turbulence dissipation rates in planetary boundary layer schemesThe biases are due to the limited and opposing stability dependence of the modeled dissipation length and are unrelated to model constantsThe dissipation length becomes self‐similar after the removal of turbulence kinetic energy associated with organized convection [ABSTRACT FROM AUTHOR]

This study analyses the departure of the velocity-variances profiles from their quasi-steady state described by the mixed-layer similarity, using large-eddy simulations with different prescribed shapes and time scales of the surface kinematic heat flux decay. Within the descriptive frames where the time is tracked solely by the forcing time scale (either constant or time-dependent) describing the surface heat flux decay, we find that the normalized velocity-variances profiles from different runs do not collapse while they depart from mixed-layer similarity. As the mixed-layer similarity relies on the assumption that the free-convective boundary layer is in a quasi-equilibrium, we consider the ratios of the forcing time scales to the convective eddy-turnover time scale. We find that the normalized velocity-variances profiles collapse in the only case where the ratio ( r ~ ) of the time-dependent forcing time scale to the convective eddy-turnover time scale is used for tracking the time, supporting the independence of the departure from the characteristics of the surface heat flux decay. As a consequence of this result, the knowledge of r ~ is sufficient to predict the departure of the velocity variances from their quasi-steady state, irrespective of the shape of the surface heat flux decay. This study highlights the importance of considering both the time-dependent forcing time scale and the convective eddy-turnover time scale for evaluating the response of the free-convective boundary layer to the surface heat flux decay. [ABSTRACT FROM AUTHOR]

This document is a note celebrating the career of Evgeni Fedorovich, the former Editor-in-Chief of Boundary-Layer Meteorology. It highlights his scientific achievements, career milestones, and impacts on the field of boundary-layer meteorology. Evgeni is known for his research on various topics, including entrainment in convective boundary layers, contaminant dispersal in urban settings, and turbulence in slope flows. He has published numerous articles and monographs and has been recognized for his contributions through awards and invitations to collaborate and lead in research and academic communities. Evgeni's career spanned several institutions, including the University of Oklahoma, where he mentored students and conducted research. The authors of the note express their gratitude to Evgeni and wish him a happy retirement. [Extracted from the article]

In this study we examined a data set of nearly two‐year collection and investigated the effects of low‐level jets (LLJ) on near‐surface turbulence, especially wind direction changes, in the nocturnal boundary layer. Typically, nocturnal boundary layer is thermally stratified and stable. When wind profiles exhibit low gradient (in the absence of LLJ), it is characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes (∆WD) in the layers near the surface. In contrast, presence of LLJs can cause dramatic changes through inducing wind velocity shears, enhancing vertical mixing, and weakening the thermal stratification underneath. Ultimately, bulk Richardson number (Rb) is reduced and weakly stable conditions prevail, leading to active turbulence, close coupling across the layers between the LLJ height and ground surface, relatively large vertical momentum and sensible heat fluxes, and suppressed ∆WD values. Rb can be a useful parameter in assessing turbulence strength and ∆WD as well. The dependence of ∆WD on Rb appears to be well defined under weakly stable conditions (0.0 < Rb ≤ 0.25) and ∆WD is generally confined to small values. However, the relationship between ΔWD and Rb breaks when Rb increases, especially Rb > 1.0 (very stable conditions), under which ΔWD varies across a very wide range and the potential for large ΔWD increases greatly. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers. Plain Language Summary: This paper investigated how low‐level jets affect the near‐surface turbulence, especially wind direction changes, in the nocturnal atmospheric boundary layers. Atmospheric stability parameter (e.g., Rb‐bulk Richardson number) can be the key in determining the wind direction changes in the nocturnal atmospheric boundary layer. Under weakly stable conditions (0.0 < Rb ≤ 0.25), in general, wind direction only changes within a small range (less than 10°) between two consecutive 10‐min intervals. In contrast, when Rb > 1.0 (very stable conditions), wind direction can vary across a very wide range and the probability for large wind direction changes also increases. Low‐level jets often enhance the turbulence near the surface, reduce the atmospheric stability and lead to small wind direction shifts. In absence of low‐level jets, nocturnal boundary layers are characterized by very weak turbulence and very large, abrupt, but intermittent wind direction changes (from a couple of 10° to 100°) in the layers near the surface. Our findings have provided important implications to the plume dispersion in the nocturnal boundary layers. Key Points: Bulk Richardson number (Rb) can be a useful parameter in assessing wind direction change (∆WD) near the surface in the nocturnal boundary layer∆WD is generally confined to small values under weakly stable conditions (0.0 < Rb ≤ 0.25) but varies across a very wide range with increasing potential for large ΔWD under very stable conditions (Rb > 1.0), which implies a complicated relationship between Rb and ΔWDPresence of the Low‐Level Jets (LLJ) enhances the turbulence activities, decreases the Rb values, leads to relatively large vertical momentum and sensible heat fluxes, and suppresses ∆WD values [ABSTRACT FROM AUTHOR]

The terms of the budget equations for turbulence kinetic energy (TKE), heat, flux, and temperature variance are evaluated observationally using 4 levels of turbulence observations made over a period of 10 months at a 30-m tower in southern Brazil. All terms are evaluated, except those associated with horizontal transport or pressure perturbations, which were not observed. The analysis is split between daytime, convective conditions and night-time, stable conditions. In both cases, the terms are initially shown in their original dimensional form as a function of the mean wind speed at 3 m, and the observations are then classified by net radiation. During the day, the TKE budget is dominated by shear and buoyant production, while dissipation is the main removal mechanism and the residual analysis indicates that the pressure transport term is positive near the surface. At night, the TKE budget is dominated by shear production and destruction by dissipation, while the buoyant destruction is about an order of magnitude smaller than both. The inferred role of pressure transport depends on the turbulence regime. In the daytime heat flux budget the dominant production mechanism varies between gradient and buoyant production, depending on mean wind speed and net radiation, while most of the destruction is mainly attributed to the pressure covariance term, inferred from the budget residual. At night, buoyancy becomes a relevant destruction mechanism, and its relationship with gradient production is related to the stable boundary layer turbulence regime. Temperature variance budget is a balance between gradient production and dissipation both at day and night, in which case the residual is larger and its possible causes are discussed. Dimensionless expressions for the dominant terms in the three budgets considered are compared to the data, as a function of the stability parameter. [ABSTRACT FROM AUTHOR]

Turbulence parameterization scheme for planetary boundary layer (PBL) based on Taylor's statistical formulation for turbulent flow is used in the Brazilian Global Atmospheric Model (BAM). Taylor's approach has been already applied to mesoscale and air pollution atmospheric models, but it is the first time that this approach is employed in a global model. The BAMmodel is operationally used to generate numerical weather forecasting by the INPE (National Institute for Space Research -- Brazil). The simulation performed with BAM model using Taylor's parameterization is compared with the results obtained with the schemes presented by Holtslag-Boville, Sungsu-Park, and Mellor-Yamada trying to reproduce the ERA5 reanalysis data of two different initial conditions of two different seasons (dry season and wet season). The obtained comparison exhibit a positive result for the new parameterization simulating global precipitation, cloud coverage, and top atmospheric thermal radiation, specially in the dry season. Positive results for the Amazon basin were also obtained. [ABSTRACT FROM AUTHOR]

The wake of an isolated model-scale wind turbine is analysed in a set of inflow conditions having free stream turbulence intensity between 3 % and 12 %, and integral time scales in the range of 0.1–10 times the convective time scale based on the turbine diameter. It is observed that the wake generated by the turbine evolves more rapidly, with the onset of the wake evolution being closer to the turbine, for high turbulence intensity and low integral time scale flows, in accordance with literature, while flows at higher integral time scales result in a slow wake evolution, akin to that generated by low-turbulence inflow conditions despite the highly turbulent ambient condition. The delayed onset of the wake evolution is connected to the stability of the shear layer enveloping the near-wake, which is favoured for low-turbulence or high-integral time scale flows, and to the stability of the helical vortex set surrounding the wake, as this favours interaction events and prevents momentum exchange at the wake boundary which hinder wake evolution. The rate at which the velocity in the wake recovers to undisturbed conditions is instead analytically shown to be a function of the Reynolds shear stress at the wake centreline, an observation that is confirmed by measurements. The rate of production of Reynolds shear stress in the wake is then connected to the power harvested by the turbine to explain the differences between flows at equivalent turbulence intensity and different integral time scales. The wake recovery rate, and by extension the behaviour of the turbine wake in high-integral time scale flows, is seen to be a linear function of the free stream turbulence intensity for flows with Kolmogorov-like turbulence spectrum, in accordance with literature. This relation is seen not to hold for flows with different free stream turbulence spectral distribution; however, this trend is recovered if the contributions of low frequency velocity components to the turbulence intensity are ignored or filtered out from the computation. [ABSTRACT FROM AUTHOR]

Reflected waves have been frequently observed via weather radar, even during sunny, cloudless days. Additionally, it is recognized that turbulence and biological scatterers can also dominate the scattering process. In previous studies, echo returns of turbulence from clear-air Bragg scattering (CABS) have been used to detect the height of the convective boundary layer (CBL). However, in a dual-frequency antenna multiplexing radar system, through which the dual-wavelength ratio (DWR) identifies the CABS clearly, CABS can be unexpectedly observed not only at the edge of the CBL but also above the CBL, highlighting the need for an expanded set of causes of clear-air echoes. It was further identified that the negative second derivative of horizontal wind speed, which is measured by a coherent Doppler Lidar, is consistent with the variation of the CABS layers' height above the CBL. These results emphasize the presence of physical processes leading to turbulence in the troposphere, with implications for Bragg scattering studies and the theory of turbulence in general. This study will help the ecology of research better understand the laws of biological activity. [ABSTRACT FROM AUTHOR]

Kelvin–Helmholtz billows (KHBs) within a rising turbulent layer during the transition period from stable to unstable stratification occurring in the morning hours in summertime at the interior of Antarctica (Dome C, Concordia station) are examined in this study. The wave pattern captured by high-resolution sodar echograms from November 2014–February 2015 exhibits regular braid-like structures, associated with Kelvin–Helmholtz shear instabilities. This phenomenon is observed in more than 70% of days in the selected period. Two main regimes of the morning evolution with KHBs are identified roughly, distinguished by the presence or absence of turbulence in the preceding night-time. The weather and turbulent conditions favouring the occurrence of these regimes are analyzed. Also, two distinct patterns of KHBs are identified: (i) quasi-periodical (with periods ≈ 8–15 min) trains containing 5–10 braids, (ii) about continuous series lasting 20–90 min containing 20–80 braids. A composite shape of KHBs is determined. The periodicity of these waves is estimated to be between 20 and 70 s, and their wavelength is estimated roughly to be 100–400 m. The vertical thickness of individual braids at the wave crests ranges between 5 and 25 m. The total depth of a rising turbulent layer containing these waves varies between 15 and 120 m, and the ratio of the wavelength to the depth of the wave layer varies from 3 to 12 with a mean value ≈ 8.2. The morphology of the turbulence structure in the ABL is studied as a function of both temperature and wind field characteristics retrieved from an instrumented 45-m tower and an ultrasonic anemometer-thermometer at 3.5 m. The observational results highlight the necessity of considering the interaction between convective and wave processes when occurring simultaneously. [ABSTRACT FROM AUTHOR]

Direct numerical simulations (DNS) at bulk Reynolds number Re = 10 4 and bulk Richardson number Ri = 0.25 of plane Couette flow are performed with the results used to analyze the structure and mixing intensity in strongly stable boundary-layer flows. The Couette flow set-up is used as a proxy for a real-world stable boundary layer flow with surface thermal heterogeneity. Along the upper and lower walls, the temperature is either homogeneous or varies sinusoidally, but the horizontal-mean surface temperature is the same in all cases. Over homogeneous surfaces, the strong stratification always quenches turbulence resulting in linear velocity and temperature profiles. However, over a heterogeneous surface turbulence survives. Molecular diffusion and turbulence contribute to down-gradient momentum transfer. The total (diffusive plus turbulent) heat flux is directed downward, but its turbulent contribution is positive, i.e., up the mean temperature gradient. Analysis of covariances of velocity and temperature, their skewness, and the flow structure suggests that counter-gradient heat transport is due to quasi-organized cell-like vortical motions generated by surface thermal heterogeneity. These motions transfer heat upwards similar to their counterparts in highly convective boundary layers. Thus, the flow over heterogeneous surface features local convective instabilities and upward eddy heat transport, although the overall stratification remains stable with downward mean heat transfer. The DNS results are compared to the results from large-eddy simulations of weakly stable boundary layers (Mironov and Sullivan in J Atmos Sci 73:449–464, 2016). The DNS findings corroborate the key role of temperature variance in setting the structure and transport properties of stably stratified flow over heterogeneous surfaces, and the importance of third-order transport of the temperature variance. [ABSTRACT FROM AUTHOR]

We present results from the multi-level mast eddy-covariance measurements conducted over flat grassland and over complex urban terrain composed of trees and small buildings. Relationships between the stability parameter and turbulent statistical properties were analysed. Over flat grassland, non-dimensional turbulent moments up to the third-order obey Monin–Obukhov similarity theory. Third-order moments increase for increasing instability, indicating nonlocal turbulent transport of heat flux due to convective motions, and in this case, the relationship between the third- and second-order moments obtained using the framework of the mass-flux approach might be applicable. In stable conditions, the turbulent transport of heat flux is positive and does not demonstrate an explicit dependence on stability but might be associated with large eddies and sweep motions transporting downwards the heat flux originating near the top of a stable boundary layer. Such a downward transport of negative heat flux by large eddies makes applicable the mass-flux approach even in stable boundary layers at higher levels. Over the urban landscape, for all wind directions the third moment corresponds well with the theoretical curve in cases of stable and unstable stratification, but there is a wide spread of data in near-neutral conditions. Skewness of the vertical velocity component is negative over the urban landscape and its value depends on wind direction. At lower levels, the standard deviation of the vertical velocity component is increasing with stability faster than expected from the theoretical curve. Over the urban landscape, large roughness elements do not affect the performance of the mass-flux approach in the convective boundary layer. Furthermore, there is a link between the second and third moments even in stable boundary layers over heterogeneous landscape. [ABSTRACT FROM AUTHOR]

This paper investigates the surface-layer processes associated with the morning transition from nighttime downslope winds to daytime upslope winds over a semi-isolated massif. It provides an insight into the characteristics of the transition and its connection with the processes controlling the erosion of the temperature inversion at the foot of the slope. First, a criterion for the identification of days prone to the development of purely thermally driven slope winds is proposed and adopted to select five representative case studies. Then, the mechanisms leading to different patterns of erosion of the nocturnal temperature inversion at the foot of the slope are analyzed. Three main patterns of erosion are identified: the first is connected to the growth of the convective boundary layer at the surface, the second is connected to the descent of the inversion top, and the third is a combination of the previous two. The first pattern is linked to the initiation of the morning transition through surface heating, and the second pattern is connected to the top-down dilution mechanism and so to mixing with the above air. The discriminating factor in the determination of the erosion pattern is identified in the partitioning of turbulent sensible heat flux at the surface. Significance Statement: The purpose of this study is to improve our understanding of the thermally driven slope circulations with a focus on the unsteady processes associated with the morning transition and the erosion patterns of the nocturnal temperature inversion, so far in the literature less investigated and understood than the evening transition. Understanding this diurnal process will advance our abilities to model it and to improve the accuracy of weather forecasting in complex terrain. [ABSTRACT FROM AUTHOR]

The connection between the heat transfer and characteristic flow velocities of planetary core-style convection remains poorly understood. To address this, we present novel laboratory models of rotating Rayleigh–Bénard convection in which heat and momentum transfer are simultaneously measured. Using water (Prandtl number, P r ≃ 6 ) and cylindrical containers of diameter-to-height aspect ratios of Γ ≃ 3 , 1.5 , 0.75 , the non-dimensional rotation period (Ekman number, E) is varied between 10 − 7 ≲ E ≲ 3 × 10 − 5 and the non-dimensional convective forcing (Rayleigh number, R a ) ranges from 10 7 ≲ R a ≲ 10 12 . Our heat transfer data agree with those of previous studies and are largely controlled by boundary layer dynamics. We utilize laser Doppler velocimetry (LDV) to obtain experimental point measurements of bulk axial velocities, resulting in estimates of the non-dimensional momentum transfer (Reynolds number, R e ) with values between 4 × 10 2 ≲ R e ≲ 5 × 10 4 . Behavioral transitions in the velocity data do not exist where transitions in heat transfer behaviors occur, indicating that bulk dynamics are not controlled by the boundary layers of the system. Instead, the LDV data agree well with the diffusion-free Coriolis–Inertia–Archimedian (CIA) scaling over the range of R a explored. Furthermore, the CIA scaling approximately co-scales with the Viscous–Archimedian–Coriolis (VAC) scaling over the parameter space studied. We explain this observation by demonstrating that the VAC and CIA relations will co-scale when the local Reynolds number in the fluid bulk is of order unity. We conclude that in our experiments and similar laboratory and numerical investigations with E ≳ 10 − 7 , R a ≲ 10 12 , P r ≃ 7 , heat transfer is controlled by boundary layer physics while quasi-geostrophically turbulent dynamics relevant to core flows robustly exist in the fluid bulk. [ABSTRACT FROM AUTHOR]

The tropical tropopause layer (TTL) is a sea of vertical motions. Convectively generated gravity waves create vertical winds on scales of a few to thousands of kilometers as they propagate in a stable atmosphere. Turbulence from gravity wave breaking, radiatively driven convection, and Kelvin–Helmholtz instabilities stirs up the TTL on the kilometer scale. TTL cirrus clouds, which moderate the water vapor concentration in the TTL and stratosphere, form in the cold phases of large-scale (> 100 km) wave activity. It has been proposed in several modeling studies that small-scale (< 100 km) vertical motions control the ice crystal number concentration and the dehydration efficiency of TTL cirrus clouds. Here, we present the first observational evidence for this. High-rate vertical winds measured by aircraft are a valuable and underutilized tool for constraining small-scale TTL vertical wind variability, examining its impacts on TTL cirrus clouds, and evaluating atmospheric models. We use 20 Hz data from five National Aeronautics and Space Administration (NASA) campaigns to quantify small-scale vertical wind variability in the TTL and to see how it varies with ice water content, distance from deep convective cores, and height in the TTL. We find that 1 Hz vertical winds are well represented by a normal distribution, with a standard deviation of 0.2–0.4 m s -1. Consistent with a previous observational study that analyzed two out of the five aircraft campaigns that we analyze here, we find that turbulence is enhanced over the tropical west Pacific and within 100 km of convection and is most common in the lower TTL (14–15.5 km), closer to deep convection, and in the upper TTL (15.5–17 km), further from deep convection. An algorithm to classify turbulence and long-wavelength (5 km < λ < 100 km) and short-wavelength (λ < 5 km) gravity wave activity during level flight legs is applied to data from the Airborne Tropical TRopopause EXperiment (ATTREX). The most commonly sampled conditions are (1) a quiescent atmosphere with negligible small-scale vertical wind variability, (2) long-wavelength gravity wave activity (LW GWA), and (3) LW GWA with turbulence. Turbulence rarely occurs in the absence of gravity wave activity. Cirrus clouds with ice crystal number concentrations exceeding 20 L -1 and ice water content exceeding 1 mg m -3 are rare in a quiescent atmosphere but about 20 times more likely when there is gravity wave activity and 50 times more likely when there is also turbulence, confirming the results of the aforementioned modeling studies. Our observational analysis shows that small-scale gravity waves strongly influence the ice crystal number concentration and ice water content within TTL cirrus clouds. Global storm-resolving models have recently been run with horizontal grid spacing between 1 and 10 km, which is sufficient to resolve some small-scale gravity wave activity. We evaluate simulated vertical wind spectra (10–100 km) from four global storm-resolving simulations that have horizontal grid spacing of 3–5 km with aircraft observations from ATTREX. We find that all four models have too little resolved vertical wind at horizontal wavelengths between 10 and 100 km and thus too little small-scale gravity wave activity, although the bias is much less pronounced in global SAM than in the other models. We expect that deficient small-scale gravity wave activity significantly limits the realism of simulated ice microphysics in these models and that improved representation requires moving to finer horizontal and vertical grid spacing. [ABSTRACT FROM AUTHOR]

There is a need for improved wind measurements inside the planetary boundary layer (PBL), including the capability to sample turbulent flow. Airborne Doppler lidar (ADL) provides unique capabilities for spatially resolved and targeted wind measurements in the PBL. However, ADL wind profiling in the PBL is challenging, as turbulence violates the flow homogeneity assumption used in wind profile retrieval and thereby introduces error in the retrieved wind profiles. As turbulence is a dominant source of error it is necessary to investigate and optimize ADL wind profiling capabilities in turbulent PBL flow. This study investigates the potential of a novel multiple fixed-beam ADL system design to provide improved wind information in turbulent PBL flow, compared to traditional single scanning-beam ADL systems. To achieve this, an LES-based airborne Doppler lidar simulator presented in Gasch et al. (2020) is employed and extended in this study. Results show that a multiple fixed-beam system with settings comparable to those of commonly used single scanning-beam systems offers distinct advantages. Advantages include overall reduced wind profile retrieval error due to turbulence and improved spatial representation alongside higher wind profile availability. The study also offers insight into the dependence of the retrieval error on system setup parameters and retrieval parameters for both fixed-beam and scanning-beam systems. When using a fixed-beam system, an order of magnitude higher wind profile resolution appears possible, compared to traditional scanning systems at comparable retrieval accuracy. Thus, using multiple fixed-beam systems opens the door towards better sampling of turbulent PBL flow. Overall, the simulator provides a cost-effective tool to investigate and optimize wind profile error characteristics due to turbulence, and to optimize system setup and retrieval strategies for ADL wind profiling in turbulent flow. [ABSTRACT FROM AUTHOR]

Doppler wind lidar measurements were used for the first time in Romania to analyse the wind and turbulence statistics for a peri-urban site located at Măgurele, southwest of Bucharest. Vertical and scanning measurements between December 2019 and November 2021 were processed using an existing toolbox. The statistics over the two-year period were performed on seasonal and diurnal cycle bases. The analyses showed a diurnal cycle for the horizontal wind speed, with lower values during daytime. In the upper part of the planetary boundary layer (PBL), the wind speed is lowest during the day and highest at night (near surface, the behaviour is reversed). The diurnal cycle has variations during the year (from approximately 500 m during midnight winter to approximately 1250 m during summer noon). The wind direction during autumn shows similarities with the summer season, with prevailing directions from east and northeast. The winter season is characterised by westerly winds. The most variable diurnal wind direction is observed during summer, with nighttime westerly winds and changing directions (from northeast to west) during daytime. The ERA5 reanalysis shows similar patterns for wind speed with Doppler wind lidar (slightly underestimated) and direction. The planetary boundary layer classes over the altitude region analysed shows the predominant convection during daytime and non-turbulent behaviour during nighttime. To a lesser extent, the intermittent turbulent class is observed during the growth and the decay of the mixing layer. [ABSTRACT FROM AUTHOR]

This study performs extensive numerical simulations to investigate how the aspect ratio (AR) of a lid-driven cavity influences the onset of elastic instability and elastic turbulence and the subsequent mixed convective heat transfer rate inside it. To this end, we utilize the finite volume method based open source code OpenFOAM along with Rheotool to solve the mass, momentum, energy, and viscoelastic constitutive equations. We find that the dependency of the cavity AR on the heat transfer rate is highly complicated depending upon the values of the Richardson (Ri) and Prandtl numbers (Pr). At low values of Ri, the heat transfer rate continuously decreases with AR irrespective of the value of the Prandtl number and the fluid type, i.e., Newtonian or viscoelastic. The same trend is also observed at high values of Ri and low values of Pr. At these combinations of Ri and Pr, the heat transfer rate is always higher in viscoelastic fluids than in Newtonian fluids due to the presence of elastic turbulence in the former fluids. However, a different trend is observed at high values of both Ri and Pr. At this combination of Ri and Pr, the heat transfer rate increases with AR in Newtonian fluids, whereas it decreases in viscoelastic fluids. Therefore, at high values of AR, Ri, and Pr, the heat transfer rate is higher in Newtonian fluids than that in viscoelastic fluids despite the presence of elastic turbulence in the latter fluids. This is in contrast to the assumption that the elastic turbulence phenomenon always increases the rate of transport processes. A possible explanation for this behavior is provided in this study. Along with the heat transfer aspects, we also provide a detailed discussion on how the cavity aspect ratio influences the corresponding flow dynamics inside the cavity. In particular, we find that the onset of the elastic instability (and the subsequent elastic turbulence) phenomenon is delayed to higher values of the Weissenberg number as the cavity aspect ratio increases. This is in line with prior experimental studies reported in the literature. [ABSTRACT FROM AUTHOR]

Cloud formation is based on the fundamental principle of atmospheric convection, which involves the vertical transport of heat and moisture into an unstable environment. Convective transfer of moisture and heat in the form of turbulent fluxes over the Bay of Bengal (BoB) has not been explored much and is not resolved in global and regional climate models (GCMs and RCMs) due to the coarser grid resolutions used. Therefore, the present study is an attempt to understand the convection phenomenon over the BoB using a high-resolution cloud-resolving large eddy simulation. Due to the lack of observational data over the BoB, initial and boundary conditions were generated using reanalysis data. We found that the LES successfully captured the cloud formation and convection phenomenon. The turbulence in the convection was analyzed by using Reynolds averaging to obtain variances and covariances. The presence of turbulence over the region was observed. The cloud characteristics were verified by conditionally averaging the output fields. The present study paves a pathway to perform various simulations at different atmospheric conditions over the region in order to create a library of high-resolution simulations. [ABSTRACT FROM AUTHOR]

We present an experimental study simulating atmospheric dust devils in a controlled laboratory experiment. The experimental facility, called the "Barrel of Ilmenau" (www.ilmenauer-fass.de) represents a classical Rayleigh‐Bénard set‐up and is believed to model the phenomena in a convective atmospheric boundary layer fairly well. Our work complements and extends the numerical work of Giersch and Raasch (2021)https//doi.org/10.1029/2020jd034334 by experiments. Dust devils are thermal convective vortices with a vertical axis of rotation visualized by entrained soil particles. They evolve in the convective atmospheric boundary layer and are believed to substantially contribute to the aerosol transport into the atmosphere. Thus, their evolution, size, lifetime, and frequency of occurrence are of particular research interest. Extensive experimental studies have been conducted by field measurements and laboratory experiments so far. Beyond that, our study is the first attempt of Rayleigh‐Bénard convection (RBC) in air to investigate dust devil‐like vortices in a laboratory experiment. Up to now, this set‐up mimics the natural process of dust devil evolution as closest to reality. The flow measurement was carried out by particle tracking velocimetry using neutrally buoyant soap bubbles. We initially identified dust devil‐like vortices by eye from the Lagrangian velocity field, and in a later, more sophisticated analysis by a specific algorithm from the corresponding Eulerian velocity field. We analyzed their frequency of occurrence, observation time, and size. With our work, we could demonstrate that turbulent RBC is an appropriate model to mimic the natural process of the evolution of dust devils in the convective atmospheric boundary layer without artificial stimulation. Plain Language Summary: We could experimentally demonstrate that dust devil‐like vortices spontaneously arise in turbulent Rayleigh‐Bénard convection. This first‐time experimental survey simulates the evolution of dust devil‐like vortices in a laboratory experiment which mimics the convective atmospheric boundary layer quite closely and gets by without any artificial input of rotation. Dust devil‐like vortices are measured and identified using the particle tracking velocimetry technique. Within an observation period of 2 hr, 56 dust devil‐like vortices could be detected in total. Their properties coincide quite well with those structures identified in very recent direct numerical simulations (DNS) by Giersch and Raasch (2021, https//doi.org/10.1029/2020jd034334). As well, they show similarity to atmospheric dust devils. The size of our experimentally generated dust devil‐like vortices starts at about 1 dm and ranges up to about 1 m. This is larger than in DNS, but still smaller than in the atmosphere or in large eddy simulation. Key Points: Dust devil‐like vortices spontaneously evolve in turbulent Rayleigh‐Bénard convection at sufficiently high Rayleigh numbers Ra > 1010We studied their properties in a large‐scale Rayleigh‐Bénard experiment using Lagrangian particle tracking velocimetryThe vortical structures in the laboratory experiment are weaker than atmospheric dust devils, but they exhibit similar features [ABSTRACT FROM AUTHOR]

The US Caribbean ocean circulation is governed by an influx of Atlantic water through the passages between Puerto Rico, Hispaniola and the Virgin Islands, and an interplay of the Caribbean Sea water with the local topography of the region. We present an analysis of the US Caribbean ocean flow simulated by the USCROMS; which is the ROMS AGRIF model configured for the US Caribbean regional ocean at a horizontal resolution of 2 km. Outputs from the USCROMS show a seasonal variability in the strength of submesoscale turbulence within a mixed layer whose depth varies from −70 to −20 m from winter to summer, and internal tides originating from the passages between the islands. Energy spectra of the simulated baroclinic velocity show diurnal and semi-diurnal maxima and several higher-order harmonic frequency maxima associated with non-linear internal waves forming over steep slopes with super-critical topography in the continental shelf. The strongest conversion rates of the depth-averaged barotropic to baroclinic tidal energy occur at localized regions in the continental shelf with super-critical topography. These regions also exhibit enhanced transport and dissipation of the depth-averaged barotropic and baroclinic tidal kinetic energy. The dissipation in these regions is nearly 3 orders of magnitude stronger than the open ocean dissipation. The energy transport terms show a seasonal pattern characterized by stronger variance during summer and reduced variance during the winter. At the benthic regions, the dissipation levels depend on the topographic depth and the tidal steepness parameter. If the benthic region lies within the upper-ocean mixed-layer, the benthic dissipation is enhanced by surface-forced processes like wind forcing, convective mixing, submesoscale turbulence and bottom friction. If the benthic region lies below the mixed-layer, the benthic dissipation is enhanced by the friction between the super-critical topographic slopes and the periodically oscillating baroclinic tidal currents. Due to bottom friction, the tidal oscillation in the lateral currents adjacent to the sloping topography generates cyclonic and anti-cyclonic vortices with O(1) Rossby number depending on the orientation of the flow. While the cyclonic vortices form positive potential vorticity (q) leading to barotropic shear instability, anti-cyclonic vortices form negative q which leads to periodically occurring inertial instability. The lateral and inertial instabilities caused by the baroclinic tidal oscillations act as routes to submesoscale turbulence at the benthic depths of −100 m to −400 m near the super-critical topography of the continental shelf, forming O(10 km) long streaks of turbulent water with dissipation levels that are 3 orders of magnitude stronger than the dissipation in the open ocean at the same depth. The magnitudes of the dissipation and q at the benthic regions over super-critical continental-shelf topography are also modulated by the spring-neap tidal signals. [ABSTRACT FROM AUTHOR]

Land surface heterogeneity in conjunction with ambient winds influences the convective atmospheric boundary layer by affecting the distribution of incoming solar radiation and forming secondary circulations. This study performed coupled large‐eddy simulation (ICON‐LEM) with a land surface model (TERRA‐ML) over a flat river corridor mimicked by soil moisture heterogeneity to investigate the impact of ambient winds on secondary circulations. The coupled model employed double‐periodic boundary conditions with a spatial scale of 4.8 km. All simulations used the same idealized initial atmospheric conditions with constant incident radiation of 700 W⋅m−2 and various ambient winds with different speeds (0 to 16 m⋅s−1) and directions (e.g., cross‐river, parallel‐river, and mixed). The atmospheric states are decomposed into ensemble‐averaged, mesoscale, and turbulence. The results show that the secondary circulation structure persists under the parallel‐river wind conditions independently of the wind speed but is destroyed when the cross‐river wind is stronger than 2 m⋅s−1. The soil moisture and wind speed determine the influence on the surface energy distribution independent of the wind direction. However, secondary circulations increase advection and dispersive heat flux while decreasing turbulent energy flux. The vertical profiles of the wind variance reflect the secondary circulation, and the maximum value of the mesoscale vertical wind variance indicates the secondary circulation strength. The secondary circulation strength positively scales with the Bowen ratio, stability parameter (−Zi/L), and thermal heterogeneity parameter under cross‐river wind and mixed wind conditions. The proposed similarity analyses and scaling approach provide a new quantitative perspective on the impact of the ambient wind under heteronomous soil moisture conditions on secondary circulation. [ABSTRACT FROM AUTHOR]

There is a rich history of studying coherent structures in the atmospheric boundary layer through the use of spatial correlations between wall shear stress and elevated velocity measurements. This work has primarily focused on neutral and convective boundary layers, while structures in the stable boundary layer (SBL) have received less attention. We use direct numerical simulations (DNSs) of turbulent channel flow across a range of static stabilities to examine the inclination angles of turbulent structures in the SBL. Angles are inferred not only from wall shear stress and velocity correlations, but also from correlations between the wall buoyancy flux and buoyancy. Results indicate that structures in the SBL have a smaller angle than those under neutral conditions, and that the difference is enhanced with increasing stratification. Specifically, stratification across the range of considered simulations decreases the angles from the neutral case by < 1 ∘ near the surface and by ∼ 1 – 3 ∘ at the top of the logarithmic region. Additionally, the angles of buoyancy structures are larger than those inferred from momentum by ∼ 3 ∘ throughout the entire depth of this layer. Further, angles increase with height until leveling off near the top of the logarithmic region, which may be the result of local z-less stratification based on analysis of the Richardson number and normalized standard deviations. The DNS data are in good agreement with both existing published data and newly reported observations from the AHATS field campaign. Both numerical and observational data exhibit an increase in angle variability with increasing stratification, which seemingly indicates that the variability is related to physical processes of the flow, such as intermittency or laminarization/bursting phenomena. In the future, large-eddy simulation surface boundary conditions will need to better capture this variability to properly represent instantaneous land-surface interactions in the SBL. [ABSTRACT FROM AUTHOR]

Doppler lidar vertical velocity retrievals were analyzed for the scale and structure of mixed‐layer turbulence over a 7‐year period, on fair‐weather warm season days (442 cases) in the U.S. Southern Great Plains. Based on the hypothesized influence of land‐surface forcing and updraft size on convective boundary layer clouds, a spectral analysis was performed to quantify effects of surface forcing (surface buoyancy flux, friction velocity, and evaporative fraction) on turbulence scale. Significant (order‐of‐magnitude) variations in spectral density were found in the energy‐production subrange and mesoscale regimes. Integral scale decreased with increasing buoyancy flux (and Monin‐Obukhov stability parameter), while spectral density in the energy‐production subrange increased, implying a transition to buoyancy‐driven cellular structures with narrower updrafts. The influence of stability parameter was limited to the neutral to convective transition, and could not explain the wide variation in spectral density in the mesoscale regime. However, high friction velocity (u∗>0.5ms−1 ${u}^{\ast } > \,0.5\,\mathrm{m}\,{\mathrm{s}}^{-1}$) was associated with larger integral scale (updraft width), and a greater portion of spectral density in the mesoscale regime. Lidar profiles and radar reflectivity for individual cases revealed evidence of roll and wave‐like structures contributing to mesoscale variability on high friction velocity days, suggesting shear instabilities on days with wetter land surface conditions and smaller buoyancy flux. The role of friction velocity emphasizes the multivariate nature of surface influences on turbulence, and implies that variance‐based turbulence closures may not be adequate for capturing effects of surface forcing on updraft size. Key Points: Turbulence spectra show wide variation in energy content in energy production and mesoscale regimes relevant to boundary layer cloudsThe portion of energy in the largest scales varies with surface friction velocity, implying shear instability in secondary circulationsFriction velocity masks the role of surface buoyancy flux in transitions from intense narrow updrafts to roll and wave‐like structures [ABSTRACT FROM AUTHOR]

In the numerical gray zone of the convective boundary layer (CBL), the horizontal resolution is comparable to the size of organized convective circulation. As turbulence becomes partially resolved, gridscale variations of the subgrid-scale (SGS) turbulent fluxes become significant compared to the mean. Previously, such variations have often been ignored in scale-adaptive planetary boundary layer (PBL) schemes developed for the gray zone. This study investigates these variations with respect to height and resolution based on large-eddy simulations. It is found that SGS fluxes exhibit maximum variability at the center of the gray zone, where the resolved and the SGS mean fluxes are approximately equal. A simple analytical model is used to associate such characteristic variations to the nonlinear interactions of the dominant energy-containing mode of CBL turbulence. Examination of the horizontal distribution of the SGS fluxes reveals their preferential location over the updraft edges surrounding the core. A priori analysis further suggests the ability of a scale-similarity closure to reproduce the unique spatial patterns of the SGS fluxes at gray zone resolutions. Four scale-adaptive PBL schemes are evaluated focusing on their representations of the modeled SGS flux variability. Their shared shortcomings as a result of their gradient diffusion–based formulation are exposed. This study suggests that a mixed model consisting of a scale-adaptive PBL scheme to represent the mean, and a scale-similarity component to account for gridscale variability to be advantageous for the gray zone. Significance Statement: As Gresho and Lee's famous quote on numerical schemes goes, "Don't suppress the wiggles. They're telling you something!" In the numerical gray zone, where the grid spacing is comparable to the characteristic length scale of the flow, turbulence is partially resolved. The "wiggles" (or gridscale variability) reflecting the resolved heterogeneity of turbulent fluxes is an outstanding feature of the gray zone. However, they are often overlooked as error bars to the mean. This study uncovers the significance of these error bars, and characterizes and explains their variations with height and grid spacing. A suitable model that captures such variations is investigated to help build better planetary boundary layer schemes. [ABSTRACT FROM AUTHOR]

The Global Atlas for Siting Parameters project compiles a suite of models into a complex modeling system, uses up-to-date global datasets, and creates global atlases of siting parameters at a spatial resolution of 275 m. These parameters include the 50-year wind, turbulence, and turbine class recommendations based on relevant generic turbines. The suite of models includes the microscale Linear Computational Model (LINCOM), a statistical, spectral correction method here revised for strong convective areas and tropical cyclone affected areas, two turbulence models with four setups, and load models. To this complexity, an uncertainty model was developed to classify the various sources of uncertainties for both the extreme wind and the turbulence calculations, and accordingly, atlases of uncertainty classification were created. Preliminary validation of the global calculations of the 50-year wind and turbulence is done through comparisons with measurements, and the results are promising. This is the first time the siting parameters are obtained with such a high spatial resolution and shared on open data portal. It is expected to benefit the global wind energy planning and development. [ABSTRACT FROM AUTHOR]

Despite extensive research on the effect of atmospheric stability, the influence of convective rolls and thermal plumes in convective boundary layer (CBL) on urban winds and turbulence remains merely understood. Using amplitude modulation (AM), we examine the multi-scale interaction among turbulence intensities, fluxes, and large-scale CBL structures in unstable urban boundary layers (UBLs). Nine sets of large-eddy simulation (LES) are conducted, contrasting the CBL behavior over idealized urban morphology from neutral to free convection. This study, comparing the stability-dependent AM coefficient, reveals a tight modulation of small-scale turbulence and fluxes at urban canopy layer (UCL) by different types of characteristic CBL structures. Wavelet analysis corroborates their interactions in the time-frequency domain. Moreover, the correlation between the AM and the coherent CBL structures in the mixed layers is unveiled. Additionally, building-induced secondary flow structures are observed to dominate the modulation of small-scale turbulence residing within. UCL building-scale turbulence exhibits a higher susceptibility to modulation compared with intermediate scales. Notably, strong convection can invert the phase relationship between large scales and UCL turbulence. These results signify the crucialness of considering CBL coherent structures in the development of land-surface-parameterization schemes as well as the cautious implementation of predictive models in urban CBLs. • Amplitude modulation in full-scale urban CBLs transitioning from neutral to strong convection is examined. • Small-scale turbulence and fluxes in UCL are significantly modulated by the large-scale CBL structures. • Building wakes attenuate the modulation by CBL structures in the building vincinity. • Building-size turbulent processes are more susceptible to the large-scale streamwise modulation. • Stability profoundly modifies the phase relationship between large- and small-scale signals in UCL. [ABSTRACT FROM AUTHOR]

This study examines boundary layer turbulence derived from high temporal resolution meteorological measurements from 40 research flights over the western North Atlantic Ocean during the 2020 deployments of ACTIVATE. Frequency distributions of various turbulent quantities reveal stronger turbulence during the winter deployment than in summer and for cloud‐topped than in cloud‐free boundary layers during the summer deployment. Maximum turbulence kinetic energy (TKE) is most often within cloud from observations in winter and summer, whereas it is mostly below cloud in both seasons by a global model turbulence parameterization. Bivariate frequency distributions are consistent with the bivariate Gaussian probability distribution functions assumed for the closure of higher‐order turbulence/shallow convection parameterizations used by some global models. Turbulence simulated by the Community Atmosphere Model version 6 and the Energy Exascale Earth System Model Atmosphere Model version 2 using such parameterizations is not as strong as observed, with more TKE going into vertical wind perturbations rather than into zonal wind perturbations as observed, suggesting that the treatment of turbulence in Earth system models still needs to be further improved. Key Points: Maximum turbulence kinetic energy is most often within cloud in observations but mostly below cloud by a global model parameterizationObservations are similar to bivariate normal probability distribution functions assumed to close higher‐order turbulence parameterizationsBoundary layer turbulence simulated by global models is weaker than observed [ABSTRACT FROM AUTHOR]

The use of ensembles for numerical weather prediction has become common during the last decade. For global models, the generation of initial‐condition perturbations has a number of well‐tested methodologies. In ensembles that predict convective storms explicitly (i.e., Δx<4$$ \Delta x<4 $$ km), the generation of physically realistic perturbations is less well posed. This study introduces a technique to generate physically coherent and spatially correlated (PCSC) initial‐condition perturbations that are calibrated to the environment. Ensembles of idealized CM1 simulations, initialized with either PCSC perturbations (EXP_PCSC), spatially coherent random perturbations (EXP_3KM), or Gaussian white‐noise random perturbations (EXP_WHITE), are run for both a linear convective line of storms and a single "supercell" storm to demonstrate the utility of this new perturbation technique in diverse environments. PCSC perturbations are extracted from high‐resolution simulations of boundary‐layer turbulence and random perturbations are calibrated to be the same in magnitude as PCSC perturbations. EXP_PCSC simulations spawn turbulence fastest in this study. The simulated turbulence is more robust than in other experiments more than 1 hr into the simulation, because horizontal convective rolls enhance power at the largest scales. Random perturbations are slow to generate turbulence; this problem is exacerbated when the base model state flow is nonturbulent. Due to robust turbulence, EXP_PCSC ensemble spread increases fastest during the first simulation hour and remains largest throughout the remainder of the simulations. Although EXP_PCSC spread is the largest, the sensitivity of convection to the initial perturbations varies at different times in the storm life cycle. Storms appear more sensitive to perturbations added near the time of convective initiation. [ABSTRACT FROM AUTHOR]

The potential effect of failing to predict nocturnal deep radiation fog on the development of the daytime convective boundary layer (CBL) is studied using large‐eddy simulations. Typical spring and autumn conditions for the mid‐latitudes are used to perform simulations in pairs. Fog formation is allowed in one simulation of each pair (nocturnal fog [NF]) and is suppressed in the other (clear sky [CS]). This allows for the identification of properties (temperature, humidity, boundary‐layer depth), conditions, and processes in CBL development that are affected by fog. Mixing‐layer temperatures and boundary‐layer depths immediately after fog dissipation in CSs are shown to be up to 2.5 K warmer and 200 m higher, respectively, than the NF counterparts. Additionally, greater water vapor mixing ratios are found in the CSs. However, owing to greater temperatures, relative humidities at the CBL top are found to be less in CSs than in the corresponding NFs. This relative humidity difference might be an indication that cloud formation is suppressed to some extent. The magnitude of the differences between CSs and NFs during the day is mainly correlated to the fog depth (in terms of duration and liquid water path), whereas the key processes responsible for differences are the atmospheric long‐wave cooling of the fog layer (for temperature development) and droplet deposition (for water vapor mixing ratio development). [ABSTRACT FROM AUTHOR]

The ocean surface boundary layer is a gateway of energy transfer into the ocean. Wind-driven shear and meteorologically forced convection inject turbulent kinetic energy into the surface boundary layer, mixing the upper ocean and transforming its density structure. In the absence of direct observations or the capability to resolve subgrid-scale 3D turbulence in operational ocean models, the oceanography community relies on surface boundary layer similarity scalings (BLS) of shear and convective turbulence to represent this mixing. Despite their importance, near-surface mixing processes (and ubiquitous BLS representations of these processes) have been undersampled in high-energy forcing regimes such as the Southern Ocean. With the maturing of autonomous sampling platforms, there is now an opportunity to collect high-resolution spatial and temporal measurements in the full range of forcing conditions. Here, we characterize near-surface turbulence under strong wind forcing using the first long-duration glider microstructure survey of the Southern Ocean. We leverage these data to show that the measured turbulence is significantly higher than standard shear-convective BLS in the shallower parts of the surface boundary layer and lower than standard shear-convective BLS in the deeper parts of the surface boundary layer; the latter of which is not easily explained by present wave-effect literature. Consistent with the CBLAST (Coupled Boundary Layers and Air Sea Transfer) low winds experiment, this bias has the largest magnitude and spread in the shallowest 10% of the actively mixing layer under low-wind and breaking wave conditions, when relatively low levels of turbulent kinetic energy (TKE) in surface regime are easily biased by wave events. Significance Statement: Wind blows across the ocean, turbulently mixing the water close to the surface and altering its properties. Without the ability to measure turbulence in remote locations, oceanographers use approximations called boundary layer scalings (BLS) to estimate the amount of turbulence caused by the wind. We compared turbulence measured by an underwater robot to turbulence estimated from wind speed to determine how well BLS performs in stormy places. We found that in both calm and stormy conditions, estimates are 10 times too small closest to the surface and 10 times too large deeper within the turbulently mixed surface ocean. [ABSTRACT FROM AUTHOR]

Natural laminar flow airfoils have achieved such a level of refinement that further optimisation and subsequent wind tunnel testing need to regard the specific free-stream turbulence to be expected during operation. This requires the characterisation of this turbulence in terms of those properties which are relevant for boundary layer receptivity and subsequent transition. These parameters of turbulence change with environmental conditions and, in case of aircraft, along the flight profile. This study investigates the free-stream turbulence relevant for the case of sailplane airfoils. In-flight measurements with a constant temperature anemometer x-wire probe were conducted during cross-country flights in Central Europe and provided 22 h of flight data, covering thermalling phases as well as straight flight legs. Longitudinal and transversal velocity fluctuations were recorded well into the dissipation range. The special challenges of operating a constant temperature anemometer probe continuously for several hours are addressed. The permanent unsteadiness of the inflow poses challenges for the evaluation, but also provides a broad database of measured turbulence levels. The quality of the measurements is shown by verifying some of the predictions of Kolmogorov's inertial range theories. Free-stream turbulence in thermalling phases is sufficiently homogeneous to be described accurately, as the dissipation range fluctuates only in a limited range and follows a log-normal distribution. On the straight flight legs, the turbulence depends on the convective activity along the flight path. In general, within the convective part of the atmosphere, turbulence levels are found to be significantly larger than in low-turbulence wind tunnels. [ABSTRACT FROM AUTHOR]

The vertical heat and moisture exchange in the convective boundary layer over mountainous terrain is investigated using large-eddy simulation. Both turbulent and advective transport mechanisms are evaluated over the simple orography of a quasi-two-dimensional, periodic valley with prescribed surface fluxes. For the analysis, the flow is decomposed into a local turbulent part, a local mean circulation, and a large-scale part. It is found that thermal upslope winds are important for the moisture export from the valley to the mountain tops. Even a relatively shallow orography, possibly unresolved in existing numerical weather prediction models, modifies the domain-averaged moisture and temperature profiles. An analysis of the turbulent kinetic energy and turbulent heat and moisture flux budgets shows that the thermal circulation significantly contributes to the vertical transport. This transport depends on the horizontal heterogeneity of the transported variable. Therefore, the thermal circulation has a stronger impact on the moisture budget and a weaker impact on the temperature budget. If an upper-level wind is present, it interacts with the thermal circulation. This weakens the vertical transport of moisture and thus reduces its export out of the valley. The heat transport is less affected by the upper-level wind because of its weaker dependence on the thermal circulation. These findings were corroborated in a more realistic experiment simulating the full diurnal cycle using radiation forcing and an interactive land surface model. [ABSTRACT FROM AUTHOR]

Modern general circulation models continue to require parameterizations of subgrid transport due to planetary boundary layer (PBL) turbulence and convection. Some schemes that unify these processes rely on assumed joint probability distributions of vertical velocity and moist conserved thermodynamic variables to predict the subgrid-scale contribution to the mean state of the atmosphere. The multivariate double-Gaussian mixture has been proposed as an appropriate model for PBL turbulence and shallow convection, but it is unable to reproduce important features of shallow cumulus convection. In this study, a novel unified PBL turbulence–convection–cloud macrophysics scheme is presented based on the eddy-diffusivity/mass-flux framework. The new scheme augments the double-Gaussian representation of subgrid variability with multiple stochastic mass-flux plumes at minimal added computational cost. Improved results for steady-state maritime and transient continental shallow convection from a single-column model implementation of the new scheme are shown with respect to reference large-eddy simulations. Improvements are seen in the cloud layer due to mass-flux plumes occupying the extreme moist, low liquid-water potential temperature tail of the joint temperature–moisture distribution. Significance Statement: Computer models of the atmosphere used to predict future climate are unable to directly represent air motion at small spatial scales because it would take too long to run the model over the entire planet. Instead, models typically use coarse model grid spacing and a simplified statistical representation of the physical processes that cause small-scale motions. This paper improves a particular simplified representation by adding a mechanism to represent statistically rare events of strong small-scale air motion that coherently transport air from near the surface to higher in the atmosphere. This increased transport also improves the representation of clouds, a particularly difficult phenomenon to simulate in models. [ABSTRACT FROM AUTHOR]

Coupling between mesoscale models and large‐eddy simulation (LES) models is increasingly used to more realistically represent the wide range of scales of atmospheric motions affecting boundary layer winds and turbulence that need to be simulated accurately for applications such as wind energy. However, such mesoscale‐to‐microscale coupled modeling frameworks are potentially affected by a large number of uncertain closure parameters. Here, we investigate the sensitivity associated with six closure parameters related to a 1.5‐order subgrid‐scale turbulence closure for an ensemble of mesoscale‐coupled LES. The simulations are performed using the Weather Research and Forecasting model nested from horizontal resolutions of greater than a kilometer down to tens of meters. Closure parameters are varied to generate perturbed parameter ensembles for two case studies of highly sheared, convective boundary layers observed in the Columbia Basin of Oregon and Washington during the Second Wind Forecast Improvement Project. Machine learning algorithms are used to explore the sensitivity of LES predictions, considering the effects of the perturbed physical parameters alongside categorical factors such as the case study identity, measurement location, and LES resolution. For the conditions we examine, a single parameter, the eddy viscosity coefficient, is the dominant source of parametric sensitivity and its importance is comparable to the categorical factors for several of the simulation response variables we examine. Key Points: We generate perturbed parameter ensembles of simulations nested from mesoscale to large eddy simulation configurationsEnsemble machine learning techniques are used to represent simulation output and assess importance of parametric and categorical factorsThe eddy viscosity coefficient dominates sensitivities to parameters of the subgrid‐scale turbulence closure for the cases considered [ABSTRACT FROM AUTHOR]

With the rapid improvement in computational resources, it has become possible to structure numerical weather model simulations to use sub‐kilometer grid spacing. Traditional turbulence‐mixing closures have become obsolete within the context of the gray zone of turbulence. Here, a new scale‐aware algorithm (SA‐UW) is developed based on the UW (University of Washington Moist Turbulence) scheme. By dividing both the local and nonlocal vertical turbulence flux into sub‐grid and resolved parts, the partition of sub‐grid turbulence is tuned by a set of well‐acknowledged scale‐dependent relationships. Idealized simulations confirm that the newly developed scheme can adequately reproduce sub‐grid turbulence transport at gray‐zone scales. The inclusion of a nonlocal transport term in the SA‐UW scheme is essential, as it vastly improves the distribution and intensity of coherent structures. The SA‐UW scheme recovers the conventional UW scheme when grid spacing is larger than the turbulence length scale, thereby evincing the sale‐aware capability of the new method. For real‐case simulations, the SA‐UW scheme can capture the diurnal cycle and provide a more accurate vertical structure of temperature and humidity. Plain Language Summary: Turbulence mixing within the planetary boundary layer is a key process which needs to be parameterized in numerical weather models. Owing to the increasing computational power, operational numerical weather prediction centers around the world have begun to carry out simulations with horizontal resolutions of the order of several kilometers to sub‐kilometers. However, traditional turbulence‐mixing parameterization schemes are not designed for these so‐called "terra incognita" or "gray zone" resolutions. This study aimed to develop a turbulence‐mixing scheme suitable for simulations from the gray zone to mesoscale models. Compared with some traditional schemes, the new scheme improves the vertical structure of the convective boundary layer, including wind speed, temperature, moisture, and turbulence fluxes. The new scheme shows promising prospects for improving the model performance at gray‐zone scales. Key Points: A scale‐aware planetary boundary layer scheme (SA‐UW) was developed to address simulation issues at gray‐zone scalesThe new scheme fully takes into account the scale‐awareness of both the local and nonlocal transport in the mixed layerIdealized and real case simulations show considerable improvement in the new scheme compared with traditional schemes [ABSTRACT FROM AUTHOR]

We hypothesize that large eddies play a major role in the entrainment of aeolian particles. To test this, wind-tunnel experiments are carried out to measure the particle entrainment rate for various sizes and flow conditions. Wind-tunnel flows are usually neutrally stratified with no large eddies, which are typically seen in convective atmospheric boundary layers. Here, a novel technique is applied, by deploying a piece of randomly fluttering cloth, to generate large eddies similar to convective eddies, here referred to as quasi-convective turbulence. The characteristics of quasi-convective turbulence are analyzed with respect to neutral turbulence in the Monin–Obukhov similarity framework, and the probability distributions of surface shear stress are examined. We show that for a given mean flow speed and in comparison with neutral flow conditions, quasi-convective turbulence increases the surface shear stress and alters its probability distribution and hence substantially enhances the entrainment of sand and dust particles. Our hypothesis is thus confirmed by the wind-tunnel experiments. We also explain why large eddies are important to aeolian entrainment and transport. [ABSTRACT FROM AUTHOR]

We consider the closure problem of representing the higher-order moments (HOMs) in terms of lower-order moments, a central feature in turbulence modeling based on the Reynolds-averaged Navier–Stokes (RANS) approach. Our focus is on models suited for the description of asymmetric, nonlocal, and semiorganized turbulence in the dry atmospheric convective boundary layer (CBL). We establish a multivariate probability density function (PDF) describing populations of plumes that are embedded in a sea of weaker randomly spaced eddies, and apply an assumed delta-PDF approximation. The main content of this approach consists of capturing the bulk properties of the PDF. We solve the closure problem analytically for all relevant HOMs involving velocity components and temperature and establish a hierarchy of new non-Gaussian turbulence closure models of different content and complexity ranging from analytical to semianalytical. All HOMs in the hierarchy have a universal and simple functional form. They refine the widely used Millionshchikov closure hypothesis and generalize the famous quadratic skewness–kurtosis relationship to higher order. We examine the performance of the new closures by comparison with measurement, LES, and DNS data and derive empirical constants for semianalytical models, which are best for practical applications. We show that the new models have a good skill in predicting the HOMs for atmospheric CBL. Our closures can be implemented in second-, third-, and fourth-order RANS turbulence closure models of bi-, tri-, and four-variate levels of complexity. Finally, several possible generalizations of our approach are discussed. [ABSTRACT FROM AUTHOR]

In large‐eddy simulations, subgrid‐scale (SGS) processes are parameterized as a function of filtered grid‐scale variables. First‐order, algebraic SGS models are based on the eddy‐viscosity assumption, which does not always hold for turbulence. Here we apply supervised deep neural networks (DNNs) to learn SGS stresses from a set of neighboring coarse‐grained velocity from direct numerical simulations of the convective boundary layer at friction Reynolds numbers Reτ up to 1243 without invoking the eddy‐viscosity assumption. The DNN model was found to produce higher correlation between SGS stresses compared to the Smagorinsky model and the Smagorinsky‐Bardina mixed model in the surface and mixed layers and can be applied to different grid resolutions and various stability conditions ranging from near neutral to very unstable. The DNN model can capture key statistics of turbulence in a posteriori (online) tests when applied to large‐eddy simulations of the atmospheric boundary layer. Plain Language Summary: Using grid‐scale state variables to represent subgrid‐scale (SGS) processes is a major step in large‐eddy simulations of turbulence, which typically rely on some physically based assumptions that do not always apply. Here we replace physically based assumptions with deep learning in SGS modeling and show the latter performs better in a priori (offline) tests of atmospheric turbulence. In addition, the deep neural networks model well captures key statistics of turbulence in a posteriori (online) tests when applied to in large‐eddy simulations of atmospheric boundary layers. Key Points: Deep learning‐based subgrid‐scale models outperform traditional eddy‐viscosity models in a priori (offline) testsDeep learning‐based subgrid‐scale models can account for density stratification effects in boundary‐layer turbulenceThe DNN model can capture key turbulence statistics in a posteriori (online) tests when applied to large‐eddy simulations of the atmospheric boundary layer [ABSTRACT FROM AUTHOR]

Modeling atmospheric turbulence in the convective boundary layer is challenging at kilometer and subkilometer resolutions, as the horizontal grid spacing approaches the size of the most energetic turbulent eddies. In this range of resolutions, termed terra incognita or gray zone, partially resolved convective structures are grid dependent and neither traditional 1D mesoscale parameterizations nor 3D large-eddy simulations closures are theoretically appropriate. Leveraging on a new set of one-way nested, full-physics multiscale numerical experiments, we quantify the magnitude of the errors introduced at gray zone resolutions in a real-case application and we provide new perspectives on recently proposed modeling approaches. The new set of experiments is forced by real-time-varying boundary conditions, spans a wide range of scales, and includes traditional 1D schemes, 3D closures, scale-aware parameterizations, and strategies to suppress resolved convection at gray zone resolutions. The study area is Riyadh (Saudi Arabia), where deep CBLs develop owing to strong convective conditions. Detailed analyses of our experiments, including validation with radiosonde data, calculations of spectral features, and partitioning of turbulent fluxes between resolved and subgrid scales, show that (i) grid-dependent convective structures entail minor impacts on the first-order characteristics of the fully developed boundary layer due to some degree of implicit scale awareness of 1D parameterizations and (ii) 3D closures and scale-aware schemes outperform traditional 1D schemes especially in the surface layer, among other findings. The new suite of experiments provides a benchmark of real simulations that can be extended to assess how new turbulence closures perform at gray zone resolutions. Significance Statement: As recent advances in high-performance computing are leading to a new era in numerical simulations, regional atmospheric models can now increase their resolution to the widely unexplored kilometer and subkilometer range. While increasing the resolution of atmospheric models is desirable to (i) have more realistic weather and air quality predictions and (ii) better represent boundary conditions for microscale models, kilometer and subkilometer grid spacings pose some theoretical challenges that need to be addressed by the atmospheric modeling community. In this work we run a set of numerical experiments for a real case study that aim to offer new perspectives on recently developed modeling strategies and identify the most promising directions that should be investigated by follow-up studies. [ABSTRACT FROM AUTHOR]

We present interannual, seasonal, and regional variations in the daytime Martian convective boundary layer (CBL). Martian CBL meteorology is driven both by the effect of diurnal and seasonal cycles as well as complex Martian topography. One of the most important components of the Martian atmosphere is its dust cycle. Here, we develop a novel semi‐interactive dust transport model within the MarsWRF framework, in which the dust is lifted, advected by model winds, mixed, and allowed to sediment, but is then scaled to match two‐dimensional maps of the observed daily column‐integrated dust opacity. This allows the vertical dust distribution and associated dust radiative heating to be controlled by model processes, while the horizontal dust distribution is constrained to follow observations. We report the impact of the dust cycle on Martian boundary layer meteorology. Enhanced dust transport lowers the global net surface heating rates, decreasing the turbulent mixing in CBL to virtually zero (within the dust storm season) and, enhances the wind shear on average by almost 50%. As a superposition of both impacts, during global dust storms (GDS) in Mars Year (MY) 25 and 34, we find that long‐lasting extremely shallow daytime boundary layers can globally form as shallow as 0.5 km (but not for the less intense GDS in MY 28), unlike the 9 km deep and highly turbulent CBL formation at GDS onset and decay. Based on our GCM results, strong CBL suppression lasts as long as approximately 67 and 57 sols during GDS events in MY 25 and 34. Plain Language Summary: In recent years, thanks to the ongoing lander and rover missions, in situ observations of Martian surface processes (water, dust, and methane transport) have been attracting more attention, especially to search for the signatures of the planet's habitability. From this emerges the significance of planetary boundary layer dynamics, which is the key governor of surface exchange processes in the lowermost portion of the atmosphere. Although the current knowledge has been advancing by observing diurnal characteristics for specific locations, seasonal and year‐to‐year variations on a global scale are still less well comprehended. Here, we examine year‐to‐year, seasonal, and regional variabilities of the Martian daytime planetary boundary layer (so‐called convective boundary layer governed by daytime radiative heating), assessing a decade‐long general circulation model (GCM) simulation from Mars Year 24–34. Our results show that global dust storms in Mars Years 25 and 34 have profound impacts on the convective boundary layer, causing a severe surface cooling as a long‐term global‐darkness state, thus resulting in a global weakening in convective activity. We find that deep and highly turbulent convective boundary layers (with a depth of 9 km) convert into extremely shallow (0.5 km) boundary layers even during the daytime, as a consequence of global dust storms. Key Points: A novel semi‐interactive dust transport model is developed within the MarsWRF general circulation model (GCM) frameworkWe report strong interannual, seasonal, and regional variations in the Martian convective boundary layer (CBL)Our GCM results show a long‐lasting extremely shallow daytime boundary layer during global dust storms [ABSTRACT FROM AUTHOR]

The International Society for Atmospheric Research using Remotely-piloted Aircraft (ISARRA) hosted a flight week in July 2018 to demonstrate unmanned aircraft systems' (UASs) capabilities in sampling the atmospheric boundary layer. This week-long experiment was called the Lower Atmospheric Profiling Studies at Elevation – a Remotely-piloted Aircraft Team Experiment (LAPSE-RATE) field campaign. Numerous remotely piloted aircraft and ground-based instruments were deployed with the objective of capturing meso- and microscale phenomena in the atmospheric boundary layer. The University of Oklahoma deployed one Halo Streamline lidar, and the University of Colorado Boulder deployed two WindCube lidars. In this paper, we use data collected from these Doppler lidars to estimate turbulence dissipation rate throughout the campaign. We observe large temporal variability of turbulence dissipation close to the surface with the WindCube lidars that is not detected by the Halo Streamline. However, the Halo lidar enables estimating dissipation rate within the whole boundary layer, where a diurnal variability emerges. We also find a higher correspondence in turbulence dissipation between the WindCube lidars, which are not co-located, compared to the Halo and WindCube lidar that are co-located, suggesting a significant influence of measurement volume on the retrieved values of dissipation rate. This dataset has been submitted to Zenodo (Sanchez Gomez and Lundquist, 2020) for free and is openly accessible (10.5281/zenodo.4399967). [ABSTRACT FROM AUTHOR]

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer--''deep-cycle turbulence''--that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ~0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 08, 1408W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear yz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator. [ABSTRACT FROM AUTHOR]