 Subjects -> METEOROLOGY (Total: 106 journals)
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- Celebrating the Career of Evgeni Fedorovich: Explorer of the
Boundary-Layer Realm and Ambassador for the Community-
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PubDate: 2023-09-11
- Understanding Thermally Driven Slope Winds: Recent Advances and Open
Questions-
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Abstract: Abstract The paper reviews recent advances in our understanding about the dynamics of thermally driven winds over sloping terrain. Major progress from recent experiments, both in the field and in the laboratory, are outlined. Achievements from numerical modelling efforts, including both parameterized turbulence and large eddy simulation approaches, up to direct numerical simulations, are also reviewed. Finally, theoretical insights on the nature of turbulence in such winds are analyzed along with applications which benefit from progress in understanding of these flows. Open questions to be faced for further investigations are finally highlighted.
PubDate: 2023-09-09
- Atmospheric Small-Scale Turbulence from Three-Dimensional Hot-film Data
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Abstract: Abstract The behavior of small-scale atmospheric turbulence is investigated using the three-dimensional Canopy Horizontal Array Turbulence Study hot-film data. The analysis relies on an in situ calibration versus simultaneous sonic anemometer measurements. The calibration is based on King’s law and geometric relationships between the individual hot-film sensors, and is able to account for the errors associated with sensors’ misalignment and the high turbulence intensity. The details of the calibration are provided, and its performance is validated by comparing results of spectra and structure functions with standard wind-tunnel data and model spectra. A single 3 h block of data was selected, containing 33 subblocks of 2 min data without error gaps, whose statistics were averaged to provide smooth results. These data were measured above canopy under stable conditions, and correspond to a Taylor Reynolds number \(Re_\lambda \approx 1550\) . The agreement with wind tunnel results for a similar \(Re_\lambda \) and with model predictions provides a validation for the in situ calibration method applied. Furthermore, the results indicate a presence of the bottleneck effect in the lateral and vertical spectra, in addition to a lack of inertial range in the second-order structure function due to the low Reynolds number. An additional analysis of the effect of Reynolds number on the inertial range is provided using atmospheric data from the literature.
PubDate: 2023-09-07
- Correction to: Wavelet Analysis of Coherent Structures above Maize and
Soybean Crops-
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PubDate: 2023-09-01
- Wind-Tunnel Reproduction of Nonuniform Terrains Using Local Roughness
Zones-
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Abstract: Abstract Wind-tunnel modeling of Atmospheric Boundary Layer flows has primarily consisted of simplified (purely uniform) upwind terrain conditions. This approach is easier to carry out but may not replicate the true wind characteristics of the site. This paper proposes a method to simulate nonuniform terrains in a wind-tunnel and investigates the wind characteristics produced by the method. The proposed method employs the local roughness zones where the given terrain is divided into sub-areas with an approximately uniform roughness length. Next, each sub-area is represented in the wind-tunnel with uniform roughness elements. However, the overall upwind fetch will be composed of roughness elements of various heights. To study the wind characteristics produced by the method, nine different real-world sites were simulated in the Boundary Layer Wind Tunnel at the University of Florida Natural Hazard Engineering Infrastructure Experimental Facility, using a self-configurable (automated) roughness element grid. Compared with the conventional equivalent uniform representation, similarities and differences in the longitudinal mean velocity, turbulence intensity, wind spectrum, and integral length scale profiles are reported and discussed. In particular, a significant difference was observed for the higher-order moments of the longitudinal velocity component, which indicates the need for further studies in wind loads under nonuniform terrains.
PubDate: 2023-08-26
- Turbulence in the Mixed Layer Over an Urban Area: A New York City Case
Study-
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Abstract: Abstract This study examines the turbulence characteristics of the boundary layer over New York City. Understanding the urban boundary layer characteristics is key to forecasting weather in cities, where most people live. Although extensive research into urban boundary layer (UBL) processes have been carried out in the past decades, majority of these studies focused on the urban surface layer; our understanding of urban mixed layer characteristics is still incomplete. Here we use Doppler lidar observations from multiple sites in New York City to study turbulent properties in the UBL and their relationship to the heterogeneous urban surface. All three sites were influenced by different levels of urbanization. By investigating turbulent properties such as velocity variances, turbulent intensities, and vertical velocity spectra throughout the UBL, our analysis shows vertical stratification in momentum transport during non-neutral stability periods. The spectral analysis of vertical velocities show vertical stratification in normalized energy density at different heights, with the degree of stratification increasing with increasingly non-neutral surface stability. A comparison of turbulent properties over the study sites reveals a degree of homogeneity in mixed layer characteristics, with similar vertical profiles of the turbulent characteristics among the sites, suggesting horizontal homogeneity in the urban mixed layer.
PubDate: 2023-08-24
- Appropriate Momentum Provision for Numerical Simulations of Horizontally
Homogeneous Urban Canopies Using Periodic Boundary Conditions-
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Abstract: Abstract Turbulent flow over urban-like roughness has been numerically studied for various purposes, such as the clarification of turbulent characteristics over rough walls, visualization of turbulent structures around block arrays, and evaluation of urban ventilation and pedestrian winds. In such simulations, a portion of the developing boundary layer is extracted as a numerical domain, assuming periodic boundary conditions in the horizontal direction to reproduce laterally homogeneous rough surfaces. However, the conditions required to drive the airflow by an artificial momentum source uniquely determine the turbulent statistics, which are different from those in developing boundary layers. Therefore, this study presents a new approach for driving the airflow over urban-like block arrays. The new method is based on spatially averaged Navier–Stokes equations to prove the necessity of height-dependent momentum provision. The turbulent flow over a cubical-block array is determined using large-eddy simulations driven by four different momentum sources. Regardless of the driving force, the vertical profiles of the streamwise velocity components are identical. In contrast, slight differences in the vertical Reynolds stress, variances in the velocity components, and turbulence kinetic energy are appropriately reproduced in the new approach. In addition, the budget equations of the turbulent statistics reveal that a change in the vertical Reynolds stress affects the energy production and its redistribution into variance components. The proposed method can contribute to the reproduction of a realistic turbulent flow and provide a theoretical understanding of the momentum provision.
PubDate: 2023-08-22
- The Atmospheric Boundary Layer Above the Marginal Ice Zone: Scaling,
Surface Fluxes, and Secondary Circulations-
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Abstract: Abstract The Arctic is undergoing rapid changes due to global warming, including the expansion of the marginal ice zone (MIZ), a zone of mixed ice and open water surfaces. To predict the atmospheric interaction with these surfaces, a critical process in climate models, this paper examines a simplified theoretical framework to non-dimensionalize the dynamics of the atmospheric boundary layer (ABL) over a mixed ice-water surface (MIZ–ABL). A heterogeneity Richardson number, \(\text {Ri}_h\) , is proposed to account for the difference in temperature between the ice and water surface in relation to the synoptic pressure gradient forcing. With the wind angle relative to the ice-water interface, \(\alpha \) , this framework hypothesizes that these two dimensionless numbers, regardless of individual dimensional variables (surface temperature and geostrophic wind speed) are sufficient to predict the MIZ–ABL dynamics. To test this framework, large-eddy simulations were employed over half-ice and half-water surfaces, with varying surface temperatures and geostrophic wind velocities. While the surface heat fluxes over ice, water, and the aggregate surface seem to be captured reasonably well by \(\alpha \) and Ri \(_h\) , the mean wind and turbulent kinetic energy (TKE) profiles were not, suggesting that not only the difference in stability between the two surface, but also the individual stabilities over each surface influence the dynamics. The wind angle had a significant impact on the results, both in terms of heat fluxes at the surface, turbulent and dispersive fluxes in the MIZ–ABL, and the structure of the secondary circulations. When wind blows perpendicular to the water-ice interface, internal boundary layers are favoured except at the highest Ri \(_h\) simulated. For cases with wind parallel to the interface, large rolls parallel to the shore emerge. The paper raises at least as many questions as it answers, highlighting the need for further studies of the MIZ–ABL.
PubDate: 2023-08-21
- Correction to: Evidence of Strong Flux Underestimation by Bulk
Parametrizations During Drifting and Blowing Snow-
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PubDate: 2023-08-01
DOI: 10.1007/s10546-023-00809-x
- Correction to: Daytime Convective Boundary-Layer Evolution on Three
Fair-Weather Days in CASES-97-
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PubDate: 2023-08-01
DOI: 10.1007/s10546-023-00813-1
- A New Stable Boundary Layer Parameterization for Numerical Weather
Prediction Models: A Heat Flux Budget Approach-
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Abstract: Abstract The present study introduces a new boundary layer parameterization for weather and forecasting models. It is implemented here as a boundary layer module in Weather Research and Forecasting (WRF) model. The main novelty in the new scheme is that it includes prognostic equations for the heat flux and temperature variance, being the first WRF boundary layer scheme with that feature. This is specially aimed at improving the representation of nocturnal stable boundary layer and of its turbulence regimes, weakly and very stable. The effort is supported by previous studies that found that the two regimes and the transitions between them are better represented by simplified numerical schemes that represent the interactions between the surface and the air adjacent to it when the heat flux and temperature variance are solved prognostically. The results show that the two regimes are adequately simulated by the new scheme. Such an evaluation is presented in terms of the relationship between the turbulence velocity scale and mean wind speed, of the dependence of the potential temperature gradient near the surface and the mean wind speed, and by the relationship between flux and gradient Richardson numbers. In the new scheme, the relationship between thermal structure and the mean and turbulent flows arises naturally from the heat flux prognostic equation, not being arbitrarily imposed by an empirical stability function.
PubDate: 2023-08-01
DOI: 10.1007/s10546-023-00810-4
- Large Eddy Simulation Study of Atmospheric Boundary Layer Flow over an
Abrupt Rough-to-Smooth Surface Roughness Transition-
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Abstract: Abstract The atmospheric boundary layer flow downstream of an abrupt rough-to-smooth surface roughness transition is studied using large eddy simulations (LES) for a range of surface roughness ratios. Standard wall models assume horizontal homogeneity and are inapplicable for heterogeneous surfaces. Two heterogeneous-surface wall models are evaluated, one based on a local application of similarity theory using a twice-filtered velocity field (BZ model) and another based on a local friction-velocity obtained by blending the upstream and downstream profiles (APA model). The wall shear stress and the turbulence intensity (TI) are sensitive to the wall model while the mean streamwise velocity and the total shear stress (TSS) are less sensitive. The APA model is more accurate than the BZ model on comparison to previous experiments. The wall shear stress obtained using the APA wall model is sensitive to the ratio of the equilibrium and the internal boundary layer (IBL) heights, while other statistics are not. The IBL height is insensitive to the turbulent quantity (TSS or TI) on which it is based. Several analytical relations for the IBL height are evaluated using the LES data. Two models are found to be accurate for different roughness ratios while one model is reasonable over the full range investigated. A phenomenological model is developed for the TI downstream of the roughness jump using a weighted average of the upstream and far-downstream profiles. The model yields reasonable predictions for all roughness ratios investigated.
PubDate: 2023-08-01
DOI: 10.1007/s10546-023-00811-3
- The Performance of a Time-Varying Filter Time Under Stable Conditions over
Mountainous Terrain-
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Abstract: Abstract Eddy-covariance data from five stations in the Inn Valley, Austria, are analyzed for stable conditions to determine the gap scale that separates turbulent from large-scale, non-turbulent motions. The gap scale is identified from (co)spectra calculated from different variables using both Fourier analysis and multi-resolution flux decomposition. A correlation is found between the gap scale and the mean wind speed and stability parameter z/L that is used to determine a time-varying filter time, whose performance in separating turbulent and non-turbulent motions is compared to the performance of constant filter times between 0.5 and 30 min. The impact of applying different filter times on the turbulence statistics depends on the parameter and location, with a comparatively smaller impact on the variance of the vertical wind component than on the horizontal components and the turbulent fluxes. Results indicate that a time-varying filter time based on a multi-variable fit taking both mean wind speed and stability into account and a constant filter time of 2–3 min perform best in that they remove most of the non-turbulent motions while at the same time capturing most of the turbulence. For the studied sites and conditions, a time-varying filter time does not outperform a well chosen constant filter time because of relatively small variations in the filter time predicted by the correlation with mean flow parameters.
PubDate: 2023-07-29
- Wind-Tunnel Experiments of Turbulent Wind Fields over a Two-dimensional
(2D) Steep Hill: Effects of the Stable Boundary Layer-
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Abstract: Abstract Flow separation caused by steep topography remains a significant obstacle in accurately predicting turbulent boundary-layer flows over complex terrain, despite the utilization of sophisticated numerical models. The addition of atmospheric thermal stability, in conjunction with steep topography, further complicates the determination of disrupted turbulent wind patterns. The turbulent separated flows over a two-dimensional (2D) steep hill under thermal stratification has not been extensively addressed in previous experimental studies. Such measurements are crucial for enhancing our comprehension of flow physics and validating numerical models. We measured the turbulent wind flows over a 2D steep hill immersed in a stable boundary layer (of the bulk Richardson Number \(\textrm{Ri}_b\) = 0.256) in a thermally-stratified boundary-layer wind tunnel. The flow separation, re-circulation zone and flow reattachment were characterized by the planar particle image velocimetry technique. Vertical profiles of mean air temperature and its fluctuations are also quantified at representative locations above the 2D steep hill and in the near wake region. Results indicate that the separated shear layer, initiated near the crest of the 2D steep hill, dominates the physical process leading to high turbulence levels and the turbulent kinetic energy production in the wake region for both stable and neutral thermal stability. Although the stable boundary layer does not dramatically change the turbulent flow pattern around the hill, the mean separation bubble is elongated by 13%, and its vertical extent is decreased by approximately 20%. Furthermore, the reduced turbulence intensities and turbulent kinetic energy of the near wake flow are attributed to the relatively low turbulence intensity and low momentum of the stable boundary layer due to buoyancy damping, compared to the neutral boundary layer. Additionally, a distinct low-temperature region—a cold pool—is extended beyond the separation bubble, reflecting the significant sheltering effect of the 2D steep hill on the downwind flow and temperature field.
PubDate: 2023-07-13
DOI: 10.1007/s10546-023-00820-2
- The Role of Vertical Diffusion Parameterizations in the Dynamics and
Accuracy of Simulated Intensifying Hurricanes-
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Abstract: Abstract Rotation in hurricane flows can significantly impact the dynamics and structure of the turbulent boundary layer. Despite this unique feature of hurricane boundary layers, the current planetary boundary layer (PBL) schemes in weather models are neither specifically designed nor comprehensively tested for intensifying hurricane flows. The objective of this paper is to bridge this knowledge gap by characterizing the role of vertical diffusion depth and magnitude in simulated hurricane intensity, size, and track. To this end, five major hurricane cases undergoing an intensification period are simulated using two widely used local and non-local PBL schemes in Weather Research and Forecasting (WRF) model. In total, eighty WRF simulations are conducted by varying the grid resolution, PBL scheme, eddy diffusivity depth and magnitude, and PBL height. By decreasing the existing vertical diffusion depth and magnitude, on average, ~ 38 and ~ 24% improvements in hurricane intensity forecasts were obtained compared to the default models. Hence, the results indicate that the current PBL schemes in WRF are overly diffusive for simulating major hurricanes since they do not account for turbulence suppression effects in rotating hurricane flows. The paper yields new insights into the role of vertical diffusion in simulated hurricane dynamics and provides some guidance to enhance the PBL schemes of NWPs for improved hurricane forecasts.
PubDate: 2023-07-12
DOI: 10.1007/s10546-023-00818-w
- Budgets of Second-Order Turbulence Moments over a Real Urban Canopy
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Abstract: Abstract This study analyses budgets of second-order turbulence moments over a real urban canopy using large-eddy simulation. The urban canopy is representative of the City of Boston, MA, United States and is characterized by a significant height variability relative to the mean building height. The budgets of double-averaged Reynolds-stress components, scalar fluxes, and scalar variances are examined with a focus on the importance of the dispersive terms above the mean building height. Results reveal the importance of the wake (dispersive) production term, in addition to the shear production term, in the turbulence kinetic energy (TKE), streamwise velocity variance and scalar variance budgets well above the mean building height. In this region, the turbulent and dispersive transport terms are smaller than the production and dissipation terms. Nonetheless, the dispersive transport terms in the TKE and scalar variance budgets can be as important as their turbulent counterparts. The subgrid-scale dissipation term is the main sink in the TKE, vertical velocity variance and scalar variance budgets. In the momentum and scalar flux budgets, the pressure-strain correlation term and the pressure gradient-scalar interaction term are the significant sink terms, respectively. Our analysis highlights the complexity associated with the budgets of second-order turbulence moments over real urban canopies and has important implications for developing urban parameterizations for weather and climate models.
PubDate: 2023-07-03
DOI: 10.1007/s10546-023-00816-y
- A Quantitative Study of Turbulent Fluxes over a Coastal Station
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Abstract: Abstract A comprehensive investigation is undertaken to discern the structure of momentum flux, turbulent kinetic energy, and scalar fluxes like heat, CO \(_2\) , and H \(_2\) O in the atmospheric surface layer (ASL) at the Thumba Equatorial Rocket Launching Station—a coastal station on the west coast of southern peninsular India. The vertical transport, transfer efficiency, and dissimilarity between flux transport are studied as a function of stability using data collected over 1 year. The transfer efficiency for heat fluxes and momentum exhibits a strong dependence on stability ( \(\zeta \) ). However, the transfer efficiency of passive scalars CO \(_2\) and H \(_2\) O displays no apparent dependence on \(\zeta \) . The correlation between fluxes and squared coherence estimates is evaluated to study the dissimilarity between flux transport. The correlation is strongest among momentum and heat fluxes and between CO \(_2\) and H \(_2\) O fluxes and shows a dependence on the prevailing stability conditions. However, the influence of stability is not evident for the various other combinations. The momentum and heat flux transport is dissimilar for unstable conditions, and it becomes similar during the transition from unstable to near-neutral conditions. The quadrant analysis is employed to study the contribution of different fluid motions to the aforementioned turbulent fluxes. Except for CO \(_2\) and H \(_2\) O fluxes, where all the quadrants have an equal contribution, ejections and sweeps are the dominating contributors for momentum and heat fluxes. The stability conditions greatly determine the ejection-sweep balance for heat flux, while some changes in duration and impact fraction are also detectable for momentum flux. Furthermore, contour maps of joint-probability function (JPDF) of vertical velocity fluctuations ( \(w'\) ) with streamwise velocity fluctuation ( \(u'\) ), temperature fluctuation ( \(T'\) ), and scalar fluctuations, respectively, are also presented. The dominance of the ejection and sweep cycles for turbulent fluxes provide evidence for the presence and importance of coherent structures in ASL.
PubDate: 2023-07-01
DOI: 10.1007/s10546-023-00802-4
- Turbulence Characteristics in the Atmospheric Surface Layer Over a
Heterogeneous Cultivated Surface in a Tropical Region-
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Abstract: Abstract The description of atmospheric turbulence on heterogeneous surfaces and more specifically during stable stratification conditions, remains nowadays a relevant and open issue in micrometeorology. To partially remedy this problem, we investigate in this study, the behaviour of turbulence above a heterogeneous cultivated surface during stable and unstable conditions. The analyses were realized according to seasons using almost seven years of eddy covariance measurements acquired at 4.95 m height. The results showed that the turbulence kinetic energy per unit mass was on average less than 0.5 m2 s−2 at night but increased during the day. It hits around local noon a peak value ~ 1.6 m2 s−2 on average. The peak value is relatively lower (~ 0.92 m2 s−2) during the wet season compared to other periods (1.6 m2 s−2). The average value of turbulence intensities are Iu = 0.44, Iv = 0.50 and Iw = 0.22 following u, v, w wind speed directions. During stable conditions, there are turbulent movements when the flux Richardson number is less than a critical value estimated approximately to 0.17. The flux-variance similarity functions that characterize the behaviour of turbulence are seasonally dependent, especially under stable conditions. However, each of these functions obey the Monin–Obukhov Similarity Theory whatever the stratification except those of temperature under near-neutral conditions. The results obtained indicate that the atmospheric turbulence is relative to the type of ecosystem and the height of measurement. This process is led by aerodynamic parameters and wind speed.
PubDate: 2023-06-24
DOI: 10.1007/s10546-023-00815-z
- The Departure from Mixed-Layer Similarity During the Afternoon Decay of
Turbulence in the Free-Convective Boundary Layer: Results from Large-Eddy
Simulations-
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Abstract: Abstract 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 ( \(\widetilde{r}\) ) 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 \(\widetilde{r}\) 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.
PubDate: 2023-06-21
DOI: 10.1007/s10546-023-00812-2
- Statistics of Wind Farm Wakes for Different Layouts and Ground Roughness
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Abstract: Abstract In this work, wakes of wind farms are investigated using large-eddy simulation with an actuator disk model for the wind turbine. The effects of streamwise turbine spacings, number of wind turbine rows and roughness lengths of ground surface on the characteristics of wind farm wakes are examined. The simulation results showed that the effects of \(S_x\) (streamwise turbine spacings) are mainly located in the near wake of wind farm (less than 20 rotor diameters downstream from the last row of the wind farm), where the turbulence intensity is higher for smaller values of \(S_x\) . In the far wake of wind farms (more than 90 rotor diameters downstream from the last row of the wind farm), the streamwise velocity deficit as well as the Reynolds stresses from cases with different streamwise turbine spacings are close to each other. For cases with more wind turbine rows ( \(N_{row}\) ) and larger roughness length of ground surface ( \(k_0\) ), faster velocity recovery and higher turbulence intensity are observed. Terms in the budget equation for mean kinetic energy (MKE) are examined. The analyses showed that the vertical MKE transport via mean convection and turbulence convection plays a dominant role in the velocity recovery in wind farm wakes, being different from the wind farm region where streamwise MKE flux due to mean convection also plays a role. Lastly, an analytical model for the velocity deficit in wind farm wake is proposed based on the Emeis model. Improvements on the model predictions are observed for all the simulated cases. The velocity deficit at one downstream location of the wind farm needs to be given is one major limitation of the analytical model of this type.
PubDate: 2023-06-02
DOI: 10.1007/s10546-023-00814-0
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