Roll vortices in the planetary boundary layer: A review
TL;DR: Roll vortices may be loosely defined as quasi-two-dimensional organized large eddies with their horizontal axis extending through the whole planetary boundary layer (PBL), and their indirect manifestation is most obvious in so-called cloud streets as can be seen in numerous satellite pictures as mentioned in this paper.
Abstract: Roll vortices may be loosely defined as quasi two-dimensional organized large eddies with their horizontal axis extending through the whole planetary boundary layer (PBL). Their indirect manifestation is most obvious in so-called cloud streets as can be seen in numerous satellite pictures. Although this phenomenon has been known for more than twenty years and has been treated in a review by one of us (R.A.Brown) in 1980, there has been a recent resurgence in interest and information. The interest in ocena/land-atmosphere interactions in the context of climate modeling has led to detailed observational and modeling efforts on this problem. The presence of rolls can have a large impact on flux modelling in the PBL. Hence, we shall review recent advances in our understanding of organized large eddies in the PBL and on their role in vertical transport of momentum, heat, moisture and chemical trace substances within the lowest part of the atmosphere.
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TL;DR: In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations.
Abstract: In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate these urban weather features for comparison with observations. WRF is also used to test the sensitivity of model simulations to different urban land use scenarios and urban building structures to investigate the impacts of urbanization on surface weather and boundary layer structures. Results show that the coupled WRF/Noah/UCM modeling system seems to be able to reproduce the following observed features reasonably well: 1) the diurnal variation of UHI intensity; 2) the spatial distribution of UHI in Beijing; 3) the diurnal variation of wind speed and direction, and interactions between mountain–valley circulations and UHI; 4) small-scale boundary layer conv...
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...Hence the aspect ratio l/H of HCRs in urban areas (;1.5) is smaller than the typical value over natural landscapes (2–15) (Etling and Brown 1993; Atkinson and Zhang 1996)....
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TL;DR: In this paper, the authors review possible mechanisms by which global climate variability and change may influence the wind energy resource and operating conditions, summarize some of the tools that are being employed to quantify these effects and the sources of uncertainty in making such projections, and discuss results of studies conducted to date.
Abstract: Expansion of wind energy installed capacity is poised to play a key role in climate change mitigation. However, wind energy is also susceptible to global climate change. Some changes associated with climate evolution will likely benefit the wind energy industry while other changes may negatively impact wind energy developments, with such ‘gains and losses’ depending on the region under consideration. Herein we review possible mechanisms by which global climate variability and change may influence the wind energy resource and operating conditions, summarize some of the tools that are being employed to quantify these effects and the sources of uncertainty in making such projections, and discuss results of studies conducted to date. We present illustrative examples of research from northern Europe. Climate change analyses conducted for this region, which has shown considerable penetration of wind energy, imply that in the near-term (i.e. to the middle of the current century) natural variability exceeds the climate change signal in the wind energy resource and extreme wind speeds, but there will likely be a decline in icing frequency and sea ice both of which will tend to benefit the wind energy industry. By the end of the twenty-first century there is evidence for small magnitude changes in the wind resource (though the sign of the change remains uncertain), for increases in extreme wind speeds, and continued declines in sea ice and icing frequencies. Thus the current state-of-the-art suggests no detectable change in the wind resource or other external conditions that could jeopardize the continued exploitation of wind energy in northern Europe, though further research is needed to provide greater confidence in these projections.
397 citations
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TL;DR: The Turbulent Ocean as discussed by the authors describes the principal dynamic processes that control the distribution of turbulence, its dissipation of kinetic energy and its effects on the dispersion of properties such as heat, salinity, and dissolved or suspended matter in the deep ocean, the shallow coastal and the continental shelf seas.
Abstract: The subject of ocean turbulence is in a state of discovery and development with many intellectual challenges. This book describes the principal dynamic processes that control the distribution of turbulence, its dissipation of kinetic energy and its effects on the dispersion of properties such as heat, salinity, and dissolved or suspended matter in the deep ocean, the shallow coastal and the continental shelf seas. It focuses on the measurement of turbulence, and the consequences of turbulent motion in the oceanic boundary layers at the sea surface and near the seabed. Processes are illustrated by examples of laboratory experiments and field observations. The Turbulent Ocean provides an excellent resource for senior undergraduate and graduate courses, as well as an introduction and general overview for researchers. It will be of interest to all those involved in the study of fluid motion, in particular geophysical fluid mechanics, meteorology and the dynamics of lakes.
380 citations
TL;DR: An overview of the atmospheric boundary layer (ABL) over both continental and ocean surfaces, mainly from observational and modelling perspectives, is given in this paper, where the authors present exciting challenges so far as improved understanding of ABL behaviour and improved representation of the ABL in numerical models of the atmosphere are concerned.
361 citations
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...Most recently, an up-to-date review of roll vortices in the ABL has appeared (Etling and Brown, 1993), following on from that by Brown (1980)....
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TL;DR: A review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere can be found in this paper, where a review of theoretical and laboratory analyses and numerical modeling of MSC is presented.
Abstract: This paper is a review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length scale of a few to a few tens of kilometers, and takes distinctive planforms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MCC), comprising three-dimensional cells. MSC is characterized by its shape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For cells the horizontal extent is their diameter, whereas for rolls it is their spacing. Rolls usually align along or at angles of up to 10° from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rolls may occur over both water surface and land surfaces. Mesoscale convective cells may be divided into two types: open and closed. Open-cell circulation has downward motion and clear sky in the cell center, surrounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50, and both occur in a convective layer with a depth of about 1 to 3 km. Both the magnitude and direction of horizontal wind in the convective layer change little with height. MSC results from a complex and incompletely understood mix of processes. These processes are outlined, and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of MSC.
331 citations
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TL;DR: In this paper, the role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations, and diagrams and graphs are provided.
Abstract: The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.
2,518 citations
1,750 citations
TL;DR: In this paper, a new large-eddy-simulation (LES) model code which uses a mixed pseudospectral finite-difference method was developed and tested with a simple vortex flow and with the Wangara day-33 data.
Abstract: A large-eddy-simulation (LFS) model explicitly calculates the large-eddy field and parameterizes the small eddies. The large eddies in the atmospheric boundary layer are believed to be much more important and more flow-dependent than the small eddies. The LES model results are therefore believed to be relatively insensitive to the parameterization scheme for the small eddies. Deardorff first applied this type of numerical model to boundary-layer turbulence. In order to continue his important work, and to take advantage of the fast Fourier transformation algorithm, a new LES model code which uses a mixed pseudospectral finite-difference method was developed. This LES model is described here and tested with a simple vortex flow and with the Wangara day-33 data. This model will be used to systematically investigate fundamental problems in the area of boundary-layer turbulence. It is hoped that three-dimensional simulations will give useful statistical information about turbulence structural and impr...
1,308 citations
TL;DR: In this paper, a simple formulation of the boundary layer is developed for use in large-scale models and other situations where simplicity is required, where some resolution is possible within the boundary layers, but where the resolution is insufficient for resolving the detailed boundary-layer structure and overlying capping inversion.
Abstract: A simple formulation of the boundary layer is developed for use in large-scale models and other situations where simplicity is required. The formulation is suited for use in models where some resolution is possible within the boundary layer, but where the resolution is insufficient for resolving the detailed boundary-layer structure and overlying capping inversion. Surface fluxes are represented in terms of similarity theory while turbulent diffusivities above the surface layer are formulated in terms of bulk similarity considerations and matching conditions at the top of the surface layer. The boundary-layer depth is expressed in terms of a bulk Richardson number which is modified to include the influence of thermals. Attention is devoted to the interrelationship between predicted boundary-layer growth, the turbulent diffusivity profile, ‘countergradient’ heat flux and truncation errors. The model predicts growth of the convectively mixed layer reasonably well and is well-behaved in cases of weak surface heat flux and transitions between stable and unstable cases. The evolution of the modelled boundary layer is studied for different ratios of surface evaporation to potential evaporation. Typical variations of surface evaporation result in a much greater variation in boundary-layer depth than that caused by the choice of the boundary-layer depth formulation.
1,195 citations
TL;DR: In this article, numerical integrations are presented for a neutrally stratified planetary boundary layer containing a passive scalar, and for three unstable cases with upward heat flux, assuming the air is assumed unsaturated.
Abstract: Results of numerical integrations are presented for a neutrally stratified planetary boundary layer containing a passive scalar, and for three unstable cases with upward heat flux. The air is assumed unsaturated. A total of either 16,000 or 32,000 grid points was used in a three-dimensional region with length and width several times the height of the boundary layer. A key result is the irrelevance of the neutral height scale, u*/f, and its replacement by the height zi of the inversion base which confines the convective mixing when m and only for the two slightly unstable cases were the vertical velocity eddies distinctly elongated as in Ekman-layer theories. At large instabilities it is shown how the friction velocity u/* loses its influence upon the turbulence intensifies and a convective velocity wale becomes important. Vertical profiles of mean wind, potential tempe...
905 citations