L. Briatore

A laboratory model of wake-affected stack's emissions

Abstract The high demand for electric energy during winter months, which coincides with the most critical conditions from the point of view of atmospheric pollution, makes it necessary to exploit at their maximum capacity even the oldest and smallest power plants, where dispersion is usually inefficient and greatly influenced by the aerodynamic wake of the buildings. In this paper the results of a series of hydrodynamic simulations, concerning the emissions of two 70 MW units of an electric power plant are presented. It was, in fact, necessary to decide whether a certain height-increase of the stacks, within the limits imposed by the building structures, would be sufficient to reduce the elevated ground-level concentrations found in some critical wind conditions, or if new stacks, external to the power plant buildings, would be needed.

A laboratory simulation of mesoscale flow interaction with the Alps

Abstract A series of laboratory experiments, aimed at the simulation of some aspects of Alpine lee cyclogenesis has been carried out in the rotating tank of the Coriolis Laboratory of LEGI-IMG in Grenoble. Dynamic and thermodynamic processes, typical of baroclinic development triggered by the orography, were simulated. The background flow simulating the basic state of the atmosphere consisted of a stream of intermediate density fluid introduced at the interface between two fluid layers. The structure of the intermediate current was established by mixing fluid obtained from the upper layer of fresh water with fluid removed from the heavier salty layer below. The dynamical similarity parameters are the Rossby ( Ro ), Burger ( Bu ) and Ekman ( Ek ) numbers, although this last, owing to its small values, need not be matched between model and prototype, since viscous effects are not important for small time scales. The flow in both the prototype and laboratory simulation is characterized by hydrostatics; this requires ( Ro 2 δ 2 / Bu )⪡1 (where δ = H / L is the aspect ratio of the obstacle) which is clearly satisfied, in the atmosphere and oceans, and for the laboratory experiment. A range of experiments for various Rossby and Burger numbers were conducted which delimited the region of parameter space for which background flows akin to that found to the northwest of the Alps prior to baroclinic cyclogenesis events, were observed. One such experiment was carried out by placing a model of the Alps at the appropriate place in the flow field. The subsequent motion in the laboratory was observed and dye tracer motions were used to obtain the approximate particle trajectories. The density field was also analyzed to provide the geopotential field of the simulated atmosphere. Using standard transformations from the similarity analysis, the laboratory observations were related to the prototype atmosphere. The flow and the geopotential fields gave results compatible with the particular atmospheric event presented.

Laboratory simulation of inertial and frictional effects on barotropic rotating flows over and past obstacles: Comparison with simple numerical and analytical models

SummaryThis paper presents the results of laboratory experiments designed to simulate some basic process of large-scale flows interacting with obstacles, also in order to better understand details of subsynoptic disturbances that are created in the lee of large topographic features. For this event, the experimental facilities of the Istituto di Cosmogeofisica of Consiglio Nazionale delle Ricerche (CNR) were exploited, consisting in a hydraulic channel mounted on a rotating platform, along whose longitudinal axis a hemispherical obstacle was towed at various speeds. Because of the conversion of potential vorticity, the experimental results showed, as expected, the existence of a region of anticyclonic circulation, located above the obstacle; however, also an asymmetric pattern of positive vorticity located downwind of the obstacle did appear, which cannot be interpreted in terms of simple quasi-geostrophic inviscid dynamics. This behaviour is not surprising, if one considers that the real flow near the obstacle could hardly ever match the conditions of inviscid quasi-geostrophy (Ro≪1,E∼0), but was similar to that characterising the zone close to the surface of the obstacle, where inertial and viscous effects are not negligible. Finally, in order to investigate the importance of these effects on the interaction processes, simple numerical and analytical models were applied, by which the consistency of some laboratory simulations, chosen among the most significant ones, could be compared.

A laboratory experiment on the development of cyclogenesis in the lee of a mountain

SummaryOne of the most intriguing problems concerning the interaction of subsynoptic and synoptic atmospheric flows with topographic features is orographic cyclogenesis. A fully satisfactory prediction theory is not yet available, but a lot of efforts have been made by theoreticians to implement reliable numerical models simulating the different phases of this complex phenomenon. An attempt to perform a laboratory experiment to simulate physically this kind of interaction has been made by us, through the generation of a baroclinic frontal system in the rotating hydraulic platform of the «Coriolis Laboratory—LEGI-IMG—Grenoble». The adopted technique consists in a device which produces, at the interface separating two water layers of different density (ϱ1 and ϱ2), a stream of stratified fluid whose density has an intermediate value ϱ1 < ϱm < ϱ2. This stream is generated at the height of the interface between the two layers; due to the rotation of the platform, the attainment of geostrophic equilibrium brings about an intermediate-water flow running along a wall, giving rise to a three-layer baroclinic structure which can represent some of the main outstanding features of an atmospheric frontal system impinging on a mountain. In a well-defined range of the Rossby and Burger numbers, the instability of this current gives rise to a couple of persistent cyclonic and anticyclonic vortices, whose horizontal dimensions and vertical extents reproduce quite faithfully the synoptic situation supporting the onset of the orographic cyclogenesis, with its characteristic cold front stretching between the two vortex structures. It is enough to place an obstacle of a suitable size in the proper geographic position, to make the cyclogenesis start. The first results of our simulations have been encouraging, showing the occurrence of lee cyclogenesis when the stream conditions in our model correspond to the synoptic features which have been recognized as the precursors of orographic cyclogenesis in the lee of the Alps.

Interaction between atmospheric flows and obstacles: Experiments in a rotating channel

We performed an experimental study using scale models in a hydrodynamic rotating channel, concerning the interactions between fluid flows and obstacles of different shapes. The study was meant to analyze the characteristics of the wakes observed on the lee side of quasi-bidimensional obstacles, in a neutral atmosphere.The obstacles were half-cylinders (with aspect ratio 0.87), placed transversally on the channel bottom and totally submerged in the fluid. We call them “quasi-bidimensional” since their width was a little smaller than the channel width, thus allowing the flow to partially go round their edges.The simulations were performed in the rotating hydraulic channel of ICG-CNR in Turin, and included various conditions of rotation period and flow speed. An interesting behaviour of the wakes was found on the lee side of subsynoptic-scale obstacles, modelled in conditions of Reynolds-Rossby similitude. More precisely, if a given threshold of flow velocity is exceeded, wake size is constant and is fully determined by the height of the obstacle.

A Wind-Speed Anomaly in the Surface Layer in the Po Valley

A wind anomaly, consisting of a relative minimum in speed at a height of 100 m, in a direction opposite to that of the local flows, has been observed on the meteorological tower of Trino Vercellese, at the western extremity of the Po Valley. An interpretation in terms of a thermal wind, due to an horizontal thermal gradient induced by sloping terrain close to the site, is suggested.

Experimental study in a rotating channel on the similarity law of tracer concentration distribution in the turbulent Ekman boundary layer

Abstract Concentration disrribution of airborne pollutants released into the atmosphere by large and elevated industrial sources or extended urban areas generally depends on the dynamical and thermal structure of the whole depth of the atmospheric planetary boundary layer (PBL). In order to be able to get meaningful predictions of pollutant dispersion from simple dispersion formulae using surface parameters as input data (like the Gaussian one), it is necessary to establish some reliable relationships between these surface parameters and the whole PBLs vertical structure. This paper presents the results of laboratory experiments in rotating hydraulic channel, aimed at investigating the influence of the Earth rotation on the dispersion of tracers which spread in a near-neutral Ekman turbulent PBL and at expressing the change of u and v components of mean wind with height in terms of rotation and turbulence scales typical of the atmospheric PBL. The results provide some evidence that in a near-neutral PBL the profiles of dilution ratio, adimensionalized by means of friction velocity u ∗ = √ u′w′ and Coriolis parameter f = 2Ω sin ϕ , as a function of an adimensional height zf χ u ∗ , where χ is the von Karman constant, can be described by a universal function for a given Rossby number of the flow. This result confirms the theoretical expectation put forth by Wippermann and Yordanov in their study on the prediction of concentration patterns in a rotating turbulent PBL.

Laboratory simulation of coriolis effects on atmospheric dispersion of airborne tracers over a complex terrain

SummaryThe experimental research described here deals with a problem of diffusion and transport on local scale (30 km) of pollutants of urban and industrial origin, emitted into the atmosphere of a complex terrain site, with noticeable orographical features. The diffusion process was visualized and analysed in detail. The results of this research allowed acquiring the capability of forecasting environmental impact on the area, due to the sources considered. On account of its spatial scale, the phenomenon was investigated in a rotating hydraulic channel, by the method of simulation on physical model at reduced scale. Typical local atmospheric conditions, taking into account diurnal evolution of stability inside a turbulent, rotating planetary boundary layer, were modeled. Seasonal variations of sources emission rates were considered. Measurements of dilution factors at several spatial points of the model and at five levels (from ground up to a height equivalent to about 1000m) allowed mapping out concentration isolines over the site in various conditions and producing evolving contour plots which visualized the diffusion processes in a very useful way for planning purposes.