Impact of the thermal stratification on the atmospheric flow over a topography

Transcription

1 Impact of the thermal stratification on the atmospheric flow over a topography A. Butet METEO-FRANCE CNRM/GMEI42 av. Coriolis Toulouse Cedex, France alain.butet@meteo.fr Abstract Chemical releases are particulary dangerous when occuring under meteorological conditions such as stable thermal stratification and light wind as defined by a small Froude number. The consequences of such releases are even worse when the source is situated near hills or mountains. At small Froude number, the buoyancy forces control the flow dynamics and prevent the flow from passing over the hill. In order to describe these phenomena and to validate a non-hydrostatic numerical model (Meso-NH), Meteo-France has taken part in a multidisciplinary study (PRIMEQUAL) on the city of Toulouse. This paper presents the field measurements performed in the winters 1995 and 1996 with an automatic weather stations network and a radiosounding to select and define the stratified meteorological conditions. Laboratory experiments at a 1:10000 scale were carried out in a towing tank filled with stratified brine to describe the transition regime around the Froude number 0.3. Finally, some comparisons between these experimental results and numerical simulations will be provided. 1 Introduction In 1995, a research project PRIMEQUAL has been initialized by the French Ministery of Environment and Research to improve the knowledge about atmospheric pollution and the control of the air quality. This project is carried out by several laboratories in a multidisciplinary action : INRETS (Institut National de Recherche sur les Transports et leur Securite) to evalute the

2 532 Air Pollution de Recherche sur les Transports et leur Securite) to evalute the pollutant emissions with TURBAN code, L.A. (Laboratoire d'aerologie de 1'universitee Paul Sabatier) to establish the physical-chemical processes, Meteo-France to evaluate a dynamic model of the atmosphere and ORAMIP (Observatoire Regional de 1'Air en Midi-Pyrenees) to plane field measurements of NO% and CO pollutants. This paper presents the contribution of Meteo-France especially in the evaluation of the atmospheric dynamics when the meteorological conditions present stable thermal stratification in low atmospheric layers. In contrast to the situations associated with a neutral atmosphere and strong wind which are propitious to disperse pollutants by turbulent mixing, these situations induce the most critical hazards. The effect of the buoyancy forces is no longer negligible compared to the inertial forces and the vertical motions are reduced. The pollution is enclosed in a thin atmospheric layer and the pollutant concentrations increase. The city of Toulouse has been used as the experimental field for this project. This medium size town is situated on the plain of the Garonne river and includes a large chemical complex close to a hill 120m high (Pech-David hill) in the southern outskirts. This study is composed of several measurement campaigns to define the atmospheric stability conditions, a set of simulations in a hydraulic channel to analyse the flow behaviour and, finally, a numerical simulation using a nonhydrostatic model. 2 Field measurements 2.1 Experimental set-up In order to obtain the flow description in the low atmospheric layers, a large experimental set-up has been put in place during the winters 1995 and 1996 (Butet[3]). It is based on a network of 8 automatic weather stations spread over a 8x6 km* domain to measure continuously (@ 0.1 Hz) temperature and wind at 10m over the ground level (Figure 1). The vertical characteristics of the atmosphere are obtained with synchronized radiosounding (mteo) and wind-radar (rvto) during the growth of the thermal stratification from IhOO UTC to 6hOO UTC (one launch each Ih30). Seven intensive observation periods (IOP) were recorded during these compaigns. They present a vertical thermal gradient reaching +3 C/100 m up to a 300 m height. The network data show that the flow is highly dependent on the flow stability defined by the Froude number F,

3 Air Pollution 533 where U, wind velocity (m/s), 0, potential temperature ( K), g, gravity module (9.81 m/s*), z, vertical coordinate (m). When the Froude number is less than 0.3, the buoyancy forces are more important than the inertial forces, the vertical motions are prevented and the flow is forced to turn around the hill. This phenomena has been already identified on a bell-shaped topography in the model experiments of Hunt et al.[3] in 1980 and Kadri et al.[6] in The IOP of March 19,1996 The flow transition is clearly illustrated by the IOP recorded on March 19, In the same night, a strong stratification (+2.5*C/100m) with a light wind defined by a Froude number of 0.3 is followed after 3h UTC by the outbreak of a strong wind associated with a Froude number of the order of 1 (Figure 2). The wind direction in altitude stays from 135 during all the period. The wind data display a stagnation flow downstream of the hill during the first part of the night. A reverse flow is visible at stations 11, 16 and 17. During the second part of the night, the wind speed increases from 4 m/s to 13 m/s at 300 m of altitude. The stagnant area disappears and the flow takes the direction of the altitude wind. Then, the turbulent mixing reduces the vertical thermal gradient and the temperature difference between the stations (13 and 14) on top of the hill and those (11, 15, 16 and 17) layed on the plain decreases from 4 C to 0.5 C. 3 Hydraulic simulation Laboratory experiments provide an accurate study of the flow behaviour on the both sides of this flow transition for the Froude numbers 0.2 and 0.4 (Butet[4]). The windfieldsare measured at four levels : 15, 50, 110 and 220 m over the plain. In order to visualize the impact of the stratification on pollutant diffusion, plumes were simulated by dye emissions. 3.1 Experimental apparatus The hydraulic simulations were performed in a towing tank facility (7 x 0.8 x 0.7 m^) filled with stratified brine. The linear stratification is obtained by a computer monitored filling process which mixes variable quantities of clear water with a saturated salt solution. A model at 1:10000 scale restitutes the topography of the Pech-David hill. In order to create the initial altitude flow, the model is towed over the channel floor at constant speed in still fluid. In hydraulic simulation, the Froude number is defined as the ratio of the advection frequency to the Brunt-Vaisala frequency N,

4 534 Air Pollution YS: (2) p oz where p,fluiddensity (kg/m3), z, vertical ccordinate (m), g, gravity module (9.81 m/s*). The Froude number expression becomes : where F = -2- (3) N A [/, towing velocity (m/s), //, Brunt-Vaisala frequency (rad/s) A, topography height (m). In all these experiments, the density gradients remained constant with a typical Brunt-Vaisala frequency around 1 rad/s. The Froude similarity is reached by tuning the towing velocity from 0.23 to 0.46 cm/s to obtain the Froude numbers 0.2 and Visualization and velocity measurement Flow visualization techniques were used extensively in obtaining both qualitative and quantitative information on the flow structure around the topography. Styren micro-particles in hydrostatic equilibrium with the surrounding brine are illuminated by a laser sheet at four levels (15, 50, 110 and 220 mm). The visualizations are recorded on VCR using a black and white high resolution CCD. Camera and laser sheet are moved with the model. The images are digitalized as soon as the stationary flow appears after the dimensionless time N t > 200. A PITV (Particle Image Tracking Velocimetry) software computes the particles motion and provide the velocity field on a regular grid of 200 m resolution. The detection of 100 particles is typical enough to calculate the velocity field. However, this amount reaches up to 300 when the flow is highly sheared and contains small-scale vortices. In order to validate the accurancy of the results, the particle trajectories are computed with the velocity field and compared to the particle streaks recorded in video. 3.3 Flows at Froude number 0.2 and 0.4 At Froude number 0.2, the flow is constrained to move in a horizontal plane and the fluid is forced to pass around the topography (Figure 3a). The downstream flow separation induces an increase of the flow velocity and a westward rotation near the north end of the hill. The wake structure displays a stagnation region at levels 15 m and 50 m. Above, the westward rotation is conserved and the stagnant area is replaced by a slow flow in the free-stream direction. At higher Froude number (Figure 3b), the kinetic energy is more

5 Air Pollution 535 signifiant and the fluid can pass over the hill. From levels 15m and 50 m, the stagnant area desappears and is replaced by a slow flow in the initial upstream direction. The downstream flow separation is still visible near the north end of the hill. At the level 110m and above, the flow takes the main direction of the free stream. 3.4 Plume dispersion The dye emission visualization allow to evaluate the impact of the stratification on accidental releases. Fluorescein dye is mixed with brine and emitted from the chemical site at 15 m level. The plume dispersion is visualized by a laser sheet at levels 15 m and 50 m (Figure 4). At Froude number 0.2, the plume undergoes a rotation up to 90 in short distance. Due to the horizontal shear flow in low layers, the plume trajectory and the forecast polluted regions are highly dependent on the initial position of the source. At Froude number 0.4, there is no deviation and the plume direction is as the altitude flow. The pollution is advected downstream of the hill. The dye discountinuity downstream of the source reveals the presence of vertical motions in the wake flow. 4 Numerical simulation The Meso-NH atmospheric simulation model has been jointly developped by Meteo-France and L.A. during the period (Bougeaultfl], Laforef?]). This research numerical model adapted to the community scientists is able to simulate the atmospheric motions ranging from the large meso-alpha scale down to the micro-scale. A set of facilities prepares the initial states from idealized cases or meteorological analyses. This model is based on the Lipps and Hemler form of the anelastic system. It allows for simultaneous simulation of several scales of motion by an interactive grid-nesting technique. It is able to calculate the transport and diffusion of passive scalars and to be coupled with a chemical module. The turbulence parametrization is one-and-a-half-order closure scheme, involving a prognostic equation for kinetic energy, computation of mixing lengths and a Richardson-number formulation for the Prandtl numbers. The validation cases demonstrate that this model is able to produce state-of-art results for a range of grid size from 1 m to 100 km. In the PRIMEQUAL framework, the Meso-NH model is used on a 20x20 km* domain with a grid of 500 m resolution and a vertical extension reaching 3500m on 35 levels (Bouzom[6]>. The initial state is provided by the radiosounding of the March 19, IOP. At Ih30 UTC (Froude number < 0.3), the computed velocity field reconstitutes the stratification effects (Figure 5). The stagnation region is well identified downstream of the topography and the westward rotation of the flow is also displayed at the end of the hill. For the 6hOO UTC initial conditions (Froude number > 0.3), the flow takes again the

6 536 Air Pollution main direction of the altitude wind and the stagnant area disappears. The numerical simulation is in good agreement with the experimental results. 5 Conclusion This study performed by Meteo-France reveals the impact of the thermal stratification on the atmospheric flow defined by a small Froude number. Initialized from real data collected in field measurements over the Pech-David hill near Toulouse, the experimental simulations describe finely the flow behaviour on the both sides of the critical Froude number 0.3. Under this value, perturbations appear even for a small topography like the Pech-David hill. The forecast trajectory of accidental releases is unusual and highly dependent on the source position. For Froude numbers larger than 0.3, the flow takes again a classical dynamics. These results have helped to validate the capabilities of the Meso-NH model to reproduce, at small scales, the effects of the stratification. The atmospheric dynamics now being understood, the next place of PRIMEQUAL project will be to include the pollutant emissions and the physical-chemical processes to evaluate the air pollution transport around a city. References 1. Bougeault, P., Belair, S., Carriere, S.M., Cuxar, I, et al., The Meso-NH atmospheric simulation system: scientific documentation, Meteo-France and CNRS, Bouzom, M., Etude des phenomenes physico-chimiques de pollution lies aux transport en agglomeration, final report Meteo-France/SCEM, Butet, A., Experience Toulouse-Sud : Documentation des situations en atmosphere stable, internal note Meteo-France/CNRM, Butet, A., Simulation hydraulique des ecoulements en atmosphere stable sur le site de Toulouse-Sud, internal note Meteo-France/CNRM, Hunt, J.C.R. & Snyder, W.H., Experiments on stability and neutrally stratified flow over a model three-dimensional hill, J. Fluid Mech. N 96, part 4, pp , Kadri,Y., Bonneton, P., Chomaz, J.M. & Perrier, M., Stratified low over three-dimensional topography, Dyn. Atmos.&Oceans N 24, pp , Lafore, J.P., Stein, J., Asencio, N., Bougeault, P., et al., The Meso-NH atmospheric system. Part 1: adiabatic formulation and control simulations, Ann. Geophysicae 16, pp , 1998

7 Air Pollution I Figure 1. Experimental domain in south of Toulouse ; (10->17) automatic weather stations, "mteo" radiosounding, "rvto" wind-radar. 1h30UTC Froude < 0.3 6hOO UTC Froude > 0.3 Figure 2. Wind at 10 m above the ground level during the Froude number transition in the night of March 19, 1996.

8 538 Air Pollution (a): Froude 0.2 Z = 15m wind direction (b): Froude 0.4 north direction Z = 110m Figure 3. Velocity fields at 15 m and 110 m elevation in the experimental simulations at Froude number: (a) 0.2 and (b) 0.4

9 Air Pollution 539 = 0.2 wind direction = 0.4 Figure 4. Plume dispersion and flow pattern in experimental simulations at Froude numbers 0.2 and /03/19 1h30 50m/Goronne 96/03/19 6hOO 50m/Garonne F < 0.3 F > 0.3 Figure 5. Velocity fields in numerical simulations around the Froude number 0.3.