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• W2296939629 abstract "The pollutant transport associated with TRANSALP experimental tracer release, conducted in South Switzerland on 29 September 1990, is simulated. The region in which experiment (i.e. release and concentration measurements) took place belongs to a complex topographical structure. The TRANSALP release was made at a critical site of terrain, i.e. at beginning and near base of a major valley (Leventina valley) while a quite dense network of ground tracer samplers was installed along valley's axis, as well as at several significant surrounding points. The simulation is made using DEMOKRITOS Transport Code System, consisting of Topography Simulator DELTA, Mesoscale Atmospheric Model ADRLA and Lagrangian Dispersion Model DIPCOT-1L The predicted concentration patterns are compared with observations, indicating that major dispersion features are well reproduced. Introduction In large mountain regions topography is usually highly complex with irregularities covering a wide range of sizes. The irregularities can influence considerably regional pressure gradients and induce mechanical effects on wind systems[l]. The air/ground interaction in terms of energy and humidity exchange is highly variable due to rapid changes on surface orientation or altitude or even land use. As a sequence wind system evolution over mountains is of mulliscale character both in space and time especially in case of thermally developed winds. A pollutant release advected by such multiscale wind systems can generate a quite complicated air pollution patterns. The question that this paper is trying to address partly is whether we are in position to predict numerically such complicated air pollution patterns especially when release is occurring in the middle of a mountain range where everything around looks quite complicated. The TRANSALP experiment having above mentioned characteristics refers to Alpine region and it was designed to study transport of atmospheric transport constituents over Alpine barrier from Western Po Valley to Swiss Plateau and vice versa. [2, 3]. The particular area in which experiment (i.e. release and concentration measurements) took place belongs to complex topographical structure of central Alpine area, Transactions on Ecology and Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541 430 Air Pollution Theory and Simulation including upper part of Italian Po valley with first hills and lakes up to main Alpine ridge and a large fraction of Swiss Plateau. This area is often subjected to well developed mesoscale atmospheric systems, generated along slopes and valleys during days of weak synopting forcing. The study is focused on September 29th, 1990. The meteorological conditions at that day was characterized by a mobile anticyclone moving from West Europe to East. The associated upper level winds at same time showed an increased intensity taking a SW direction. Moist air from Adriatic Sea and Po valley was advected into region contributing some low level clouds over Agno south of release which affected full thermal wind development over area[2]. The tracer was released near Giornico at beginning and near base of Leventina valley (see fig.l) at altitude of 360 m. The source was located 8 meters above ground. The tracer used was perfluoromethylhexane PP2 (C; Fjj). The material was released at a rate of 8g/s starting at 10:00 LST together with development of valley breeze and lasted for two hours. Swiss Coordinates (Km) Figure 1: Geographical simulation of area of interest including positions of ground concentration samplers. Transactions on Ecology and Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution Theory and Simulation 431 Methodology The simulation of above mentioned experiment has been performed by utilizing DEMOKRITOS Transport Code System [3]. The calculation domain covers an area 121x101 kmas shown in Fig 1. It includes upper part of Po Valley in Italy, with first hills and lakes up to main Alpine ridge, as well as a large fraction of Swiss Plateau, up to city of Luzern and region around Zurich. The area includes network of ground measurements and two main valleys, located in middle of simulated area. The horizontal grid selected for atmospheric calculations is symmetrical around station Giornico (G), starting with a horizontal mesh of 1km (within first 5km in both axes) and increasing towards boundaries with a geometrical progression relation of ratio 1.15 approximately, up to maximum value of 5km. In vertical axis, a non-homogeneous grid spacing is set, with minimum interval of 50m (over ground) and maximum of 500m (near top). The height of domain is about 9.5km. The Cartesian grid has been selected inslend of a terrain following one since it allows for surged resolution giving ability to include subgrid details in structures permitting more realistic air/ ground interaction especially in energy budget related directly to thermal wind systems evolution. The topographical information was provided by DELTA/GAiA module, with a sub grid resolution of 16 triangles per bottom-boundary surface to allow more detailed description of ground topography[3]. For wind flow prediction mesoscale nohydrostatic compressible mesoscale model ADREA-I has been utilized [4,5]. The wind flow prediction details are given elsewhere [6]. We should mentioned here that calculations were performed, using typical initial conditions for wind, temperature and humidity, corresponding to those prevailing during examined day. The initial wind was set equal to 2m/s (measurements within boundary layer). This low value was kept for whole domain, so that daytime mesoscale effects could be reproduced with minimum large-scale effect. The simulation started at 0700 LST (near sunrise) and continued until 1800 LST (approximately time at which daytime mesoscale systems are dispersed). The wind flow model provided dispersion model with velocity components and substance diffusion coefficients based on 1-Eq turbulent model [4]. The particular option of Lagrangian particle model DIPCOT-1I utilises consept for eddy diflusivity to produce random walk of particles. The complete trajectory of a particle can be given as [7]. Where is u; mean Eulerian velocity in th direction, kj is eddy diffusiviry along same direction and d% is a random term generated from a suitable statistical distribution such as Gaussian or rectangular. Transactions on Ecology and Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541 432 Air Pollution Theory and Simulation Results Concerning wind field predictions, details of data comparisons are given in [6]. The model has quite accurately reproduced major features of day time wind systems especially in area where pollutant cloud was present. Indicative are results of figure 2 illustrating predicted surface wind field at 12:00 LST . The above figure shows development of up valley wind which is in qualitative agreement with fact that stations that measured non-zero concentrations where lying mainly within valley upward release point (see fig. I). Figure 2: Surface Wind Field at area of interest (12:00 LST). Figure 3 is scattergraph of maximum concentrations at stations shown in Figure 1. In most of stations results are quite satisfactory. The good prediction of maximum concentration can be attributed mainly to i) wind prognostic model which reproduced well wind direction and ii) turbulent model used which provided reasonable values for 3-D eddy diffusivity coefficients. On other hand arrival times proved to be less satisfactory. A model time delay of two hours was rather typical more or less for all station. Figure 4 shows concentration time history in station FOPPA (F) included in figure 1. The reason is that model generally undepredicis wind speed especially in area near source. Low space resolution as well as lack of good ground description could be a reason for such a discrepancy. Transactions on Ecology and Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution Theory and Simulation 433" @default.
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• W2296939629 title "The TRANSALP Experiment Tracer ReleaseAnd Transport Simulation" @default.
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