WaTV. ^mo JP, 2P700? zaczmza,

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1 Model simulations of industrial plumes mesoscale interactions in complex coastal area G. Tinarelli,* P. Faggian,* S. Finardi,* G. Brusasca,* G. Carboni', *CISE Divisione Ambiente, via Reggio Emilia 39, Segrate WaTV. ^mo JP, 2P700? zaczmza, Abstract The interaction and possible overlap between the pollution effects generated by different industrial sources influenced by the mesoscale circulation is studied by means of model simulations. A mesoscale, hydrostatic circulation model is used to reproduce the complex structure of both the atmospheric mean flow and turbulence in a region of Central Italy. The pollutant dispersion of two thermal power plants located along the Thyrrenian coast is described by a model integrating the advection-diffusion equation for passive scalars onto an Eulerian grid. A three days simulation has been performed in order to describe a complete diurnal/nocturnal cycle in a situation representing the typical sea/breeze scenario. An intensive measurement program gave the data set used to define both initial and boundary conditions and to validate model results. The model capabilities to reproduce the main features of the atmospheric flow are analysed and results are compared with field measurements not used by the

2 386 Air Pollution Modelling, Monitoring and Management model The simulations have been performed in real meteorological conditions with an hypothetical emission scenario. Results from the dispersion model are also discussed in order to assess the possible interactions of the different sources in the area of interest. 1 Introduction The behaviour of pollutant dispersion emitted by large thermal power plants is a problem of particular interest both at local and regional scales. In this last case the flow complexities induced by mesoscale structures, such as topography irregularities or sea/land discontinuities, generate diffusion patterns with peculiar characteristics, often very difficult to simulate. The emitted plume can actually be trapped into these structures superposed to the synoptic flow, sometimes generating important impact episodes. The situation could become particularly critical when more than one single emission is present on the same area. In this case, different plumes can interact even if they are emitted at considerable distance between them and their overlap should be strongly influenced by local circulations and located in hardly predictable regions. Such complexities can be satisfactorily simulated using a meso scale prognostic model, which integrates the equations needed to correctly describe the air motion at mesoscale (Pielke [1]) over a limited area. Mean flow and turbulence fields generated by these models can be used to produce dispersion simulations, using either Eulerian or Lagrangian approaches (Calbo et al. [2], Anfossi et al. [3]). In this paper, the pollution effect generated by the separated sources in a typical sea/breeze scenario is investigated by means of model simulations. The region of interest is located in Central Italy, along the Thyrrenian coast, where two large thermal power plants are sited at Montalto di Castro and Torvaldaliga, as shown in figure 1. Model results of simulated flow are compared with the available experimental data not used by the simulation itself, to check the representativeness of the reproduced scenario with respect to the typical breeze conditions at the site. Dispersion simulations have been performed in order to reproduce a possible SOz emission configuration. Results have been analysed in order to verify both the behaviour of patterns induced by mesoscale structures and the interactions between the different plumes, identifying possible critical regions into the area. 2 Description of the area The flow simulation area is a 160 x 160 knf square whose topography structure is represented in figure 1. About one half of the area is occupied by the Thyrrenian Sea with the coast line oriented in north-west/south-east direction. The whole domain is located between the Toscana and the Lazio regions of Central Italy. The coastline extends southward from about 30 km north of the Argentario hill to the so called 'Roman shore'.

3 Air Pollution Modelling, Monitoring and Management E x 100 < 150 LEGEND: Mo= MONTALTO (SODAR profiles + ground station) Ca= CANINO (SODAR profiles) To= TORVALDALIGA (SODAR profiles^ s = synop data * = ECMWF data Figure 1: Plant view of the 160x160 Knf indicated. interested area. Emission and measuring points are The topography is quite flat along the coast, except for the Angentario hill site and the Torvaldaliga region, besides which the Tolfa mountains are present. Moving towards the northern and eastern boundaries the Appennini mountains are encountered, reaching maximum heights of about 1400m a.s.l. The two simulated power plants, located at Montalto di Castro and Torvaldaliga, are spaced by a distance of 30 km The dispersion simulation region is a 120 x 120 krrf square nested inside the flow simulation area leaving out a 20 km wide border. 3 Description of the data An intensive measurement program has recently been performed in order to recognize and catalog different flow regimes potentially critical for the dispersion due to the power plant emissions. Data were carried out at the positions indicated in figure 1. Three Doppler Sodars, installed at Montalto di Castro, Torvaldaliga and Canino sites, supplied about one year of hourly wind vertical profiles, starting from April 1995.

4 388 Air Pollution Modelling, Monitoring and Management About 50 percent of the profiles reaches 500 m a.g.l. with a vertical resolution of 25 m allowing a good description of the vertical structures induced by the mesoscale circulation. Ground level wind, temperature, humidity and precipitation were available at Montalto di Castro for the same period and with the same frequency. The total solar radiation, net radiation and sensible heat flux were also measured allowing to check both the radiation and ground interface parametrization schemes inside the flow model. Different flow regimes were identified and cataloged through the analysis of the whole available data set and a three day period of May 1995 was selected for the simulation. During this period a very significant sea/breeze regime characterized by weak low level synoptic driving flow was present, possibly generating recirculations and overlaps of the emitted plumes due to the diurnal/nocturnal cycles. Local measurements have been integrated by wind and temperature vertical profiles from the ECMWF 0.5 grid analysis and synop observations, located as in figure 1. These last data, available every six hours starting from 00:00 UTC, describe the synoptic driving forcing into the area, used to build initial and boundary conditions for the circulation model. 4 Simulations 4.1 Hermes model The model used is the hydrostatic mesoscale HERMES code, developed by Electricite de France (Perdriel [4]). It resolves the set of conservation equations for momentum, energy and mass in a flux form using a finite-difference approximation over a staggered grid with a vertical reduced-pressure coordinate a. The closure is obtained using a first order scheme and the k diffusion coefficients are determined by the Louis parametrization (Louis [5]). A forcerestore two-level prognostic model, balancing all the ground heat fluxes is coupled to the atmospheric code for the determination of the ground temperature. Hermes model also permits the integration of the advectiondiffusion equation for passive scalars, so the simulation of the pollutant dispersion from different sources is possible. In our flow simulation, the 160 x 160 knr computational domain was horizontally discretized into 40 x 40 square cells whereas 25 unevenly spaced a levels were adopted in the vertical, with the top level at about 500 hpa and the minimum grid spacing of about 20m close to the ground. The time step used was 5 s. The model was initialized using the wind and temperature fields generated by the mass-consistent model Minerve (Geai [6]). This last takes as input the vertical wind and temperature profiles from ECMWF analysis and ground level synop observations, producing interpolated fields of wind and temperature over a terrain-following grid firstly. Then a mass-conservation adjustment is applied for the wind field producing the input data for Hermes initialisation. The same procedure is repeated every 6 hours to produce the driving boundary conditions. The 120 x 120 knr nested

5 Air Pollution Modelling, Monitoring and Management 389 computational domain for dispersion simulations was horizontally discretized into 60 x 60 2 km size grid cells with a vertical extension up to 850 hpa and 25 non-equidistant vertical levels, with the first layer about 20 m high. 4.2 Simulated period The simulation started at 19:00 h 7 May 1995 (local solar time) and lasted for three days, ending at 19:00 h 10 May As aforementioned, this period has been chosen as representative of the typical sea/breeze conditions into the selected area. Upper level winds persisted with a direction towards north-east and velocities around m/s. At lower levels the presence of diurnalnocturnal cycles becomes evident. The time history of ground level wind velocity and direction collected at the Montalto station is shown in figure 2. A cycle is repeated three times with diurnal wind directions from the sea orthogonal to the shoreline and higher velocities (up to 5 m/s). During the night the wind blows almost parallel to the shoreline, from south-east to north-west, with a lower velocity (about 2.5 m/s). Surface wind speed and obs wind speed sim wind speed obs wind direction sim wind.direction direction at Montalto : 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: May 1995 Figure 2: Time series of the observed and simulated wind variables at the Montalto site. The situation at the Torvaldaliga site is slightly different. Here the presence of the To If a mountains just behind the coast tend to cover the low level sea/breeze cycle determining more persistent winds. A low level cycle is anyhow present as demonstrated by the 50 m a.g.l. wind measured by the Sodar and shown in figure 3. This cycle tends rapidly to disappear at higher levels and it is almost absent at 500 m. 4.3 Emission scenario The two thermal power stations considered are located about in the middle of the computational domain. The Montalto power station has two separate

6 390 Air Pollution Modelling, Monitoring and Management sections: the first one is a standard oil fired thermal power unit, while the second is made of gas turbines that could be coupled with the previous one in a repowered configuration. Torvaldaliqa height: 50. m obs wind speed stm wind speed obs wind direction sim wind direction : 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: May 1995 Torvaldaliqa height: 350. m obs wind speed 400 sim wind speed obs wind direction sim wind direc" 200 "O CD 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: May 1995 Torvaldaliqa height: 500. m obs wind speed 400, sim wind speed obs wind direction sim wind *** direction ^ : 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: 6: 12: 18: 0: May 1995 Figure 3: Time series of the observed and simulated wind variables at the Torvaldaliga site. The three panels refer to different vertical levels as indicated in the graph titles. Actually, this power plant is only partially operational The Torvaldaliga power station is composed of several standard thermal oil fired thermal power units. Several operational combinations of the different sections are therefore possible and have been foreseen. In order to simulate the future operational configuration and to study the interaction between the two emissions, an hypothetical scenario has been considered emission is present. In this scenario all the sections producing significant SOz were taken into account. The following table summarizes the main characteristics of both the stacks and emissions:

7 Air Pollution Modelling, Monitoring and Management 391 section power (MW) stack height (m) stack diameter (m) emiss. temp. C emiss. vertical speed (m/s) SOz emission (S/s) TORVALDALIGA x x x x4 MONT ALTO x Simulation results Flow simulation As mentioned earlier, the Hermes model was initialized and driven at boundaries using only the ECMWF profiles and ground level synop data. This means that all the mesoscale structures in the domain interior had to be reconstructed directly by the model itself. In this respect we expected a realistic reconstruction of a sea/breeze cycle without the exact reproduction of all the details. 800 F~ 600 ~- 400 ~- 200 r o ~ E Net radiation and! obs net radiation _ sim net radiation... sim senbible hea sensible heat flux at Montalto 18: 0: 6: 12: 18: 6: 12: 18: 6: 12: 18: 0: 6: 12: 18: May 1995 Figure 4: time series of the observed and simulated net radiation and the simulated sensible heat flux at the Montalto site. This can be verified both examining the global structure of the simulated flow fields and comparing the time histories of some of the measurements not used by the model (i.e. Sodar profiles and Montalto ground level data) with simulation results at corresponding points. The comparison between computed an measured net radiation at Montalto is shown in figure 4, together with simulated sensible heat flux. The agreement between observed and simulated data is quite good suggesting that the correct amount of energy is supplied to the system To have an overall view of the flow structure generated by the model, the low level diurnal and nocturnal wind fields close to the ground are

8 392 Air Pollution Modelling, Monitoring and Management shown infigures5a and 5b. These figures refers to 2:00 of May 9* and 16:00 of May 10*. In thefirstone the typical nocturnal structure is represented, with the wind blowing along the coast on the shoreline whereas in the second one a sea breeze situation is well developed (a) X? 4750-bt ^ ' 1700 ' 1750 x (km) f 10 m/s *(km) Figures 5a and 5b: Wind fields near the ground at 02:00 (a) and 16:00 (b) of May 10" 1800 The comparison between the ground level wind velocity and direction measured at Montalto and simulated by Hermes model is shown in figure 2. Here some discrepancies are evident, in particular the model tends to underestimate the maximum velocity peaks in the afternoon and to rotate more clockwise, but the breeze cycles are satisfactorily reproduced. The flow of the second simulated day, during which the synoptic driving circulation was weaker and a stronger breeze is present, seems to be better reproduced with respect to the other two days. The comparison between some of the Torvaldaliga Sodar time series and the corresponding model simulations is shown in figure 3. Here wind velocities and directions at 50 m, 200 m, and 500 m a.g.l. are shown. The model tends to reproduce quite well the situation at 50 m. The weakening of the sea breeze signal with respect to that of Montalto is satisfactorily reconstructed and the vertical extension of the simulated and measured signals seems to be similar, even if a general tendency to an underestimation of the wind module is present Concentration simulation The dispersion simulation started at the same initial time of the flow simulation, i.e. 19:00 of May 7*, and lasted for three complete days, until 19:00 of May 10*. During this period, stationary emissions with the characteristics described in the previous table have been considered with plume rises described by a Briggs's formulation. As a general comment, the pollution level is quite low, with SOz maximum ground level concentration below 100 ig/m\ although some interesting recalculation and overlapping episodes happen. During the night, the lower part of the emitted plumes, particularly those coming from the lower stacks of the Torvaldaliga power plant, tends to be transported along the

9 Air Pollution Modelling, Monitoring and Management 393 coast and, when the nocturnal breeze is stronger such as in May Thyrrenian Sea. 9*\ towards the g (a) x(km) (b) %(km) g Figures 6a,b,c,d: Ground level concentration fields at different times and for different contributions, as indicated in the text. The scale represented is M-g/m^ In this way the effects of the two power plants can overlap along the coast or on the open sea, with low concentration levels. The upper plumes are instead transported north-eastward by the synoptic flow. The case of May 9* night and morning is particularly interesting. As mentioned before, during this period the nocturnal breeze is well developed and the lower plumes are transported towards the Thyrrenian Sea. In the morning, these plumes tend to come back to the coast, determining a situation in which three different glc maxima are present. The first two, located in the north-eastern part of the domain, are the direct effect of the higher plumes generated by the two power stations and the third one is due to the return of the nocturnal lower emissions, now transported

10 394 Air Pollution Modelling, Monitoring and Management northward. This situation is illustrated in figure 6a,b,c. In particular, figure 6a shows the ground level concentration field at 7:00 May 9^ due to the Torvaldaliga sources, giving evidence of the two separated plume transported by the low level and high level winds. Figure 6b shows the contribution of the Montalto power plant only, whereas figure 6c illustrates the total contribution. The three concentration maxima are clearly evident, having almost the same values. This particular aspect of the dispersion patterns is presumably hard to reproduce using simpler models like Gaussians, unable to take into account most of the needed details. In the afternoon, when the diurnal breeze and the turbulent PEL are well developed, all the plumes are transported towards northeast, the maximum glc are higher, but the impacts of the two power plants are clearly separated, as indicated in figure 6d. 5 Conclusions A three days simulation in a typical sea/land breeze condition has been performed by the Hermes atmospheric circulation model over a North Thyrrenian area. The mean flow has been reproduced and used to simulate the dispersion of pollutant due to separated large thermal power plants located on the shoreline in an hypothetical emission scenario. The output flow fields have been compared with experimental data showing reasonable agreement; ground level concentration patterns have been also analyzed in order to verify the overlap conditions of the different emitted plumes, pointing out some interesting episodes. The obtained results show the potential offered by the used tools. Other different weather conditions and emission scenarios are going to be simulated in the next future. Some useful advices are expected to be obtained to modulate and balance the emission amounts between the two power plants, in order to avoid critical environmental conditions due to the overlapping or reciculation of the plumes. Keywords: mesoscale modeling, Eulerian models, pollutant dispersion. References 1. Pielke R. A. Mesoscale Meteorological Modeling, p.612, Academic Press, New (1984) York, 2. Calbd J, Baldasano J.M., Costa M. Modelling the dispersion of CO over Barcelona area with the the mesoscale model PROMETEO, Air Pollution III, Vol. 3 (ed C.A. Brebbia, N. Moussiopoulos, H. Power), pp 25-32, Wessex Publishing, Anfossi D., Desiato F, Tinarelli G., Brusasca G., Ferrero E., Sacchetti D. TRANS ALP 1989 Experimental Campaign - part II: Simulation of a tracer experiment with Lagrangian particle models, Atmospheric Environment (in press) 4. Perdriel S.. Note de principe du code Hermes, EDF/DER Report HE-33/90.04, 1990 (available from E.D.F., 6 Quai Waitier, 78400, Chatou, France). 5. Louis J.F. A parametric model of vertical eddy fluxes in the atmosphere, Boundary Layer Meteorology, 1979, 17, Geai P. Methode d'interpolation and reconstitution tridimensionelle d'un champ de vent: le code d'analyse objective MINERVE, EDF/DER report HE-34/87.03, 1987