Abstract. 1 Introduction

Transcription

1 Nested dispersion simulation over the Lisbon region R. Kunz,* M. Coutinho,^ C. Borrego^ N. Moussiopoulos' "Institute for Technical Thermodynamics, University of Karlsruhe, Karlsruhe, Germany ^Department of Environment and Planning, University ofaveiro, 3810 Aveiro, Portugal ^Laboratory of Heat Transfer and Environmental Engineering, Aristotle University Thessaloniki, Thessaloniki, Greece Abstract The city of Lisbon is situated near the estuary of the river Tejo. The estuary separates the urban region west of the estuary from a more rural area in the eastern part. A new bridge will be built over the estuary modifying the traffic patterns of the region. As the city of Lisbon is built in a very complex region, the wind flow and the dispersion of pollutants are influenced by phenomena belonging to different mesoscale subscales. In view of the different scale influences, a nested grid technique is applied to simulate the wind flow and the dispersion of prospective traffic emissions over the Lisbon region. The simulation is performed using the EUMAC Zooming Model (EZM). 1 Introduction The Greater Lisbon Area (GLA) is one the largest conurbations in southern Europe. It has a total population of 3.5 million inhabitants and is the home of the most important Portuguese industries. The city of Lisbon is built in a very complex topographic region near the Atlantic coast. The region is dominated by a 320 km large estuary and multiple hills, surrounded by small mountain ranges reaching heights over 400 m above sea level (Figure 1). Uptill now, urban development was predominantly limited to areas west and southwest of the estuary. A new bridge will be built over the estuary which will probably encourage the develoment of new residential areas at the earstern coast of the estuary. The present study analyses the impact of potential

2 60 Urban Pollution i- >. / Figure 1: Location of the coarse, medium and fine grids. Residential areas are squared, industrial areas are solid and forest and agricultural areas are marked with diagonals. emissions from new settlements east of the estuary over the air quality of the GLA. The wind flow and the dispersion of pollutants in the GLA is influenced by phenomena with different characteristic length scales. Within prognostic simulations it is therefore necessary to account for the different scale influences by an appropriate spatial resolution. In the present study a nested grid simulation is performed to achieve a higher resolution in a part of the whole model domain. The simulations were performed with the non-hydrostatic mesoscale model MEMO. The model is one of the core models of the HUM AC Zooming Model (EZM) and was developed at the University of Karlsruhe to study wind flow over complex terrain. A description of the model is given in Flassak [1] and Moussiopoulos et al. [2]. Within the model MEMO the conservation equations for mass, momentum and scalar quantities like energy, humidity or pollutant concentrations are solved numerically. To solve these equations the initial state and boundary conditions have to be specified for each quantity. The initial state is obtained by a diagnostic model based on measurements. Boundary conditions at the lower boundary are calculated taking into account solar radiation and heat fluxes at the ground. To determine lateral boundary values

3 Urban Pollution 61 the solution of the model domain itself is considered as well as the large scale distribution of the quantities in the model environment. In case of a nested grid simulation the large scale distribution of the inner grids follow from the larger simulation whereas for the outermost grid it is derived from measurements. The nesting technique which is used in the model MEMO is based on the expanded radiation boundary condition proposed by Carpenter [3]. As it is a one-way interactive scheme the simulations for the different grids can be performed consecutively which enables its use on even small computers like workstations. A detailed description of the nesting technique can be found in Kunz [4]. The deposition of the passive pollutant is calculated through a 'bigleave' multi resistance model which is based on the similarity theory. 2 Case specification The presented simulation consists of three nested model domains: the coarse grid domain (CG), the medium grid domain (MG) and the fine grid domain (FG). The CG, MG and the FG domain cover an area of 150 x 150 knf, 60 x 60 km^ and 30 x 30 knf at a horizontal grid resolution of 5, 2 and 1 km, respectively. The orography of the CG domain and the location of the two inner grids are given in Figure 1. Each numerical grid has a non-equidistant vertical grid spacing increasing with height and a minimum value of 20 m at the bottom. The model top is fixed at 6000 m in each grid. The meteorological conditions represent a typical summer situation over the Iberian peninsula [5]. A thermal low is created at the high and arid plateau of the peninsula inducing a relatively strong (3-4 m s~*) N-NE wind over Portugal. The initial state for all grids and the large scale portion of the lateral boundary conditions for the outermost grid were derived from vertical soundings taken on the 4^ of August In order to model the impact of emissions from future sources east of the estuary, the dispersion of CO is studied taking into account its typical diurnal cycle. Due to the lack of a more detailed emissions inventory in the present study only a single area source of 5x5 km is modeled. The source is intended to represent an area with traffic emissions and emissions from small factories. The location of the source (AS) is marked in Figure 1. All the calculations were done on an IBM RS 6000 at the Institut fur Technische Thermodynamik of the University of Karlsruhe. Each grid comprised 30 x 30 grid meshes in horizontal direction and 35 vertical layers. The simulation period was 24 hours starting at 0 Local Standard Time (LST).

4 62 Urban Pollution Figure 2: Ground wind field at 4 and 16 LST for the medium grid. 3 Results Figure 3: Ground wind field at 4 and 16 LST for the fine grid. In this section results of the dispersion calculations are presented. The calculated flow fields are very similar to those shown in Coutinho et al. [6] and are therefore only briefly presented here to show the basic characteristics of the circulation. Figures 2 and 3 show the near ground wind field at 4 and 16 LST calculated for the medium and the fine grid, respectively. For reasons of clarity only one fourth of all wind vectors are plotted. During the morning hours the synoptic pressure gradient enforces mainly northerly winds which can be seen

5 Urban Pollution 63 in the results for both numerical grids. Near the Atlantic coast the wind turns towards northeastern to eastern direction due to the influence of the land breeze. In the afternoon the picture changes: as a result of the sea breeze westerly to northwesterly winds occur over the Ocean and over the coast. The MG results reveal that air masses are transported over the Sintra mountains barely reaching the city of Lisbon. A considerable part of the wind flow over the estuary is governed by the sea breeze which can be seen in the results of the FG domain. At the western coast of the estuary a lake breeze is established. At the eastern coast the lake breeze is suppressed by the synoptic forcing and northerly winds prevail. Results of the dispersion calculations for the MG and FG domain are shown in Figures 4 and 5, respectively. Horizontal cross sections of the CO concentration are given at about 10 m above ground level for 4, 8, 12, 16, 20 and 24 LSI. The results calculated for the two grids are similar but with a different spatial resolution. In the morning hours CO emissions are driven towards the sea by northeasterly winds. At 8 LST emission levels are higher due to the morning rush hour. Thus, the extension of the plume is wider than at 4 or 12 LST. During the midday hours the wind direction over the southwestern part of the estuary changes from northeast to nearly west. Therefore, pollutants are transported towards south and later on towards southeast. In the afternoon intense turbulent mixing leads to a vertical exchange of pollutants. As a consequence near ground concentration levels are lower and their extension is more limited than during the morning hours. The analysis of the vertical profiles reveals that during the morning CO concentrations are confined to a narrow layer near the surface while in the afternoon considerable concentrations occur up to 1000 m above ground level. In the evening the pollutants are transported towards south-east by westerly winds coming from the Atlantic coast. 4 Conclusions The simulation results show an increase of pollutant concentrations southeast of the estuary for the considered meteorological conditions which are most frequent over the GLA. Thus, the city of Lisbon and the estuary of the river Tejo seem not to be strongly affected by potential emissions due to a prospective urbanization of the east part of the estuary.

6 64 Urban Pollution Figure 4: CO concentration fields at approximately 10 m above ground level calculated with MEMO on the medium grid at 400, 800, 1200, 1600, 2000 and 2400 LST of August 4*, 1992.

7 Urban Pollution 65 Figure 5: CO concentration fields at approximately 10 m above ground level calculated with MEMO on the fine grid at 400, 800, 1200, 1600, 2000 and 2400 LSI of August 4*, 1992.

8 66 Urban Pollution However, it has to be pronounced that within the limited frame of the present study only a single area source was considered. Any synergetic effects based on the emissions of other sources in the GLA are beyond the current investigation. Acknowledgement The study was prepared under the framework of the Luso-German Scientific Integrated Actions. References [1] Flassak,Th. (1990) Bin nichthydroststisches mesoskaliges Modell der planetaren Grenzschicht, Fortschr.-Ber. VDI, Reihe 15, Nr. 74, pp 204. [2] Moussiopoulos, N., Flassak, Th., Berlowitz, D., Sahm, P. (1993) Simulations of the Wind Field in Athens With the Nonhydrostatic Mesoscale Model MEMO, Environmental Software 8, [3] Carpenter, K. M. Note on the paper 'Radiational condition for the lateral boundaries of limited-area numerical models' by Miller, M.J. and Thorpe, A.J. (Q.J. 107, ), gwarf. J & M;f. Sbc, 1982,108, [4] Kunz, R. (1995) Entwicklung einer Mehrgebietsmethode fur hochauflosende prognostische Modelle der atmospharischen Grenzschicht, Dissertation Universitat Karlsruhe. [5] Coutinho, M., Rocha, A., Borrego, C. (1994) Numerical Simulation of Meso-Meteorological Circulations in the Lisbon Region, Air Pollution Modeling and its Application X, (Gryning, S. and Millan, M. eds), Plenum Press, [6] Coutinho, M., Kunz, R., Borrego, C, Moussiopoulos, N. (1994) Nested Wind flow simulation over the Lisbon Region, Air Pollution II, Vol. 1 (C. Brebbia, H. Power, P. Zanetti, eds), Wessex Publishing,