Abstract. 1 Introduction

Transcription

1 Simulation of nocturnal drainage flows and dispersion of pollutants in a complex valley D. Boucoulava, M. Tombrou, C. Helmis, D. Asimakopoulos Department ofapplied Physics, University ofathens, 33 Ippokratous, GR Athens, Greece Abstract Nocturnal drainage flows in Megalopolis valley are simulated by a two dimensional vertically averaged dynamic model. The calculated wind field is compared with hourly observations taken at four stations situated in the central part of the valley. Ground level concentrations of SC>2 which is emitted from the stacks of a Power Plant are also estimated. 1 Introduction Nocturnal drainage flows are a very common feature of mountainous regions. Cooling by nocturnal radiation causes the layer of the air adjacent to the slope to be cooled by conduction and turbulence. The temperature difference between this layer and the ambient air at the same altitude over the centre of the valley causes buoyant forces generating a complex circulation which can be modified or even eliminated when synoptic winds are strong. The purpose of this study is the simulation of the wind field of the Megalopolis Valley during some nights favourable for drainage flows. The ground level concentrations of SC>2 which is emitted from a Power Plant situated on the valley floor is also calculated. Details on the stacks characteristics can be found in Lalas et al. [1]. The Megalopolis valley is situated in the central part of Peloponnisos (Greece). Its length is 20km and its width 18km. It is surrounded by hills about m higher than its floor whose elevation reaches 400m above sea-level. Three openings are present, at its SW,SE and NW corners, (see figures (1,2) )The vegetation of the valley is relatively low and dispersed. In the time period , an experiment is conducted Hourly data of wind speed and direction at the height of 4m, from 4 meteorological masts are available at four different locations :

2 224 Air Pollution Theory and Simulation Power Plant (PP), Voutsaras (V), Tripotamo (T), Karvounari (K). Figure (2). At the Power Plant location an acoustic sounder is in operation, and a few tethersonde ascents are also taken. In addition, measurements of hourly concentrations of SC>2 in Megalopolis town are available. In this study, the two dimensional vertically averaged hydrostatic dynamic drainage flow model 2DFLOW developed by the Savannah River Laboratory Garrett & Smith[2,3] is used. The model is based on conservation equations of momentum and mass in a terrain following coordinate system. The required input data are local topography, ambient temperature, an initial constant ambient and geostrophic wind. The drainage velocity is assumed uniform over the drainage depth which varies in space and time. The buoyancy force is driven by a temperature deficit (0^) which can roughly be assumed to be constant in space and time as drainage flows reach a quasi steady state during the night. Dispersion calculations are also performed by a simple Monte Carlo model described by Hanna et al.[4]. 2 Methodology The model has been adjusted to accept spatial variation of the ambient wind, which is provided by the mass-consistent model AIOLOS [5] in order that our study should take into account the modification of the wind field due to the local topography. The Monte Carlo model which is two dimensional, assumes that the material is instantly diluted in the drainage layer As a result of this, it calculates pollutant concentrations counting the number of particles in a volume dx*dx*h where dx is the horizontal grid mesh and h the drainage depth. In our study, we are counting the particles that reach 10m above the ground, assuming that each particle has a vertical velocity, which equals the sum of the mean plus the turbulent component w/, in order to count ground level concentrations for comparison with measured values. The mean vertical velocity, which is not provided by the model, is set equal to zero. This assumption is valid since the vertical velocities in nocturnal flows are usually small except in regions of strong subsidence (Luhar & Rao [6]) The vertical velocity variance is calculated according to Hanna et al.[4] o^=1.3u*(l-z/h) (1) where u* is the friction velocity and h the top of the drainage layer. The assumption that the vertical velocity fluctuation w/ can be described by a white noise process is valid, when the vertical lagrangian time scale is small [6] In nocturnal flows it is typically of the order losec so w/ can be given by: w/=cyr (2) where R is a random number with mean zero and unit variance. The contribution of each stack is taken into account only when its effective height becomes lower than the drainage depth which increases with time reaching a quasi-steady state. The initial particle positions are assumed equal to the effective height. The effective height for each stack is estimated according

3 Air Pollution Theory and Simulation 225 Figure 1: Relief Map of Megalopolis Valley r 17000^ isooo^ IJOOOH -I3OOO-I o o o o o o Figure 2: Contour map of the Megalopolis Valley The locations of the measuring wind stations and Megalopolis Town are also indicated.

4 226 Air Pollution Theory and Simulation to Briggs [7] for stable conditions and found to range from 150 to 450m. Finally, particles are assumed to reflect at the ground and the top of the drainage layer. Only few rawinsonde ascents are available during the experimental time period. Because of the relatively strong winds associated with these particular nights, they prove to be unfavourable for drainage flows. In most of the nights of the year the general meteorology acts so as to eliminate the locally driven drainage winds. The selection of the nights presented here is based on examination of synoptic charts and rawinsonde ascents from Helliniko airport. Nights with low wind speeds (< 5 m/sec) and fair weather conditions have been selected. Also, during these particular nights, according to the acoustic sounder measurements, a surface inversion is present. The winds of Helliniko at the geostrophic level are found to approximate the geostrophic winds above Megalopolis, after a close examination of synoptic charts and comparison of some upper level wind measurements of the rawinsonde ascents taken in Megalopolis with those from Helliniko airport. 3 Discussion of the results The selected nights presented in this study are 18/3/84 and 11/4/84. The simulations are performed in a horizontal grid of 34x35 km with a grid mesh of 500m which is selected in order to cover most of the part of the valley. On 18/3/84 the model runs with an imposed geostrophic wind of 5 m/sec from the NNW. where AIOLOS produces an ambient wind of NW direction. The model simulation begins at local sunset and reaches a quasi-steady state after 3 hours of simulation slower than in [3] because of the more complex initial wind field. After 8 hours, (figure 3), the drainage flows on the slopes converge towards the center of the valley producing an along valley circulation. The ambient wind interacts with the locally driven winds resulting in a complex flow depending on the valley's complicated topography. The tendency of the flow to escape from the valley's openings is apparent and affected by the ambient wind field. This particular wind direction enhances the flow exit from the SE side while it tends to eliminate or even reverse the flow on the SW side which has a NE direction when the model runs with zero ambient winds. The results of the model are compared to average nighttime values measured during the above mentioned night, (table 1). There is generally quite good agreement between modelled and observed values. However, discrepancies are expected because the model is vertically averaged and the wind vertical structure is eliminated. In addition, the model is hydrostatic and performs better in less complex terrain. The measured wind speeds are generally very low, resulting in a certain ambiguity of their values as, apart from instrument errors, they can be affected from trees and local topography. At the power plant location the near zero values are accurately simulated.

5 Air Pollution Theory and Simulation 227 Table 1 : Comparison between measured and observed winds on 18/3/84 at the four locations. The wind speeds (WS) are given in m/sec and the wind directions (\VD) in degrees TIME Simulated WS WD WS WD WS WD WS WD WS WD VOUTSARAS TRIPOTAMO P. PLANT KARVOUNARJ The constant wind direction at Voutsaras location reflects the well organized flow shown in figure 3 while in Tripotamo the directions are more variable. The general structure of the wind in the vicinity of each station must be considered more representative of its wind field instead of one particular grid value. The modelled drainage depths vary from 10m over the mountains to 400m at the Power Plant location The measured temperature inversion depth at the Power Plant is lower (250m). An oveprediction appears as the drainage depth is not always confined to the surface inversion depth as it can also embody an isothermal layer [8]. The second night presented in this study is the 11/4/84. According to Helliniko rawinsonde ascents, the geostrophic wind is 3m/sec from NW direction. The windfield,after 8 h of simulation is represented infigure4 while comparison between predicted and observed values is shown in table 2. The slope winds are more pronounced than the previous day due to the lower ambient winds The agreement with the observed values at Tripotamo are not as good as in thefirstnight Table 2: As in Table 1 but for 11/4/84 TIME Simulated WS WD WS WD WS WD WS WD WS WD VOUTSARAS TRIPOTAMO P.PLANT 1, KARVOUNARI The results of the dispersion calculations for the above mentioned nights after 8h are shown in figures 5,6. On 18/3/84 the apparent NW flow tends to transport the material towards this direction. The maximum ground level concentrations reach 300pg/m^ at a distance of 10km SE of the Power Plant. The Megalopolis town which is situated SE of the Power Plant is apparently unaffected. This is consistent with the measured nighttime values which are

6 228 Air Pollution Theory and Simulation Figure 3: 18/5/84. Wind field after 8 hours of simulation 1 m/sec Figure 4: As in figure 3 but for 11/4/84

7 Air Pollution Theory and Simulation 229 Figure 5: 18/3/84 Calculated hourly ground level concentrations in jjg/r 8 hours of simulation. Contours are given at 50 ug/m^ intervals after Figure 6: As in figure 4 but for 1 1/4/84.Contours at 10 intervals

8 230 Air Pollution Theory and Simulation practically zero. The results are in accordance with the calculated values from the VALLEY model [9] during some particular nights of the experimental time period.on 11/4/84 the picture is quite different. Because of the absence of an organized flow and the relatively light wind speeds which cause calculated velocity variances to remain at low levels, the concentrations reach a maximum value of 70ug/m^ S of the Power Plant The measured values in Megalopolis town are also nearly zero apart from some traces during midnight. The predicted value of 10pg/m^ is considered as a good approximation 4 Conclusions In spite of the model's simplicity the predicted wind fields are in accordance with the measured values. A nested grid and a variable temperature deficit would probably improve the results. The measured SC>2 concentrations are very limited, but the general picture of the predicted concentrations is satisfactory. References 1. Lalas, DP, Asimakopoulos, D, Petrakis & Helmis, K, M, Air Pollution impact Assessment in Megalopolis Valley, Athens, Garrett, A,& Smith, F, Two Dimensional Simulations of drainage winds and diffusion compared to observations, J, Climate Appl. Meteor.,1984,23, Garrett, A. & Smith, F. A., Two-dimensional dynamical drainage flow model with Monte Carlo transport and diffusion calculations DP-1639, Hanna, G.A., Briggs, & R. P., Hosker, Handbook on atmospheric diffusion. DOE/TIC-11223, Lalas, DP,Wind energy estimation and siting in complex terrain/w/. J. Solar Energy,1985,3, Luhar, A.K., Rao,K S, Lagrangian Stochastic dispersion model simulations of tracer data in nocturnal flows over complex terrain, Atmospheric Environment,1994,28, Briggs, G.A., Plume Rise USAEC Critical Review Series, NTIS, Springfield Va., S.Barr, S.,& Orgill, M, Influence of external meteorology on nocturnal valley drainage winds, J.Appl. Meteor.., 1989,31, Burt, E.W., VALLEY model user's guide Report EPA-450/ Environmental Protection Agency, 1977.