- Email: [email protected]
Atmospheric flows over Athens: first tests with the DREAMS model
Environmental Software 8 (1993) 65-71
Atmospheric flows over Athens: first tests with the D R E A M S model J a c q u e s Ehinger Observatoire Cantonal, Neuchdtel, Switzerland and Institute of Geophysics, Universityof Lausanne, BFSH-2, CH-1015 Dorigny/Lausanne, Switzerland
& M a r t i n Beniston Department of Geography, Swiss Federal Institute of Technology ( ETHZ), Winterthurstrasse 190, CH-8057 Zarich, Switzerland
Abstract The paper gives a brief description of results from a 3-D numerical model over the region of Athens. The model simulations form part of the APSIS intercompadson experiment, which is aimed at comparing the capabilities of different models in reproducing meteorological and air quality episodes in the Athens area. A full diurnal cycle has been simulated for the test-case of May 25, 1990, and this includes as its essential feature an alternating land and sea breeze circulation; the strength and inland incursion of the sea breeze seems in acc¢~l with observations, even though the diurnal surface heating and other initial conditions have been specified rather rudimentarily on the basis of the available data. A correct description of the land and sea breeze patterns is of course essential in describing the behavior of pollutants in the study region, so that the present simulations of meteorological conditions of the APSIS experiment are an encouraging first step towards a coherent simulation of air pollution episodes. Key words: Atmospheric dynamics, mesoscale model, numerical modeling, land and sea breeze
Software Availability Name of the Software DREAMS: Differential equation REgional Atmospheric Modeling f~ystem Brief descdmion DREAMS-1 is a three-dimensioual atmospheric model which makes use of the finite difference technique to solve a series of time-dependent eqeations for prcr.esses acting typically on a regional scale. The model was originally developed in the early eighties to investigate organized cloud convection over a maritime area, such as the Rayleigh-B6nard type convection frequently observed as cold air masses move over a wanner ocean surface. It has also been used to investigate some aspects of cloud and radiation interactions, as well as the dynamic and thermodynamic influence of a water surface on the regional atmosphere. At a later stage of its development, DREAMS-1 has been used in complex terrain to study air pollution on a local scale, through coupling with DREAMS-2. A number of papers related to this system have been published in the peer-reviewed literature. The size of the model domain is essetially limited by the computer resources on which the model code is implemented. The characteristic spatial scales of the phenomena to be investigated should also be borne in mind, so that an optimal model domain size can be determined prior to a numerical 65 Environmental Software 0266-9838/93/$06.00 © 1993 Elsevier Science Publishers Ltd
J. Ehinger, M. Beniston
66
investigation. For a supercomputer such as the CRAY-2 or equivalent, the enormous memory capacity allows a generous domain size; DREAMS-1 has been run with up to 2 million grid-points on such a machine. On less advanced machines, the model code can generally run with a 32,000-point grid mesh distributed in the three cartesian directions, thus allowing a balance to be found between domain size and computing time. Typical grid interval is 1,000 m and up in the horizontal, and vertical levels can be unevenly distributed, allowing higher resolution of the lower levels of the ABL. In general, the fast model level is located at 50 m above the reference level, with subsequent even spacing of between t00 and 250 m. The DREAMS-2 model code contains schemes allowing the study of the physical behavior of pollution with both the Eulerian and the Lagrangian techniques; it is at present used for the study of passive pollutants and uses the dynamic and thermodynamic data generated by DREAMS-1 to compute the characteristics of pollutant transport and dispersion. A number of papers related to the complete coupled system have appeared in the literature. Author Contact Dr. Martin Beniston, Dept. of Geography, ETH-Zurich, Winterthurerstrasse 190, CH-g057 Ziirich, Switzerland. Phone: (+41) 1 257 52 11; Fax: (+41) 1 362 51 97; E-Mail: [email protected] Harware Reauirements Any machine with a FORTRAN compiler. Ideally, access to a vector supercomputer (CRAY-2, CRAY-Y/MP, NEC-SXf3) Software Reouirements Graphics post-processing software. Small routines to read topography data bases and to initialize pressure distribution, separate from the computational code Avvroximate Costs None: Basically a research model
I. Introduction
The summertime photochemical smog over Athens has become a major environmental problem in the last decade and has the reputation of leading to one of the most severe air quality problems of any European capital. In order to further understand the fundamental proceses involved, the APSIS model intercomparison study was initiated with the aim of simulating observed episodes. In this manner, different model codes could be tested for the atmospheric dynamics (APSIS Part A) and air chemistry (APSIS Part B) over this region, thereby allowing rigorous evaluation of their capability in reproducing conditions resulting in the severe smog episodes. Validated models could subsequently be used for scenario testing, in particular to assess possibilities of mitigation in the context of photochemical smog. DREAMS-1 is a three-dimensional atmospheric model which makes use of the finite difference technique to solve a series of time-dependent equations for processes acting typically on a regional scale. The model was originally developed in the early eighties to investigate organized cloud convection over a maritime area, such as the Rayleigh-Btnard type convection frequently observed as cold air masses move over a warmer ocean surface (Benistonl,2,3). It has also been used to investigate some
aspects of cloud and radiation interactions (Beniston and Schmetz4; Schmetz and Beniston5), as well as the dynamic and thermodynamic influence of a water surface on the regional atmosphere (Beniston6). At a later stage of its development, DREAMS-1 has been used in complex terrain to study air pollution on a local scale, through coupling with DREAMS-2. A number of papers related to this system have been published in the literature, such as Beniston 7, Beniston et al. 8, and Beniston 9, with description of the models and the results of case studies. The size of the model domain is essetially limited by the computer resources on which the model code is implemented. The characteristic spatial scales of the phenomena to be investigated should also be borne in mind, so that an optimal model domain size can be determined prior to a numerical investigation. For a supercomputer such as the CRAY-2 or equivalent, the enormous memory capacity allows a generous domain size; DREAMS-1 has been run with up to 2 million gridpoints on such a machine. On less advanced machines, the model code can generally run with a 32,000-point grid mesh distributed in the three cartesian directions, thus allowing a balance to be found between domain size and computing time. Typical grid interval is 1,000 m and up in the horizontal, and vertical levels can be unevenly distributed, allowing higher resolution of the lower levels
67
Atmospheric flows over Athens: first tests with the D R E A M S model
of the ABL. In general, the fhst model level is located at 50 m above the r e f e r e n c e level, with subsequent even spacing of between 100 and 250 m. The problem of determining concentrations of a particular pollutant at a given location by numerical analysis is exceedingly complex. The Eulerian technique is poorly adapted to point sources, since the source becomes specified over an entire grid volume which, even in the case of a fine grid mesh, is an invalid representation of the point source. The-most commonly used Gaussian dispersion models yield elegant analytical solutions to the emission-transport-immision problem, but contain so many restrictive assumptions that they are unable to produce correct solutions to dispersion in complex terrain under real meteorological conditions. More recently, researchers have focused on Lagrangian solutions to the dispersion problem. The advantage of the Lagrangian approach is that individual trajectories of particles (which represent a particular pollutant) are independent of grid size; also any time increment may be used according to the time resolution desired. Furthermore, a Lagrangian particle technique is the only method by which a complex flow situation, in particular strong wind shear or flow reversals, may be adequately taken into account. Both continuous and instantaneous releases can be considered. Among the f'~t workers in this field were McNider, whose "conditioned particle technique" has been widely used by Pielke and coworkers (Pielkel0). Lagrangian particles as tracers for pollution dispersion studies have also been designed by Legg and Raupach 11, and adapted to heavy gas dispersion by Schoi'ling-, i.e., here the particles are no longer passive elements of the flow but have their own mass. The DREAMS-2 model code contains schemes allowing the study of the physical behavior of pollution with both the Eulerian and the Lagrangian techniques; it is at present used for the study of passive pollutants and uses the dynamic and thermodynamic data generated by DREAMS-1 to compute the characteristics of pollutant transport and dispersion. A number of papers related to the complete coupled system have appeared in the literature, such as for example Beniston and Pielke 13.
2.
Model
initialization
The APSIS participants had access to the following data: - Topography: a detailed kilometric altitude grid, for a 72 x 72-point raster, and a lower resolution 36 x 36 grid over the same area. This data file also contains the land-use characteristics of the domain - Meteorological data at 10 m height: five recording stations distributed over the Attica Peninsula, provided information on the following parameters: wind speed and direction, temperature, dew-point temperature, and pressure, for May 25 and 26, 1990. Data is available each three hours over this period, although four of the stations have gaps in the data sets
- Vertical soundings: Four vertical soundings during this period give an idea of the vertical structure of the atmosphere above Hellinikon Airport, from the ground to about 10,000 m above sea level (ASL).
DREAMS-1 was initialized on the 1-km grid using data at 00:00 hours on May 25, 1990. The Hellinikon upper-air sounding provided the wind, temperature, and humidity data for each of the 20 model levels. The vertical grid interval is staggered with height, having an interval of 100 m within the Atmospheric Boundary Layer, and then an increasing separation up to the model top which lies at 4,400 m above sea-level. The data for each level is assigned to each grid-point at that level, thereby leading to initially homogeneous horizontal dynamic and thermodynamic conditions. A synoptic pressure gradient, computed from the available data, provides one of the forcing terms which drives the dynamics of the system. An additional, and probably dominant, forcing term is the thermal perturbation generated by the land and sea temperature difference. Sea temperatures are kept at a constant 20 ° C, while the ground surface undergoes a sinusoidal warming/cooling with an amplitude of about 9° C. The temperature difference between the land and the sea creates a pressure perturbation (given by the hydrostatic equation and the equation of state), leading to the creation of a local dynamic system along the coast which tends to move inland with time (sea breeze); with cooling nocturnal temperatures, the system is halted and reversed, resulting in air flowing out to sea from the coastal regions (land breeze). The intensity and horizontal extent of the land and sea breeze depends essentially on the strength of the land/sea temperature difference.
3.
Scenarios
for
APSIS
Experiment
A
Scenario A-1
First computations were based on the following principles: - 1: Initial data is taken from the upper-air sounding at Hellinikon on May 25, 1990, at 12:00 - 2: Land and sea breezes are generated through a regular sinusoidal temperature fluctuation at the ground 3: The synoptic flow is considered to be constant throughout the duration of the simulation. Any observed changes in atmospheric parameters with height are a result of interactions between the sea breeze and the synoptic flow - 4: A 30-hour simulation was attempted, with data output each 6 hours - 5: Wind speed and direction was not adjusted to the reference profile. The synoptic flow was established in an approximate manner, based on the experience of previous test computations. Scenario A-2
Points 1 - 3 remain the same as for scenario A-1 - 4: A first simulation of 6 hours' duration is undertaken,
J. Ehinger, M. Beniston
68
with a surface heat forcing of 7.6 ° C (in close agreement with measurements) - 5: Wind profiles are adjusted to the observed profile at Hellinikon Airport, for May 25, 1990, at 12:00
4. First results
Scenario A-1 The first evidence of the sea-breeze circulation is illuslrated in Figure 1. This occurs at about 12:00 local time for the
10
20
10
-A
20
30
30
simulated event of May 25. 1990, when the model reaches the peak wanning situation. A sea breeze can be observed along the entire coastal region, and corresponds quite well to what may be expected from a thermal forcing of the amplitude imposed in the model. The surface warming generates a mass of warm air which rises above the ground; this is compensated for by an influx of cooler maritime air into the region. The resulting convective cells have a diameter of over 10 km and a vertical extent of 2 - 3 km. As the period of warming continues into the afternoon, the sea-breeze intensifies and moves inland, as seen in Figure 2.
40
40
50
50 r
60
60
V e l o c i t y , pt: 1,1 = 4.29 m / s
.AC Figure 1: Circulation patterns at 12:00 local time
70
70
69
Atmospheric flows over Athens: first tests with the D R E A M S model
10
20
10
20
30
30
40
50
40
60
50
60
70
70
Velocity, pt: 1,1 = 4.24 m/s
.AC Figure 2: Maximum intensity of the sea-breeze circulation.
Athens is subjected to a sea breeze whose intensity fluctuates between 0.5 and 2 m/s close to the coast. Because of insufficient observational evidence, it is uncertain whether the simulated wind is in agreement with reality, and what would be the behavior of pollution embedded within the sea-breeze. The wind flow is perpendicular to the coast in the SE sector of the city, which implies that the smog could be transported and dispersed only if it crosses the hills behind the city. It would be of interest to know whether this occurs in reality.
Scenario A-2 In this simulation, the computed wind profile has been adjusted to the measured profile at Hellinikon on May 25, 1990 at 12:00 (on grid point i, j --- 35, 33 in the Cartesian reference frame of the 72 x 72-point raster).. The precision of the adjustment is within 2 ° for the wind direction and within 0.2 m/s for the wind velocity. In the absence of other vertical profiles measured simultaneously over the Attica Peninsula, it is not
70
J. Ehinger, M. Beniston
Table 1: Comparison of observational and simulated data Station
Eleusis Filadelfla Hellinikon Piraeus Tatoi
Grid Point 19, 35, 35, 26, 40,
51 50 33 37 55
Vm [m/s]
Vc [m/s]
DV m-c
DIR m [°]
DIR c[ °]
DDIR m-c
2.6 3.1 3.6 3.1 3.1
3.8 1.0 3.1 5.6 1.8
-1.2 +2.1 +0.5 -2.5 +1.3
190 230 220 160 180
200 220 230 200 225
-10 + 10 -10 -40 -45
possible to go beyond this form of adjustmenL However, a measure o f assessment is provided by the 10-m meteorological data; precautions are necessary when comparing computed and measured data, since the representativity of the weather stations for a 1-km grid are not known. The results are given in Table 1. Vm and Vc are the measured and computed velocities, respectively, DIRm and DIRc the corresponding directions, and DV and DDIR are the differences between measured and computed values. This first intercomparison is encouraging, especially concerning the wind directions. Simulated wind direction does not deviate by more than 45 ° from its observed value; however, wind velocities seem less precise, although out of five stations, three are within + 1.5 m/s which is often within the range of validity of the measurement itself.
5.
Conclusions
The first tests of the DREAMS-1 model have provided an insight as to its capability in reproducing observed seabreeze patterns in the Athens area. While not perfect, the results are encouraging and appear to be consistent with the observations (i. e., there is no significant deviation of the observed wind from reality. Improvements will be sought in future experiments, but these are linked to a number of problems which first need to be resolved. These include: - Sparse or missing data: A more comprehensive data set would be required to enable more c o h e r e n t intercomparisons between data and reality to be made. It would be ideally necessary to have vertical profiles at other locations in addition to the single sounding at Hellinikon; furthermore, additional surface data would be desirable, epecially for the period of May 24, 1990, which would enable a better initialization to be made for May 25, 1990 - Limits o f computation: The most severe restriction at present seems to be with respect to the time variations of the synoptic wind for periods of 24 hours or more. Based on the Hellinikon soundings, the uppermost model level (at 4,400 m ASL) would need to undergo velocity changes of between 5 and 20 m/s and direction changes of nearly 60 ° over a 24-hour period in order to correspond to reality. Such fluctuations would need to be taken into account by the model for more realistic simulations of the time frame of cad May 1990 over the Athens area.
- Future work: Although the figures illustrated here seem consistent with reality, a number of additional controls will be made, in particular: - Assessment of the influence of the eastern model boundary on flow characteristics near the SE Attica Peninsula, which lies close to this boundary - Assessment of the height of the model top on processes taking place at lower model levels
Such tests are standard numerical procedures and will not form part of any specific test case. However, further work on the APSIS experiment will focus on a third scenario (A-3), which will force the ground temperature according to an irregular sinusoidal function (in order to take into account non-equal day and night duration).. Scenario A-4 will atempt to incorporate changes in synoptic forcing over a 24-hour period.
References
1. Beniston M. Mesoscale model for the study of threedimensional convective phenomena. Model description and preliminary results from a KonTur simulation. Hamburg Geophys. Monogr., 1983 . B2, 1-62. 2. Beniston M. A numerical study of atmospheric cellular convection. Dyn. Atmos. Oceans, 1984. 8, 223-242.
3. Beniston M. Organization of convection in response to initial and lower boundary conditions in a mesoscale numerical model. Contr. Attn. Phys., 1985. 58, 31-52. 4. Beniston M. and J. Schmetz. A three dimensional study of mesoscale model response to radiative forcing. Boundary Layer Meteorol., 1985.31, 149-175. 5. Schmetz J. and M. Beniston, 1986 : Relative effects of solar and infrared radiative forcing in a mesoscale model. Boundary Layer Meteorol., 34, 137-155. 6. Beniston M., 1986 : The influence of a water surface on mesoscale dynamics as a function of atmospheric stability. Boundary Layer Meteorol., 36, 19-37. 7. Beniston M., 1987 : A numerical study of atmospheric pollution over complex terrain in Switzerland. Boundary Layer Meteorol., 41, pp. 75-96 8. Beniston M . , D. Ruffieux, and LA.Hertig, 1989: A combined numerical and wind-runnel study of ventlation and air pollution episodes in a rural valley of Switzerland. Boundary Layer Meteorol., 48, 129-156.
Atmospheric flows over Athens: first tests with the DREA M S model 9. Beniston M., J. P. Wolf, M. Beniston-Rebetez, H. J. K61sch, P. Rairoux, and L. W6ste, 1990: Use of LIDAR measurements and numerical models in air pollution research. J. Geophys. Res., 95, D7, 9879-9894 10. Pielke, R. A., 1984: Mesoscale Meteorological Modeling. Academic Press, New York 11. Legg, B. L, and M. R. Raupach, 1982: Markov chain simulations of particle dispersion in inhomogeneous flows. Boundary-Layer Meteorol., 24, 3 - 13
71
12. Schorling, M., 1987: Lagrangian simulation of neutral and heavy gases in complex terrain and streets. Proc. International Conf. on Energy Transformations and Interactions with Atmospheric Processes (ICEA - 87), Lausanne, Switzerland. 13. Beniston M. and R.A. Pielke (Editors). Interactions between Energy Transformations and Atmospheric Phenomena: A Survey of Recent Research. D. Reidel Publishing Co., Dordrecht/Boston, 1987. 426 pp. (Now Kluwer Academic Publishers)
Atmospheric flows over Athens: first tests with the D R E A M S model J a c q u e s Ehinger Observatoire Cantonal, Neuchdtel, Switzerland and Institute of Geophysics, Universityof Lausanne, BFSH-2, CH-1015 Dorigny/Lausanne, Switzerland
& M a r t i n Beniston Department of Geography, Swiss Federal Institute of Technology ( ETHZ), Winterthurstrasse 190, CH-8057 Zarich, Switzerland
Abstract The paper gives a brief description of results from a 3-D numerical model over the region of Athens. The model simulations form part of the APSIS intercompadson experiment, which is aimed at comparing the capabilities of different models in reproducing meteorological and air quality episodes in the Athens area. A full diurnal cycle has been simulated for the test-case of May 25, 1990, and this includes as its essential feature an alternating land and sea breeze circulation; the strength and inland incursion of the sea breeze seems in acc¢~l with observations, even though the diurnal surface heating and other initial conditions have been specified rather rudimentarily on the basis of the available data. A correct description of the land and sea breeze patterns is of course essential in describing the behavior of pollutants in the study region, so that the present simulations of meteorological conditions of the APSIS experiment are an encouraging first step towards a coherent simulation of air pollution episodes. Key words: Atmospheric dynamics, mesoscale model, numerical modeling, land and sea breeze
Software Availability Name of the Software DREAMS: Differential equation REgional Atmospheric Modeling f~ystem Brief descdmion DREAMS-1 is a three-dimensioual atmospheric model which makes use of the finite difference technique to solve a series of time-dependent eqeations for prcr.esses acting typically on a regional scale. The model was originally developed in the early eighties to investigate organized cloud convection over a maritime area, such as the Rayleigh-B6nard type convection frequently observed as cold air masses move over a wanner ocean surface. It has also been used to investigate some aspects of cloud and radiation interactions, as well as the dynamic and thermodynamic influence of a water surface on the regional atmosphere. At a later stage of its development, DREAMS-1 has been used in complex terrain to study air pollution on a local scale, through coupling with DREAMS-2. A number of papers related to this system have been published in the peer-reviewed literature. The size of the model domain is essetially limited by the computer resources on which the model code is implemented. The characteristic spatial scales of the phenomena to be investigated should also be borne in mind, so that an optimal model domain size can be determined prior to a numerical 65 Environmental Software 0266-9838/93/$06.00 © 1993 Elsevier Science Publishers Ltd
J. Ehinger, M. Beniston
66
investigation. For a supercomputer such as the CRAY-2 or equivalent, the enormous memory capacity allows a generous domain size; DREAMS-1 has been run with up to 2 million grid-points on such a machine. On less advanced machines, the model code can generally run with a 32,000-point grid mesh distributed in the three cartesian directions, thus allowing a balance to be found between domain size and computing time. Typical grid interval is 1,000 m and up in the horizontal, and vertical levels can be unevenly distributed, allowing higher resolution of the lower levels of the ABL. In general, the fast model level is located at 50 m above the reference level, with subsequent even spacing of between t00 and 250 m. The DREAMS-2 model code contains schemes allowing the study of the physical behavior of pollution with both the Eulerian and the Lagrangian techniques; it is at present used for the study of passive pollutants and uses the dynamic and thermodynamic data generated by DREAMS-1 to compute the characteristics of pollutant transport and dispersion. A number of papers related to the complete coupled system have appeared in the literature. Author Contact Dr. Martin Beniston, Dept. of Geography, ETH-Zurich, Winterthurerstrasse 190, CH-g057 Ziirich, Switzerland. Phone: (+41) 1 257 52 11; Fax: (+41) 1 362 51 97; E-Mail: [email protected] Harware Reauirements Any machine with a FORTRAN compiler. Ideally, access to a vector supercomputer (CRAY-2, CRAY-Y/MP, NEC-SXf3) Software Reouirements Graphics post-processing software. Small routines to read topography data bases and to initialize pressure distribution, separate from the computational code Avvroximate Costs None: Basically a research model
I. Introduction
The summertime photochemical smog over Athens has become a major environmental problem in the last decade and has the reputation of leading to one of the most severe air quality problems of any European capital. In order to further understand the fundamental proceses involved, the APSIS model intercomparison study was initiated with the aim of simulating observed episodes. In this manner, different model codes could be tested for the atmospheric dynamics (APSIS Part A) and air chemistry (APSIS Part B) over this region, thereby allowing rigorous evaluation of their capability in reproducing conditions resulting in the severe smog episodes. Validated models could subsequently be used for scenario testing, in particular to assess possibilities of mitigation in the context of photochemical smog. DREAMS-1 is a three-dimensional atmospheric model which makes use of the finite difference technique to solve a series of time-dependent equations for processes acting typically on a regional scale. The model was originally developed in the early eighties to investigate organized cloud convection over a maritime area, such as the Rayleigh-Btnard type convection frequently observed as cold air masses move over a warmer ocean surface (Benistonl,2,3). It has also been used to investigate some
aspects of cloud and radiation interactions (Beniston and Schmetz4; Schmetz and Beniston5), as well as the dynamic and thermodynamic influence of a water surface on the regional atmosphere (Beniston6). At a later stage of its development, DREAMS-1 has been used in complex terrain to study air pollution on a local scale, through coupling with DREAMS-2. A number of papers related to this system have been published in the literature, such as Beniston 7, Beniston et al. 8, and Beniston 9, with description of the models and the results of case studies. The size of the model domain is essetially limited by the computer resources on which the model code is implemented. The characteristic spatial scales of the phenomena to be investigated should also be borne in mind, so that an optimal model domain size can be determined prior to a numerical investigation. For a supercomputer such as the CRAY-2 or equivalent, the enormous memory capacity allows a generous domain size; DREAMS-1 has been run with up to 2 million gridpoints on such a machine. On less advanced machines, the model code can generally run with a 32,000-point grid mesh distributed in the three cartesian directions, thus allowing a balance to be found between domain size and computing time. Typical grid interval is 1,000 m and up in the horizontal, and vertical levels can be unevenly distributed, allowing higher resolution of the lower levels
67
Atmospheric flows over Athens: first tests with the D R E A M S model
of the ABL. In general, the fhst model level is located at 50 m above the r e f e r e n c e level, with subsequent even spacing of between 100 and 250 m. The problem of determining concentrations of a particular pollutant at a given location by numerical analysis is exceedingly complex. The Eulerian technique is poorly adapted to point sources, since the source becomes specified over an entire grid volume which, even in the case of a fine grid mesh, is an invalid representation of the point source. The-most commonly used Gaussian dispersion models yield elegant analytical solutions to the emission-transport-immision problem, but contain so many restrictive assumptions that they are unable to produce correct solutions to dispersion in complex terrain under real meteorological conditions. More recently, researchers have focused on Lagrangian solutions to the dispersion problem. The advantage of the Lagrangian approach is that individual trajectories of particles (which represent a particular pollutant) are independent of grid size; also any time increment may be used according to the time resolution desired. Furthermore, a Lagrangian particle technique is the only method by which a complex flow situation, in particular strong wind shear or flow reversals, may be adequately taken into account. Both continuous and instantaneous releases can be considered. Among the f'~t workers in this field were McNider, whose "conditioned particle technique" has been widely used by Pielke and coworkers (Pielkel0). Lagrangian particles as tracers for pollution dispersion studies have also been designed by Legg and Raupach 11, and adapted to heavy gas dispersion by Schoi'ling-, i.e., here the particles are no longer passive elements of the flow but have their own mass. The DREAMS-2 model code contains schemes allowing the study of the physical behavior of pollution with both the Eulerian and the Lagrangian techniques; it is at present used for the study of passive pollutants and uses the dynamic and thermodynamic data generated by DREAMS-1 to compute the characteristics of pollutant transport and dispersion. A number of papers related to the complete coupled system have appeared in the literature, such as for example Beniston and Pielke 13.
2.
Model
initialization
The APSIS participants had access to the following data: - Topography: a detailed kilometric altitude grid, for a 72 x 72-point raster, and a lower resolution 36 x 36 grid over the same area. This data file also contains the land-use characteristics of the domain - Meteorological data at 10 m height: five recording stations distributed over the Attica Peninsula, provided information on the following parameters: wind speed and direction, temperature, dew-point temperature, and pressure, for May 25 and 26, 1990. Data is available each three hours over this period, although four of the stations have gaps in the data sets
- Vertical soundings: Four vertical soundings during this period give an idea of the vertical structure of the atmosphere above Hellinikon Airport, from the ground to about 10,000 m above sea level (ASL).
DREAMS-1 was initialized on the 1-km grid using data at 00:00 hours on May 25, 1990. The Hellinikon upper-air sounding provided the wind, temperature, and humidity data for each of the 20 model levels. The vertical grid interval is staggered with height, having an interval of 100 m within the Atmospheric Boundary Layer, and then an increasing separation up to the model top which lies at 4,400 m above sea-level. The data for each level is assigned to each grid-point at that level, thereby leading to initially homogeneous horizontal dynamic and thermodynamic conditions. A synoptic pressure gradient, computed from the available data, provides one of the forcing terms which drives the dynamics of the system. An additional, and probably dominant, forcing term is the thermal perturbation generated by the land and sea temperature difference. Sea temperatures are kept at a constant 20 ° C, while the ground surface undergoes a sinusoidal warming/cooling with an amplitude of about 9° C. The temperature difference between the land and the sea creates a pressure perturbation (given by the hydrostatic equation and the equation of state), leading to the creation of a local dynamic system along the coast which tends to move inland with time (sea breeze); with cooling nocturnal temperatures, the system is halted and reversed, resulting in air flowing out to sea from the coastal regions (land breeze). The intensity and horizontal extent of the land and sea breeze depends essentially on the strength of the land/sea temperature difference.
3.
Scenarios
for
APSIS
Experiment
A
Scenario A-1
First computations were based on the following principles: - 1: Initial data is taken from the upper-air sounding at Hellinikon on May 25, 1990, at 12:00 - 2: Land and sea breezes are generated through a regular sinusoidal temperature fluctuation at the ground 3: The synoptic flow is considered to be constant throughout the duration of the simulation. Any observed changes in atmospheric parameters with height are a result of interactions between the sea breeze and the synoptic flow - 4: A 30-hour simulation was attempted, with data output each 6 hours - 5: Wind speed and direction was not adjusted to the reference profile. The synoptic flow was established in an approximate manner, based on the experience of previous test computations. Scenario A-2
Points 1 - 3 remain the same as for scenario A-1 - 4: A first simulation of 6 hours' duration is undertaken,
J. Ehinger, M. Beniston
68
with a surface heat forcing of 7.6 ° C (in close agreement with measurements) - 5: Wind profiles are adjusted to the observed profile at Hellinikon Airport, for May 25, 1990, at 12:00
4. First results
Scenario A-1 The first evidence of the sea-breeze circulation is illuslrated in Figure 1. This occurs at about 12:00 local time for the
10
20
10
-A
20
30
30
simulated event of May 25. 1990, when the model reaches the peak wanning situation. A sea breeze can be observed along the entire coastal region, and corresponds quite well to what may be expected from a thermal forcing of the amplitude imposed in the model. The surface warming generates a mass of warm air which rises above the ground; this is compensated for by an influx of cooler maritime air into the region. The resulting convective cells have a diameter of over 10 km and a vertical extent of 2 - 3 km. As the period of warming continues into the afternoon, the sea-breeze intensifies and moves inland, as seen in Figure 2.
40
40
50
50 r
60
60
V e l o c i t y , pt: 1,1 = 4.29 m / s
.AC Figure 1: Circulation patterns at 12:00 local time
70
70
69
Atmospheric flows over Athens: first tests with the D R E A M S model
10
20
10
20
30
30
40
50
40
60
50
60
70
70
Velocity, pt: 1,1 = 4.24 m/s
.AC Figure 2: Maximum intensity of the sea-breeze circulation.
Athens is subjected to a sea breeze whose intensity fluctuates between 0.5 and 2 m/s close to the coast. Because of insufficient observational evidence, it is uncertain whether the simulated wind is in agreement with reality, and what would be the behavior of pollution embedded within the sea-breeze. The wind flow is perpendicular to the coast in the SE sector of the city, which implies that the smog could be transported and dispersed only if it crosses the hills behind the city. It would be of interest to know whether this occurs in reality.
Scenario A-2 In this simulation, the computed wind profile has been adjusted to the measured profile at Hellinikon on May 25, 1990 at 12:00 (on grid point i, j --- 35, 33 in the Cartesian reference frame of the 72 x 72-point raster).. The precision of the adjustment is within 2 ° for the wind direction and within 0.2 m/s for the wind velocity. In the absence of other vertical profiles measured simultaneously over the Attica Peninsula, it is not
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J. Ehinger, M. Beniston
Table 1: Comparison of observational and simulated data Station
Eleusis Filadelfla Hellinikon Piraeus Tatoi
Grid Point 19, 35, 35, 26, 40,
51 50 33 37 55
Vm [m/s]
Vc [m/s]
DV m-c
DIR m [°]
DIR c[ °]
DDIR m-c
2.6 3.1 3.6 3.1 3.1
3.8 1.0 3.1 5.6 1.8
-1.2 +2.1 +0.5 -2.5 +1.3
190 230 220 160 180
200 220 230 200 225
-10 + 10 -10 -40 -45
possible to go beyond this form of adjustmenL However, a measure o f assessment is provided by the 10-m meteorological data; precautions are necessary when comparing computed and measured data, since the representativity of the weather stations for a 1-km grid are not known. The results are given in Table 1. Vm and Vc are the measured and computed velocities, respectively, DIRm and DIRc the corresponding directions, and DV and DDIR are the differences between measured and computed values. This first intercomparison is encouraging, especially concerning the wind directions. Simulated wind direction does not deviate by more than 45 ° from its observed value; however, wind velocities seem less precise, although out of five stations, three are within + 1.5 m/s which is often within the range of validity of the measurement itself.
5.
Conclusions
The first tests of the DREAMS-1 model have provided an insight as to its capability in reproducing observed seabreeze patterns in the Athens area. While not perfect, the results are encouraging and appear to be consistent with the observations (i. e., there is no significant deviation of the observed wind from reality. Improvements will be sought in future experiments, but these are linked to a number of problems which first need to be resolved. These include: - Sparse or missing data: A more comprehensive data set would be required to enable more c o h e r e n t intercomparisons between data and reality to be made. It would be ideally necessary to have vertical profiles at other locations in addition to the single sounding at Hellinikon; furthermore, additional surface data would be desirable, epecially for the period of May 24, 1990, which would enable a better initialization to be made for May 25, 1990 - Limits o f computation: The most severe restriction at present seems to be with respect to the time variations of the synoptic wind for periods of 24 hours or more. Based on the Hellinikon soundings, the uppermost model level (at 4,400 m ASL) would need to undergo velocity changes of between 5 and 20 m/s and direction changes of nearly 60 ° over a 24-hour period in order to correspond to reality. Such fluctuations would need to be taken into account by the model for more realistic simulations of the time frame of cad May 1990 over the Athens area.
- Future work: Although the figures illustrated here seem consistent with reality, a number of additional controls will be made, in particular: - Assessment of the influence of the eastern model boundary on flow characteristics near the SE Attica Peninsula, which lies close to this boundary - Assessment of the height of the model top on processes taking place at lower model levels
Such tests are standard numerical procedures and will not form part of any specific test case. However, further work on the APSIS experiment will focus on a third scenario (A-3), which will force the ground temperature according to an irregular sinusoidal function (in order to take into account non-equal day and night duration).. Scenario A-4 will atempt to incorporate changes in synoptic forcing over a 24-hour period.
References
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