Athens Validation Scenarios

Aristotle University of Thessaloniki,
Laboratory of Heat Transfer and Environmental Engineering

Nicolas Moussiopoulos, Kostas Karatzas

A. Introduction

Photochemical air pollution over urban areas is formed as a result of a complex non-linear interaction of chemistry and meteorology. For the Athens case, and in order to define appropriate meteorological conditions concerning the scenario formation for the application of modeling tools in the frame of ECOSIM, four meteorological conditions were chosen representing a resent synoptic classification (Kassomenos 1993, Kallos et. al. 1993). In addition a fifth case representing typical summer sea breeze circulation, taking into account the development of sea breeze in Athens (Prezerakos 1986) was also selected. All four conditions were chosen among air pollution episode cases. The largest number of air pollution episodes was found during the transient seasons with winter following. It worthies noting that a similar methodology of meteorological classification resulted in analogous results concerning ozone formation behaviour in similar cases (Wakamatsu et. al. 1990). In addition to the four cases a fifth was also chosen

B. Synoptic classification

The four meteorological synoptic categories (a, b, c and d) have the following distribution between seasons for an analysis from 1983 to 1990 (Kallos et. al., 1993):

C. Selected meteorological cases

Case 1: 25/26 May 1990.

This case represents category a. Main characteristics of this category are high pressure systems starting to develop over north-east Mediterranean and/or south Europe. The anticyclonic system moves eastward towards Greece. Warm air masses are advected toward Greece and the lower tropospheric layers are very stable. According to available data the synoptic conditions showed rather weak pressure gradients prevailing over Greece while warm advection occurred aloft. A strong temperature inversion was observed at night which did not brake up during the day. As the local circulation systems remained shallow due to the inversion, weather conditions observed in Athens could be classified as stagnant. The mixing height hardly exceed 100 m at night and 200 m in the afternoon hours.

Case 2: 9/10 December 1990.

This case represents category b. A low pressure system is located over the Central Mediterranean and moves eastward. The anticyclonic system over the Balkan peninsula starts weaken. Greece is inside the warm section of the low. The advection of warm air masses stabilizes the lower troposphere. For the specific case available observations show low western winds not exceeding 8 m/s for the upper atmosphere, while surface temperature over land was 12,2 ^oC during the morning. Lower atmosphere was relatively stable.

Case 3: 28/29 December 1990.

This case represents category c, where a cold front moves from the NW toward the Balkan Peninsula. After the passage of the cold front, relatively cool air masses are observed over Greece. The synoptic flow is relatively weak. Clear sky conditions support the formation of surface temperature inversions. According to available observation these days lower atmosphere was stable while surface temperature over land was from 12^oC in the morning to 15.2^oC in the afternoon.

Case 4: 2/3 July 1990.

This case corresponds to usual summer conditions, with an anticyclonic system covering most of the Mediterranean and northeast Europe. A pressure gradient is established over the Aegean Sea and consequently over Athens. After the eastward extent of the anticyclonic system, this pressure gradient weakens and local circulation starts to develop. According to available observations N-NW winds were blowing over Athens while the surface temperature over land was from 25^oC to 30^oC. Lower atmosphere was stable.

In addition to the above four cases a typical summer sea breeze case was selected as

Case 5: 7 July 1994.

The weather situation on that day was characterized by relatively light winds. Observations during the early morning hours reveal NW winds with a strength up to 9 m/s, while the temperature reached 26^oC. Late in the morning NEE winds were observed at Spata (SPA in Fig. 1) at heights up to 500 m above ground level (AGL) with a strength reaching 7 m/s. At noon, and while the temperature reached 32^oC, a sea breeze circulation developed with a wind speed not exceeding 5 m/s. After 16:00 LST, the wind veered to NE. At the same time and in upper levels (above 1000 m AGL), rather strong winds with speeds up to 12 m/s were prevailing. As stability is concerned, the conditions of the atmosphere over the GAA could be characterized as synoptically stable. This can be classified as a typical sea breeze case during weak synoptic-scale pressure gradients (Prezerakos, 1986).

References:

• Kallos G., Kassomenos P. and Pielke R. (1993), Synoptic and mesoscale weather conditions during air pollution episodes in Athens, Greece, Boundary Layer Met. 62, 163-184.

• Kassomenos P. (1993), Study of atmospheric conditions during the occurrence of air pollution episodes in the Greater Athens Area, PhD Thesis, Athens.

• Kunz R. and Moussiopoulos N. (1995), Simulation of the wind field in Athens using refined boundary conditions, Atmos. Environ. 29, 3575-3591.

• Moussiopoulos N. (1993), Athenian photochemical smog: intercomparison of simulations (APSIS), background and objectives, Env. Software 8, 3-8.

• Moussiopoulos N., Flassak Th., Sahm P. and Berlowitz D. (1993), Simulations of the wind field in Athens with the nonhydrostatic mesoscale model MEMO, Environmental Software 8, 29-42.

• Moussiopoulos N., Sahm P. and Kessler Ch. (1995), Numerical simulation of photochemical smog formation in Athens, Greece-a case study, Atmos. Environ. 29, 3619-3632.

• Moussiopoulos N., Sahm P. Karatzas K., Papalexiou S. and Karagiannidis A. (1997), Assessing the impact of the new Athens airport to urban air quality with contemporary air pollution models, Atmos. Environ. 31, 1497-1511.

• Pregerakos N. (1986), Characteristics of the sea breeze in Attica, Greece, Boundary-Layer Meteorology 36, 245-266.

• Wakamatsu S., Uno I. and Suzuki M. (1990), A field study of photochemical smog formation under stagnant meteorological conditions, Atmos. Environ. 24A, 1037-1050.