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During the observation period the synoptic weather in Switzerland was dominated by westerly winds (Fig. 9a and 9b), bringing relatively clean air from the Atlantic ocean. The wind speeds in the height of 5 km above sea level were approximately 7 ms - 1 during 29 July and up to 20 ms - 1 in the afternoon of 30 July. Due to the clean-air advection, peak ozone concentrations at most locations reached 60 ppb during 29 July. On 30 July, the concentrations were rising to about 80 ppb. This was probably due to the fact that the air was now advected more from the South and was passing the large urban area of of Lyon in eastern France before entering Switzerland. In the afternoon of 30 July a frontal system passed the Swiss Plateau with thunderstorms, some local showers and a significant decrease in temperatures. Since thunderstorms can not yet be modeled appropriately with Metphomod the simulations were stopped at that time.
The influence of the synoptic situation is considered in the model by setting boundary conditions. In the present study the data from the radio soundings of Payerne located at the upwind boundary of the model domain was used to set wind, temperature, humidity, and ozone concentrations at the boundary. During the intensive observation period, radio sounding measurements were taken every three hours. Ozone was measured every six hours. The synoptic air pressure gradient was estimated from the weather maps of the Swiss Meteorological Office and radio soundings.