In 1985, the first Doppler weather radar to operate in Canada was established by the Research Directorate of the Atmospheric Environment Service (AES) at a site in IntroductionThe King City weather radar facility located north of Toronto, Ontario, was established as a research centre by the Atmospheric Environment Service for cloud physics and radar meterology in 1985. It is the first Doppler radar system to be designed and operated in Canada specifically for the weather observing role. The system was developed from hardware components that were commercially available and by in-house design of interfacing and software. Over the past several years, a program of research and development of the system and its applications has progressed to the point where the system output has been fully integrated into the operational program of the Ontario Weather Centre, which provides full weather services to their forecast region. At the same time, a broad research and development program continues, with the emphasis on meteorological applications. A close liaison of the research and operational sectors has allowed the immediate transfer of technology into operations with mutual benefits.

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SummaryBackground ozone concentration displayed distinct seasonal variations at all stations, with a maximum of about 40 ppb in spring and a minimum of about 23 ppb in autumn. The monthly and yearly mean diurnal variations of ozone at urban stations exhibited bi-modal characteristics not evident at rural sites. Two daily maxima and minima were recorded at urban stations, but only one maximum and one minimum at rural sites. The second minimum in the evening at urban stations was shown to be related to high levels of nitric oxide, while nighttime heat island was considered the main cause of the second maximum. One-day resultant trajectories show that the most frequent and highest ozone concentrations were associated with a relatively light to moderate south to southwesterly flow but not with light or with strong winds. Although there were several recognizable meteorological regimes associated with elevated ozone concentrations over Ontario, those with weak pressure gradients favourable for the onset of lake breeze or with southwesterly flows on the west side of slow-moving high pressure centres crossing the eastern United States were the most frequent and resulted in the highest ozone concentrations. Ozone maxima over Ontario may mostly be attributed to long distance transport of emissions from major industrial areas south of the border. However, tropospheric and stratospheric contributions cannot be ruled out and may even be a major source at times. A case study of possible stratospheric ozone influx into the troposphere is presented. Zusammenfassung Grundlagen der Ozonklimatologie der

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