Synoptic and local effects on the climate of the Waimate area, South Canterbury
The township of Waimate, with a population of approximately 3200 residents, is situated in a small hollow at the base of coastal alpine foothills, and experiences significant air pollution problems in winter because of its close proximity to both mountains and the sea. This problem is caused by the interaction of mesoscale atmospheric conditions (both airflow and boundary layer stability structure) with the local terrain. There are also significant topoclimatological variations which are of importance to agricultural and horticultural activities. Field experiments were conducted during the winter of 1989 (April to July), and included measurements of airflow, atmospheric stability, air pollution and temperature close to the ground. Results indicate that under weak to moderate synoptic pressure gradients the research area is dominated by nocturnal cold air drainage from the alpine foothills. This cold air is channelled onto the coastal plain through narrow gorges, whereupon its velocity is reduced and the air begins to stagnate along stream channels and in the hollow where Waimate is located. This airflow pattern is associated with the development of temperature inversions over the township which contribute to trapping of air pollution, levels of which may exceed those experienced by larger urban areas. The occurrence of cold air drainage is part of a diurnal airflow and stability variation which may also include the development of onshore winds during the day. These onshore winds sometimes act to increase atmospheric dispersion, but in some cases appear to help maintain pollution concentrations. Analysis of results to date clearly indicates the uniqueness of a local atmospheric environment in close proximity to both significant mountains and the sea.