The surface wind field during winter smog nights in Christchurch and coastal Canterbury, New Zealand

In Christchurch and other Canterbury towns on the east coast of New Zealand's South island, poor dispersion conditions and high emissions of particulate matter and carbon monoxide regularly lead to the build-up of smog during anticyclonic weather conditions in wintertime. This study analyses surface wind fields during smog nights using data collected during the Christchurch Air Pollution Study 2000 (CAPS2000). Westerly land breezes and drainage winds from the foothills of the Southern Alps and the Canterbury Plains are evident in all coastal Canterbury towns, but local terrain features often complicate the wind fields. This holds particularly for Christchurch, where Banks Peninsula causes flow splitting of both the drainage winds on the Canterbury Plains and any superimposed larger scale winds.' Furthermore, stagnation of airflow over Christchurch is often caused by the convergence of localized southeasterly drainage winds down the western part of the Banks Peninsula (Port Hills) with the regional-scale northwesterly drainage winds from the Canterbury Plains. Results illustrate the interaction of cold air drainage over Christchurch, although the unusually high frequency of synopticscale easterly winds during the study period appeared to reduce the effects of both these sources of cold air drainage on air pollution concentrations. The effects of topographically induced flow splitting and non-stationary drainage wind convergence on urban air pollution dispersion identified here have not previously been studied in detail internationally and should receive greater attention in the future. (C) Copyright 2004 Royal Meteorological Society.