干旱区生态环境知识资源中心

A wind field cluster analysis was performed for the summer ozone seasons (May-October) of 1996-2004 for the Central California domain. The data consisted of hourly surface wind speed and direction measurements collected by a number of surface monitoring networks. For each air basin, we identified days sharing similar diurnal cycles. Each day and basin was categorized among these recurring meteorological patterns associated with distinct 500-hPa conditions. We show how summer air quality is consistently driven by these synoptic (large-scale) influences-anticyclonic and cyclonic weather systems that develop over the Pacific Ocean and the southwestern United States. Two such anticyclonic patterns are shown to trigger the majority of the ozone exceedences. For all Central California air basins, an eastward sweeping ridge of high pressure originating offshore is associated with rapidly building ozone levels. For the inland San Joaquin and Sacramento Valleys, the northward migration of an anticyclone forming over Mojave Desert and/or Four Corners triggers a number of episodes. These episodic scenarios are illustrated by examples demonstrating these meteorological triggers.