Analysis of transport patterns during an SCOS97-NARSTO episode

Objectives for the current study included use of SCOS97-NARSTO data to understand transport factors in the occurrence of high ozone concentrations during 4–7 August 1997. Meteorological data for the case study included observations at 110 SCOS97-NARSTO surface sites, and upper-air measurements from 12 rawinsonde and 26 profiler sites. Analysis showed that the peak ozone resulted from an infrequent combination of large-scale upper-level synoptic forcing associated with a weak local coastal 700 mb ridge. Its movements over the California South Coast Air Basin lowered and strengthened the coastal subsidence inversion and also rotated the upper-level synoptic background flow from its normal westerly onshore direction to a less common offshore easterly flow during the nighttime period preceding the peak-ozone hours. Resulting easterly upper-level background winds produced easterly surface flow directions at inland sites, so that a surface frontal convergence zone formed where this flow met the westerly onshore combined sea breeze and upslope flows. The maximum inland penetration of the convergence zone on the peak-ozone day was to the western side of the San Gabriel Mountains, the location of maximum ozone concentrations.