Preliminary Meteorological analysis of a 1997 Southern California ozone study (SCOS'97) episode

An analysis of the synoptic and mesoscale meteorological conditions over the Los Angeles basin during the August 1997 SCOS'97 ozone episode is presented. This field study, sponsored by the California Air Resources board (CARB), produced an exten-sive mesometeorological data set from, for example, about 250 surface sites, 11 rawinsondes, and 26 profilers. The first goal of the current analyses was to document the characteristics of the elevated nocturnal residual turbulence layer and the subsequent early morning fumigation of its residual ozone to the surface. The second goal of the study was to simulate these turbulence processes with a modified version of MM5, so as to provide input meteorological fields for UAM-V ozone simulations.

Mesoscale analyses show how changes in the synoptic weather pattern over southern California produced altered mesoscale thermal and flow patterns that resulted in the observed ozone episode of 5 August. In particular, time-height turbulence cross-sections show nocturnal residual layers at profiler sites located west of the mountains ringing the western edge of the L. A. basin. These elevated layers resulted when the westward moving sea breeze flow was first lifted and then turned eastward by the mountains, and then finally died out. MM5 simulations with and without four dimensional data assimilation have been carried out, and results will be compared to observed meteorological patterns.