A series of modeling studies are performed using a mesocale dynamics and photochemical modeling system to (1) elucidate the interactions of synoptic, mesoscale, and boundary-layer processes responsible for the downward transport of ozone within the troposphere and (2) evaluate the relative contribution of stratospheric and tropospheric sources of ozone over eastern North America. The model results are compared to ozonesonde profiles at Cape Race, Newfoundland obtained by NOAA scientists once or twice a day during a two-week period in the summer of 1991 (Oltmans et al. 1996, JGR). Layers of high ozone concentrations ( > 100 ppb) were frequently observed in the middle and upper troposphere. To simulate the meteorological mechanisms responsible for these high ozone concentrations, a nested grid approach is used in which the outer grid encompasses North America with a horizontal grid spacing of 120 km and the inner grid encompasses the northeastern United States and eastern Canada with a grid spacing of 60 km. In contrast to previous studies, a relatively fine vertical resolution is employed (maximum grid spacing of 500 m) to better represent the vertical transport of ozone in the lower stratosphere and upper troposphere. A four-dimensional data assimilation technique that incorporates rawinsonde observations is employed by the mesoscale model to limit the forecast errors in the meteorological fields throughout the 14-day simulation period. The initial 3-D ozone field and the lateral boundary conditions for ozone on the outer grid are based on the potential vorticity values. Simulations with and without chemistry are performed to determine the relative contribution of transport from the stratosphere and tropospheric chemistry to the layers of ozone in the middle troposphere.
The simulation without chemistry produced peak ozone concentrations between 5 and 8 km MSL that were very similar to the ozonesonde measurements over Cape Race, suggesting that transport was largely responible for the high ozone concentrations. An analysis of the 3-D ozone and meteorological fields indicated several tropopause folds occurred during the 14-day period that transported ozone from the stratosphere into the mid troposphere. Air parcel trajectories also revealed that the downward velocities associated with the stratospheric intrusions brought ozone-rich air from the lower stratosphere into the mid troposphere over a 1-day period and that different geographic source regions contributed to the high ozone concentrations over Cape Race. The specific meteorological processes will be described and the contributions of stratospheric and tropospheric sources of ozone to the tropospheric and boundary layer ozone concentrations during the 14-day period will be quantified.
Symposium on Interdisciplinary Issues in Atmospheric Chemistry