Tropopause folding is a dominant mechanism for stratosphere/troposphere exchange (STE). While STEs have been studied extensively in the past, the chemical transport of stratospheric air into the lower troposphere over the southeastern United States has rarely been documented. One such event presented itself on November 2010 and the current study uses WRF/CMAQ to examine the extent of this event.

On November 6, 2010, ozonesonde measurements at Huntsville, Alabama, indicated a strong stable and extremely dry layer of air with high ozone concentration right above the boundary layer from 2-km to 3-km altitude. This layer had all the characteristics of stratospheric air. Subsequent investigation using model simulation, satellite observations, and lidar measurements at the site was able to explain this event and indicated that the high ozone was indeed of stratospheric origin and the stratospheric ozone intrusion was due to a tropopause folding event.

WRF/CMAQ modeling system was utilized in this study. The lateral boundary condition that played a crucial role in explaining this event was provided from Mozart. Model results were evaluated by lidar measurements of November 5th and 6th and exhibited good agreement suggesting that the model was able to reasonably capture the event. Further examination of the model results shows the extent of the stratospheric incursion over the southeastern United States and indicates that the high ozone observed at Huntsville is only a small fragment of the high ozone that was transported over the Gulf of Mexico. The results from this case study show that periodic ozone transport events due to tropopause folding can significantly contribute to the mid-latitude ozone burden in the lower troposphere.