Forecasting Stratospheric Intrusion Events over the Western US for Exceptional Event Demonstration

During the late winter and early spring tropopause folds frequently allow stratospheric air to descend and introduce ozone-rich air into the troposphere. These stratospheric intrusion (SI) events can occasionally result in elevated surface ozone at higher elevation monitoring sites in the western US, particularly when they occur in conjunction with deep planetary boundary layers and efficient turbulent mixing. SI can lead to exceptional events, which according to the U.S Environmental Protection Agency, are considered "unusual or naturally occurring events that can affect air quality but are not reasonably controllable using techniques that tribal, state or local air agencies may implement in order to attain and maintain the National Ambient Air Quality Standards." In this talk we present results from nested global-to-regional scale air quality modeling studies showing the potential for forecasting SI events and discuss how these modeling studies can be used to provide weight of evidence for exceptional event analysis. The modeling studies utilize global ozone analyses from the Real-time Air Quality Modeling System (RAQMS) that are used to initialize high resolution (8-12km) Weather Research and Forecasting - Chemistry (WRF-CHEM) regional predictions of SI events that occurred during May 2010, and May/June 2012. These events resulted in elevated ozone at surface monitors in Colorado and Wyoming. The impact of assimilation of stratospheric ozone profiles from real-time Microwave Limb Sounder (MLS) retrievals and total ozone column from real-time Ozone Monitoring Instrument (OMI) retrievals on forecasting SI events is assessed through data denial experiments and comparison with surface measurements from the US EPA AirNow monitoring network.AQMS 36hr 40N ozone cross section forecast valid 00Z May 25, 2012 showing pronounced tropopause fold forming a "double tropopause" with high O3 and turbulent mixing of ozone within the planetary boundary layer over the higher terrain of Colorado between 110-105E.