Stratospheric Intrusions over the U.S.: Impacts on Tropospheric Composition from NASA Aircraft Campaigns, MERRA-2, and the GEOS-5 Model

Abstract:

Intrusions of ozone-rich stratospheric air periodically enhance surface ozone levels in western U.S. states and can cause exceedances of the current National Ambient Air Quality Standard. Aircraft observations and ozonesonde profiles collected during the Maryland DISCOVER-AQ field campaign indicate the presence of stratospheric air just above the boundary layer on several days raising the question of whether or not such stratospheric intrusions (SI)s have a similar influence in the eastern U.S. and to what degree they influence policy-relevant background ozone levels. Better understanding the processes controlling the variability of tropospheric ozone is critical for both air quality and climate applications. High-resolution (50 km or greater) global atmospheric models and reanalyses have been demonstrated in several studies to realistically reproduce the observed vertical structure and temporal evolution of a limited number of SI events, but more research is needed to evaluate their ability to simulate the seasonal, spatial, and interannual variability of SIs. In this study, we used simple, computationally-efficient stratospheric tracers implemented in the Goddard Earth Observing System Model, Version 5 (GEOS-5) model, along with aircraft data and ozonesonde profiles to determine the frequency and extent of SIs over the United States during the observing period of NASA's Aura satellite (2004-2014). High-resolution simulations of stratospheric tracers are compared with ozone fields from NASA’s Modern-Era Retrospective Analysis for Research and Applications Version-2 (MERRA-2), which includes assimilation of observed ozone from MLS and OMI to constrain upper troposphere/lower stratosphere mixing ratios, to assess the representation of SIs in the lower troposphere. We will discuss 1) the implications for using reanalyses to assess interannual variability of SIs and 2) the potential of operational forecasting of SIs using simple tracers in conjunction with ozone assimilation.