NASA's High-Resolution GEOS Forecasting and Reanalysis Products: Impact of Stratospheric Intrusions on Surface Ozone Air Quality

A new approach to the study of stratospheric intrusions (SIs)– the introduction of ozone-rich stratospheric air into the troposphere – uses NASA's Goddard Earth Observing System Model (GEOS) model and assimilation products with an objective feature tracking algorithm to investigate the atmospheric dynamics in the UTLS region that generate SIs and the different mechanisms through which SIs may influence tropospheric chemistry and surface air quality. SIs have been the interest of decades of research for their link with surface ozone air quality exceedances, especially at the high elevations in the western USA in springtime; however, the impact of SIs in the remaining seasons and over the rest of the USA is less clear.

Using the tracking algorithm of Hodges (1995, 1999), a catalog of stratospheric intrusions identified in NASA’s Modern-Era Retrospective Analysis for Research and Applications Version-2 (MERRA-2) reanalysis was produced for the period since 2005. We focus on the latter half of the MERRA-2 period, since in October 2004 the ozone observing system in the data assimilation switches in MERRA-2 from SBUV measurements to the finer resolution satellite retrievals of total column ozone from the Ozone Monitoring Instrument (OMI) and stratospheric ozone profiles from the Microwave Limb Sounder (MLS), both aboard NASA’s Aura satellite. During the ‘Aura’ period, when there is direct influence of stratospheric ozone into the troposphere, such as a stratospheric intrusion, we can expect MERRA-2 to realistically represent both atmospheric dynamics and composition. While the spring over the western USA does exhibit the largest number of SIs affecting the lower troposphere (about 18 per season), we demonstrate that the number of intrusions in the remaining seasons (8-15 per season) and over the eastern USA (2-9 per season) is sizable.

Using this nearly 15-year catalogue as the base to assess the frequency of events, we extend the analysis to the high-resolution GEOS forecasting products: GEOS-FP and GEOS-CF. The operational GEOS weather forecasting system, GEOS-FP, has a similar ozone observing system to MERRA-2, while NASA’s new global high-resolution air quality forecast system, GEOS-CF, combines the operational GEOS weather forecasting model with the state-of-the-science GEOS-Chem chemistry module (version 12), simulating a wide range of additional air pollutants and tracers which strengthens this detailed analysis of the intrusions and the sources for the high ozone concentrations. The GEOS products are able to resolve the fine-scale features of the SI ozone filaments, previously misrepresented in models. Using a multitude of observational datasets, including lidar, air craft, ozonesondes and air quality monitoring surface sites, in combination with the GEOS forecast and reanalysis products, we aim to provide the public with tools which are available in near-real time to enhance their capability to identify the impact of stratospheric air on surface ozone concentrations separate from anthropogenic sources. In particular, improved understanding of the connections between large-scale climate variability and local-scale dynamically-driven air quality events may support improved seasonal prediction of SI events.