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Recent field campaigns, observational studies, and modeling work have demonstrated that extratropical tropopause-overshooting convection has a substantial and previously underestimated impact on stratospheric water vapor concentrations. This necessitates improved understanding of how tropopause-overshooting convection will respond to a warming climate. A growing body of research indicates that environments conducive to severe thunderstorms will occur more often and be increasingly unstable in the future, but no study has examined how this may be related to increased overshooting. To rectify this, this study leverages an existing pseudo–global warming (PGW) experiment to evaluate potential future changes in tropopause-overshooting convection over North America. We examine two 10-yr simulations consisting of 1) a retrospective period (2003–12) forced by ERA-Interim initial and boundary conditions (the control simulation) and 2) the same retrospective period with CMIP5 ensemble-mean high-end emission scenario climate changes added to the initial and boundary conditions (the PGW simulation). Tropopause-overshooting convection in the control simulation is validated against observed overshoots from both ground-based radar observations in the United States and GOES observations over North America. The model is shown to effectively simulate the observed regional distribution, annual cycle, and diurnal cycle of tropopause-overshooting convection. In the PGW simulation, tropopause-overshooting convection is found to increase more than 250% across the model domain, and the projected seasonal period of frequent tropopause-overshooting convection is shown to extend into late summer. Additionally, tropopause-overshooting convection with extreme tropopause-relative heights (>4 km) is more frequent in a warmed climate scenario. [ABSTRACT FROM AUTHOR]

Stratospheric water vapor (H 2 O) is a substantial component of the global radiation budget and therefore important to variability in the climate system. Efforts to understand the distribution, transport, and sources of stratospheric water vapor have increased in recent years, with many studies utilizing long-term satellite observations. Previous work to examine stratospheric H 2 O extrema has typically focused on the stratospheric overworld (pressures ≤ 100 hPa) to ensure the observations used are truly stratospheric. However, this leads to the broad exclusion of the lowermost stratosphere, which can extend over depths of more than 5 km below the 100 hPa level in the midlatitudes and polar regions and has been shown to be the largest contributing layer to the stratospheric H 2 O feedback. Moreover, focusing on the overworld only can lead to a large underestimation of stratospheric H 2 O extrema occurrence. Therefore, we expand on previous work by examining 16 years of Microwave Limb Sounder (MLS) observations of water vapor extrema (≥ 8 ppmv) in both the stratospheric overworld and the lowermost stratosphere to create a new lower-stratosphere climatology. The resulting frequency of H 2 O extrema increases by more than 300 % globally compared to extrema frequencies within stratospheric overworld observations only, though the percentage increase varies substantially by region and season. Additional context is provided for this climatology through a backward isentropic trajectory analysis to identify potential sources of the extrema. We show that, in general, tropopause-overshooting convection presents itself as a likely source of H 2 O extrema in much of the world, while meridional isentropic transport of air from the tropical upper troposphere to the extratropical lower stratosphere is also possible. [ABSTRACT FROM AUTHOR]