| dc.description.abstract | The troposphere and the stratosphere are two separate layers of the atmosphere whose dynamics, composition, and chemistry are fundamentally different. This leads to the upper troposphere and lower stratosphere (UTLS) being a complex region of the atmosphere that is critically important to both weather and climate. The upper troposphere is separated from the lower stratosphere by an identified ‘tropopause’, and any transfer of air across this interface is therefore considered to be stratosphere- troposphere exchange (STE). The difference in composition between the troposphere and the stratosphere makes processes that facilitate STE essential to the climate system. Specifically, the transport of water vapor from the relatively moist troposphere to the much drier lower stratosphere, where water vapor functions as a powerful greenhouse gas, can contribute substantially to the warming climate at the surface. The sources of stratospheric water vapor are still a topic of debate in the scientific community, where the specific contributions of larger-scale processes like the global atmospheric circulation and smaller-scale processes like tropopause-overshooting convection remain unclear, though recent evidence has demonstrated the latter to be more important than was previously thought. This dissertation seeks to clarify the role that tropopause-overshooting convection has in modulating the lower stratospheric water vapor budget in both the present and in the future.
The first component of this dissertation is the creation of a climatology of extreme water vapor concentrations within the lowermost stratosphere, with a complementary analysis exploring the sources and transport pathways of these extreme concentrations. Stratospheric water vapor is a substantial component of the global radiation budget, and therefore important to variability of 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 water vapor 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 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 water vapor feedback. Moreover, focusing on the overworld only can lead to a large underestimation of stratospheric water vapor extrema occurrence. Therefore, this dissertation expands 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 H2O 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 to this climatology through a backward isentropic trajectory analysis to identify potential sources of the extrema. It is shown that, in general, tropopause-overshooting convection presents as a likely source of H2O 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.
The second dissertation component takes a step back to examine challenges related to definition of the tropopause. Any study which examines cross-tropopause transport, like the first component of this dissertation, is reliant on an accurately identified tropopause in order to correctly assess STE. Thus, proper definition of the tropopause has far reaching implications for our understanding of Earth’s radiation budget and climate. Definition of the tropopause has remained a focus of atmospheric science since its discovery near the beginning of the 20th century. Few universal definitions (those that can be reliably applied globally and to both common observations and numerical model output) exist and many definitions with unique limitations have been developed over the years. The most commonly used universal definition of the tropopause is the temperature lapse-rate definition established by the World Meteorological Organization (WMO) in 1957 (the LRT). Despite its widespread use, there are recurrent situations where the LRT definition fails to reliably identify the tropopause. Motivated by increased availability of coincident observations of stability and composition, this study seeks to re-examine the relationship between stability and composition change in the tropopause transition layer and identify areas for improvement in stability-based definition of the tropopause. In particular, long-term (40+ years) balloon observations of temperature, ozone, and water vapor from six locations across the globe are used to identify co-variability between several metrics of atmospheric stability and composition. The results demonstrate that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which is used to propose a new universally applicable potential temperature gradient tropopause (PTGT) definition. Application of the new definition to both observations and reanalysis output reveals that the PTGT largely agrees with the LRT, but more reliably identifies tropopause-level composition change when the two definitions differ greatly.
The final component of this dissertation examines the response of tropopause-overshooting convection to a warming climate. Recent field campaigns, observational studies, and modeling work, in addition to the first component of this dissertation, have demonstrated that extratropical tropopause-overshooting convection has a substantial, and previously underestimated impact on UTLS composition, especially stratospheric water vapor. This necessitates improved understanding of how tropopause-overshooting convection may change in 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. The PGW technique applies monthly, three-dimensional projected climate changes in state variables (temperature, humidity, wind, etc.) from global climate models to a weather and research forecasting (WRF) convection-allowing model simulation with a 4-km grid. Specifically, I examine two 10-year simulations consisting of (1) a retrospective period (2003 – 2012) 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 is identified as model cloud tops exceeding the potential temperature gradient tropopause, with overshooting in the control simulation validated against observed overshoots from both ground-based radar observations in the United States and GOES satellite observations over North America. The model is shown to effectively simulate the observed regional distribution, annual cycle, and diurnal cycle of tropopause-overshooting convection. The projected response of tropopause-overshooting convection in the PGW simulation is found to be a more than 250% increase across the model domain, and the projected seasonal period of frequent tropopause-overshooting convection was shown to extend into late-summer. Additionally, tropopause-overshooting convection with extreme tropopause-relative heights (> 4 km) are more frequent in a warmed climate scenario.
In summary, this dissertation (1) examines extreme water vapor concentrations in the lowermost stratosphere and how they relate to tropopause-overshooting convection, (2) introduces an improved stability-based tropopause definition to improve future studies of stratosphere-troposphere exchange, and (3) investigates for the first time how tropopause-overshooting convection will respond to climate change. | en_US |
