Moist convection frequently reaches the tropopause and alters the distribution and concentration of radiatively important trace gases in the upper troposphere and lower stratosphere (UTLS), but the overall impact of convection on regional and global UTLS composition remains largely unknown. To improve understanding of convection‐driven changes in water vapor (H2O), ozone (O3), and carbon monoxide (CO) in the UTLS, this study utilizes 13 years of observations of satellite‐based trace gas profiles from the Microwave Limb Sounder (MLS) aboard the Aura satellite and convection from the operational network of ground‐based weather radars in the United States. Locations with and without convection identified via radar are matched with downstream MLS observations through three‐dimensional, kinematic forward trajectories, providing two populations of trace gas observations for analysis. These populations are further classified as belonging to extratropical or tropical environments based on the tropopause pressure at the MLS profile location. Extratropical regions are further separated by tropopause type (single or double), revealing differing impacts. Results show that convection typically moistens the UT by up to 300% and the LS by up to 100%, largely reduces O3 by up to 40%, and increases CO by up to 50%. Changes in H2O and O3 are robust, with LS O3 reduced more by convection within tropical environments, where the median concentration decrease is 34% at ∼2 km above tropopause, compared to 24% in extratropical environments. Quantification of CO changes from convection is less reliable due to differences being near the MLS measurement precision and accuracy.

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. We found that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which we use 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.

Abstract. Stratospheric water vapor (H2O) 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 H2O 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 H2O feedback. Moreover, focusing on the overworld only can lead to a large underestimation of stratospheric H2O 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 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. We show 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.

<p>Definition of the tropopause has remained a focus of atmospheric science since its discovery near the beginning of the twentieth century. An accurate identification of the tropopause is a vital component to upper troposphere and lower stratosphere research, especially for studies that seek to assess and quantify the two-way exchange of air across the tropopause, which in turn impacts our understanding and prediction of Earth’s radiation budget and climate. 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, we reexamine the relationship between stability and composition change in the tropopause transition layer and identify areas for improvement in a stability-based definition of the tropopause. Six locations with long-term (up to 40+ years) balloon observations of temperature, ozone, and water vapor were selected for analysis to offer a variety of environments, including tropical, subtropical, extratropical, and polar environments. Data from these sites are then used to identify covariability between several metrics of atmospheric stability and composition. We show that the vertical gradient of potential temperature is a superior stability metric to identify the greatest composition change in the tropopause transition layer, which we use to propose a new universally applicable potential temperature gradient tropopause (PTGT) definition. A comparison of the PTGT and LRT applied to both observations and reanalysis output will be shown. Overall, our results reveal that the PTGT largely agrees with the LRT, but more reliably identifies tropopause-level composition change when the two definitions differ greatly.</p>