Convection-Driven Stratosphere-Troposphere Exchange: Observations and Simulations of Midlatitude Thunderstorms and Tropical Cyclones

Abstract

The troposphere and stratosphere are two layers of the atmosphere with distinct composition and dynamics, separated by the tropopause region. Exchange between these layers, known as stratosphere-troposphere exchange (STE) occurs often, when dynamical processes impact the tropopause region. STE alters the composition and thermodynamics of the upper troposphere and lower stratosphere (UTLS), which irreversibly modifies the distribution of greenhouse gases, such as water vapor (H2O) and ozone (O3). This change in greenhouse gas distribution affects the radiation budget and is therefore important for Earth's climate. UTLS composition change occurs through multiple STE processes at various temporal and spatial scales. Tropopause-overshooting convection is the most rapid of such processes, as it enables irreversible mixing between the troposphere and stratosphere through the transport of air in minutes to hours. Previous efforts examining the contribution of overshooting convection to the lower stratospheric H2O budget have primarily considered the tropics, but more frequent and deeper midlatitude overshooting convection has gained increasing attention. Midlatitude overshooting convection can result in the formation of above-anvil cirrus plumes (AACPs) that result in extreme hydration of the stratosphere. While tropical convection has received most prior attention with respect to stratospheric H2O, tropical cyclones (TCs) remain understudied for their contribution to convection-driven UTLS composition change and compared to other recognized STE processes. Given that TCs often have long transit paths have a relatively longer lifetimes (days to weeks), TC-driven UTLS composition change could have unique impacts on the climate system compared to storms that are short-lived. Nonetheless, all convective phenomena pose unique challenges towards understanding their contributions to STE given tropopause altitude, wind shear, tropospheric instability, and stratospheric stability in convective environments are known to vary considerably and impact transport occurrence and/or efficiency. This dissertation seeks to improve understanding of convection-driven STE and concomitant UTLS composition change through isolating several of these inter-dependencies. The first component of this dissertation utilizes idealized simulations to improve understanding of how the lower stratospheric environment can impact cross-tropopause transport from midlatitude overshooting convection. Previous work has primarily focused on troposphere-to-stratosphere transport (TST) rather than stratosphere-to-troposphere transport (STT), including for AACP-producing storms which are recognized to have unique dynamics. Understanding of how transport driven by overshooting convection and AACP properties are modified by variations in UTLS environments is limited, especially that for the extent of STT (i.e., downward) transport. Therefore, AACP development and UTLS transport sensitivities to lower stratosphere stability and the UTLS wind environment are evaluated using four idealized simulations. Varying these attributes results in overshooting storms with and without AACPs, each including a single and double tropopause. It is found that greater TST and stratospheric hydration occurs in storms with AACPs, while STT is greater in storms without AACPs. This STT is facilitated by a mechanical oscillation induced by the overshoot. AACP-producing storms have increased downward transport of stratospheric overworld air to the lowermost stratosphere, which is enhanced within a double tropopause environment. More expansive AACPs, deeper overshoots, and greater TST also occurs in double tropopause environments. The second component of this dissertation uses recent observations to examine STE and UTLS composition change within an active AACP-producing storm. In particular, novel in situ observations from the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign are used to examine UTLS impacts from AACP-generating overshooting convection. The DCOTSS research flight on 31 May 2022 sampled active convection over the state of Oklahoma for more than three hours with the NASA ER-2 high-altitude research aircraft. An AACP was bisected during this flight, providing the first such extensive in situ sampling of this phenomenon. The convective observations reveal pronounced changes in air mass composition and stratospheric hydration up to altitudes 2.3 km above the tropopause and concentrations more than double background levels. Unique dynamic and trace gas signatures were found within the AACP, including enhanced vertical mixing near the AACP edge and a positive correlation between H2O and O3. Moreover, the H2O enhancement within the AACP was found to be limited to the saturation mixing ratio of the low temperature overshoot and AACP air. Comparison with all remaining DCOTSS flights demonstrates that the 31 May 2022 flight had some of the largest tropospheric tracer and H2O perturbations in the stratosphere and within the AACP. The third component of this dissertation examines the contributions of identified STE processes within TCs to STE and how STE varies with important TC environmental factors using simulations. Prior work examining TCs and the stratosphere has been limited to coarse spatial resolution, specific ocean basins, or short time periods, resulting in limited understanding of fine-scale processes impacting UTLS composition change. Previous case studies have identified multiple STE processes within TCs, but it remains unclear how these individual processes contribute to the total STE in TCs and vary with common TC environmental factors. To improve upon these limitations, three idealized simulations are executed and analyzed to provide a more thorough understanding of the importance of STE in TCs, the role of various STE processes, and their sensitivity to different TC environmental factors, including sea surface temperature (SST) and deep-layer vertical wind shear (VWS). The simulations suggest substantial hydration of the lower stratosphere occurs within the TC inner core. H2O enhancements reach greater altitudes and magnitudes with no VWS, but a greater spatial extent beyond the inner core when VWS is included. Overshooting eyewall convection is the dominant stratospheric hydration process, and near-tropopause cooling is an important constraint for hydration. The shallower TC that develops with a lower SST facilitates less TST than the other simulations, but similar STT. Downward transport of stratospheric air occurs i) within the upper portion of the near-tropopause outflow and ii) via subsidence within the eye. The final component of this dissertation evaluates observations of TC impacts on temperature in the UTLS and middle stratosphere. There are limited observations within TCs and their environments over the open ocean to confirm model outcomes, such as TC impacts on temperature structure and concomitant H2O transport in the UTLS. Satellite-based radio occultation (RO) data offer a modern global observational pathway for fine-scale evaluation UTLS temperatures, of which a special collection of RO data during a Northern Hemisphere TC season, the Radio Occultation Modeling Experiment (ROMEX), is used to investigate UTLS temperature impacts of TCs. ROMEX is particularly transformative as RO data from all operational public and private satellites was made publicly available resulting in nearly 5X as much data and far more profiles per individual TC than that otherwise available in the recent RO record (since late 2019). Here, ROMEX profiles are extracted along TC tracks in the North Atlantic, East Pacific, and West Pacific basins to extensively evaluate observed TC-induced UTLS temperature change. Robust temperature anomalies are seen, including the expected tropospheric warm core and tropopause cooling, along with at least two higher-altitude stratospheric features revealed for the first time: cooling near 20 km and warming near 30 km. This analysis suggests that TCs perturb the middle atmosphere far deeper than previously recognized. The ubiquitous cold and warm layers in the low-to-middle stratosphere are found to be insensitive to most considered TC and environmental factors, however, impacts are larger in weaker TCs. These anomalies are similarly captured in ERA5 reanalysis data. The inferred deep stratospheric circulations may have impacts on TC evolution and forecasting that are not captured in current simulations. This dissertation seeks to provide insight towards the importance of environmental factors on convection-driven STE in the midlatitudes and tropics. In particular, this dissertation examines the dynamical and physical processes leading to STE in midlatitude AACP-producing storms and their sensitivity to the lower stratospheric environment, how individual processes contribute to STE in TCs and how they vary by TC intensity and shear, and how TCs impact temperature in the UTLS and middle stratosphere. Overall, this dissertation has shown that environmental factors, such as thermodynamic and dynamic characteristics, have considerable influence on STE driven by high-impact convective phenomena. One such characteristic is wind shear, as it impacts STE for both AACP-producing storms and TCs. This dissertation has also emphasized that UTLS temperature is an important constraint for transporting H2O in convective phenomena in the midlatitudes and tropics.