Simulated Tropical Cyclone Impacts on Upper Troposphere Lower Stratosphere Composition

Tropical cyclones (TCs) have received notable attention for their damaging hazards and impacts on the climate system. One under-investigated climate impact is stratosphere-troposphere exchange (STE) from TCs and accompanying upper troposphere and lower stratosphere (UTLS) composition change. STE irreversibly modifies the distribution and concentration of greenhouse gases such as water vapor and ozone in the UTLS, which is important for the radiation budget and climate. Prior case studies have individually identified multiple STE processes that can occur in TCs, however it remains unclear how these individual processes contribute to total STE in TCs, and how contributions vary by TC intensity and deep-layer shear. This study uses multiple idealized simulations conducted with Cloud Model 1 (CM1) to provide a more thorough understanding of STE in TCs, including the role of various STE process and how they vary based on TC intensity and deep-layer shear. UTLS composition change and STE is assessed using water vapor concentrations and a suite of custom passive tracers. Simulations suggest substantial hydration of the lower stratosphere occurs within the TC inner core, reaching up to 20 ppmv (4x stratospheric background) at 18.5 km (2 km above the tropopause). This hydration is spatially limited to the inner core of the TC due to surrounding cold temperatures near the tropopause. Overshooting tops within the TC and its inner core are shown to lead to substantial two-way transport. Downward transport of stratospheric air occurs (a) via subsidence within the eye and (b) in the upper portion of the near-tropopause outflow. Additional simulations reveal that TC intensity and deep-layer environmental wind shear are also important for TC STE.