Idealized Simulations of Tropical Cyclones: Examining Stratosphere-Troposphere Exchange Processes and Composition Change

Tropical cyclones (TCs) have been recognized for their damaging hazards and impacts on the climate system. One possible, yet under-investigated climate impact is stratosphere-troposphere exchange (STE) and accompanying upper troposphere and lower stratosphere (UTLS) composition change. Previous case studies have individually identified multiple STE processes that can occur within TCs, however it remains unclear how much STE occurs in individual storms and how each of these individual processes contribute to the total STE in TCs. This study aims to provide a more thorough understanding of the importance of STE in TCs and the role of various STE processes through the utilization of idealized simulations. The Bryan Cloud Model (CM1) is used to simulate 3-D TCs and evaluate UTLS composition change and accompanying transport pathways. Two simulations are analyzed here, both of which include 2-km horizontal and 200-m vertical grid spacing, with one simulation including no background flow and the other including a weak easterly background flow at all altitudes. STE and composition change is assessed using water vapor concentrations and a suite of custom passive tracers. These passive tracers include: fractional boundary layer, tropospheric, and stratospheric tracers, tracers designed to represent ozone and carbon monoxide composition profiles within the tropics, and a convective tracer that is initialized in simulated convection at each model integration time step. The simulations suggest that substantial hydration of the lower stratosphere occurs within the inner core of the TC. Specifically, water vapor reaches nearly 20 ppmv at altitudes of approximately 18.5 km (>2 km above the tropopause). Downward transport of stratospheric air occurs i) within the upper portion of the near-tropopause outflow and ii) via subsidence within the eye, where ≥ 5% stratospheric air reaches an altitude as low as 8 km. Additionally, substantial two-way transport is found within overshooting tops within the TC and its inner core.