Examining Stratosphere‐Troposphere Exchange in an Idealized Simulation of a Tropical Cyclone

Abstract Tropical cyclones (TCs) have been recognized for their damaging hazards and impacts on the climate system. One under‐investigated climate impact from TCs is stratosphere‐troposphere exchange (STE) and accompanying upper troposphere and lower stratosphere (UTLS) composition change, which alters the radiation budget. Previous case studies have identified multiple STE processes within TCs, but it remains unclear how much STE occurs in individual storms and how these individual processes contribute to the total STE in TCs. This study utilizes an idealized simulation to provide a more thorough understanding of the importance of STE in TCs and the role of various STE processes. The 3‐D TC version of the Bryan Cloud Model (CM1) is used to simulate a TC and evaluate UTLS composition change and accompanying transport pathways. STE and composition change are assessed using water vapor concentrations and a suite of custom passive tracers. These passive tracers include: boundary layer and stratospheric tracers, and a convective tracer. The simulation suggests substantial hydration of the lower stratosphere occurs within the TC inner core. Specifically, water vapor reaches nearly 20 ppmv at an altitude of approximately 18.5 km (2 km above the tropopause) with a spatial extent limited to 250 km from TC center. Downward transport of stratospheric air occurs (a) within the upper portion of the near‐tropopause outflow and (b) via subsidence within the eye, where approximately 10% stratospheric air reaches altitudes as low as 7 km. Additionally, substantial two‐way transport is found within overshooting tops in the TC and its inner core.