Stratosphere-troposphere exchange, or STE, is far more than a technical curiosity. It governs the global atmospheric budget of chemical constituents, including the water vapor and aerosols that shape the planet’s radiative balance. When air rises from the surface into the stratosphere through the so-called troposphere-to-stratosphere transport pathway, it can carry anthropogenic pollutants along with it, altering stratospheric chemistry and feeding back on climate. In the reverse direction, ozone-rich stratospheric air can sink into the troposphere, where it threatens human health and ecosystems. Because these exchanges span scales from planetary circulations down to small-scale turbulence, scientists have struggled to fully qualify, let alone quantify, the contributions of individual mechanisms. Tropical cyclones, with their towering convective thunderstorms, have recently emerged as one important piece of the puzzle.

Earlier work had established that roughly 10 to 15 percent of the global lower stratospheric water vapor budget can be traced to overshooting convection in the tropics, and that around 15 percent of that overshooting convection occurs inside tropical cyclones. Satellite observations have revealed pronounced moistening of the lower stratosphere above storm centers and even a downward flux of ozone through the eye. Yet the exact dynamics of how air crosses the tropical tropopause, a transition that in the tropics takes place across an extended layer called the tropical tropopause layer, or TTL, remain incompletely understood. A team led by Massimo Martina and Petr Šácha of Charles University in Prague suspected a missing ingredient: the mountains beneath the storm.

The researchers’ hypothesis rested on a well-known but underexplored coupling. Typhoons, the most intense manifestation of tropical cyclones, pack maximum winds of at least 33 meters per second and drive deep convection that spans the entire troposphere. When such winds encounter mountains rising more than 2,000 meters, as they do across the Philippine archipelago, they generate orographic gravity waves, atmospheric ripples that propagate upward from the terrain. These waves are known to initiate precipitation and modulate convection, and in the extratropics their role in stratosphere-troposphere exchange is well documented. In the tropics, however, the possibility that mountain waves could boost transport during a typhoon landfall had received virtually no attention.

To test the idea, the team ran a high-resolution simulation of Molave’s landfall using the Weather Research and Forecasting model, configured with 180 vertical levels and a horizontal grid spacing of 3 kilometers, fine enough to resolve convection explicitly rather than approximating it with a parameterization. The simulation, driven by ERA5 reanalysis data and initialized with high-resolution terrain information, accurately reproduced the typhoon’s observed track across the Philippines, along with its minimum central pressure of 965 hectopascals and mean winds of roughly 38 meters per second, according to Japanese Meteorological Agency records. Some expected intensity biases appeared, but the simulation captured the essential spatiotemporal behavior of the storm, making it a trustworthy laboratory for studying terrain-storm interactions.

Into this simulated storm the researchers released four million virtual air parcels from an idealized surface source near the typhoon’s path, using the Lagrangian particle dispersion model FLEXPART-WRF. Each parcel’s trajectory was tracked, and a newly developed algorithm called AirIntrusions flagged the precise moment and location where any parcel crossed the lapse rate tropopause, the cold point tropopause, or the planetary boundary layer. The algorithm even recorded transition times and residence times, sorting intrusions into classes ranging from brief excursions of under three hours to prolonged stays exceeding nine hours. The results were striking. As Molave made landfall on 25 October 2020, intrusions spiked abruptly, and in total about 36 percent of all parcels reached the TTL, while roughly 13 percent penetrated into the stratosphere itself. The fastest parcels made the journey from the boundary layer to the stratosphere in just three to six hours.

Mapping where these crossings clustered revealed a telling pattern. The distribution of intrusions did not spread uniformly along the typhoon’s track; instead, it broke into several distinct hotspots, many of them aligned with subtle features of the underlying topography. In regions directly beneath the storm’s strongest updrafts, parcels made deep, one-way penetrations across both tropopause boundaries, driven by the ferocious overshooting convection at the cyclone’s core. But in hotspots over mountain ridges away from the center, a different and previously unrecognized mechanism dominated: parcels executed multiple shallow, back-and-forth crossings, oscillating around the tropopause boundaries. Approximately 73 percent of all parcels entering the TTL or stratosphere engaged in such repeated crossings, some more than ten times, suggesting a persistent two-way mixing process rather than a single convective punch.

Analysis of the Richardson number and turbulent kinetic energy along the parcel trajectories pointed the finger squarely at orographic gravity waves. In cross-sections over the mountains, the team identified vertically propagating waves with vertical wind amplitudes of about 3 meters per second that modulated both tropopause definitions and extended into the stratosphere. Where these waves broke, they generated pockets of dynamical instability and enhanced turbulence, precisely the conditions needed to mix parcels already hovering near the tropopause across the boundary. Over the typhoon center, by contrast, the cross-sections showed convective instability at the crowns of overshooting updrafts, where first-time deep intrusions occurred. The two mechanisms acted in concert: the typhoon’s core lifted boundary layer air rapidly toward the tropopause, and the mountains, through wave breaking and orographic lifting, provided additional gateways for entry and continued exchange.

The implications extend well beyond a single storm. Parcels that reached the TTL tended to settle into that layer for the remainder of the simulation, hinting at a possible role for orography-typhoon interactions in feeding phenomena such as the Asian Tropopause Aerosol Layer, a persistent veil of aerosols near the tropical tropopause with climate relevance. Parcels that crossed into the stratosphere proper can subsequently be swept around the globe by the Brewer-Dobson circulation, spreading tropical surface emissions far from their source in a region of the atmosphere where the Philippines sits squarely among documented pollution hotspots. Even modest errors in representing these pathways could translate into significant biases in modeled stratospheric composition and radiative forcing.

The study also carries a pointed message for climate model developers. Global chemistry-climate and earth system models cannot resolve individual mountains or gravity waves, relying instead on parameterizations of subgrid-scale orography. The new results demonstrate that even fine variations in terrain leave discernible fingerprints on the geography of tropopause crossings, meaning that inadequate representation of these effects could bias modeled transport in the upper troposphere and lower stratosphere, a region critical to the global radiation budget. The authors note that their conclusions rest on passive tracers without deposition processes and on a turbulence scheme not originally designed for the strongly stable stratification near the tropopause, and they call for follow-up experiments at even finer resolution. Still, the core message stands: typhoons do not just devastate the ground beneath them. When they collide with mountains, they can punch surface air, and everything it carries, into the stratosphere, at speeds and by routes the atmosphere’s accountants had never tallied.

Subject of Research: How orography and orographic gravity waves enhance troposphere-to-stratosphere transport during a tropical typhoon landfall

Article Title: The impact of orography on the troposphere-to-stratosphere transport during a typhoon event in the tropics

Article References: Martina, M., Villalba-Pradas, A., Bartoň, Š., & Šácha, P. (2026). The impact of orography on the troposphere-to-stratosphere transport during a typhoon event in the tropics. Atmospheric Chemistry and Physics, 26(19), 13845-13860. https://doi.org/10.5194/acp-26-13845-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13845-2026

Keywords: typhoon, stratosphere-troposphere exchange, orographic gravity waves, tropical tropopause layer, Philippines, Typhoon Molave, WRF model, FLEXPART-WRF, convection, pollution transport, turbulence, climate modeling