Stratosphere-Troposphere Exchange during a Typhoon event: A Lagrangian approach.

The characterization of the stratosphere-troposphere exchange (STE) is fundamental to its role in the global atmospheric budget of chemical constituents. The troposphere-to-stratosphere transport can inject anthropogenic pollutants from the Earth’s surface into the stratosphere, changing its chemical composition and influencing the radiative processes. For instance, the Asian Tropopause Aerosol Layer, with a main known pathway via the Asian summer monsoon, has been shown to have a radiative cooling effect on the surface. Here we propose a previously underreported potential pathway contributing to STE via tropical cyclones, typhoons in particular. We focus on one episode of a typhoon crossing over the Philippines, which is located in the highly polluted Eastern Asia-Western Pacific region.  A case study for typhoon Molave (2020), combining a Lagrangian modeling tool with the Weather Research and Forecasting model (FLEXPART-WRF), demonstrates that the typhoon can result in rapid transport of pollutants from the surface to the upper troposphere-lower stratosphere (UTLS) region, inducing strong STE. Using a Lagrangian model, it has been possible to characterize the intensity of the air intrusion from the boundary layer to the free troposphere and stratosphere by computing their residence times. Furthermore, we try to disentangle the role of convection, orographic lifting, and gravity waves inducing this type of rapid transport. Overall, our study indicates that typhoon episodes can play an important, intermittent and previously insufficiently considered role in STE, influencing emerging topics of the highest importance such as the long-range dispersion of microplastics.