Transport to the Extratropical Stratosphere by Overshooting Storms in Idealized Simulations

Abstract Deep convection is a significant source of water to the extratropical stratosphere which can alter radiative properties and contribute to ozone loss. Previous studies find it responsible for 40% of mid‐latitude water vapor above 380K. However, the amount of hydration from individual storms and the mechanisms that initiate mixing is less understood. We use an idealized large eddy simulation with 100 m horizontal and vertical grid spacing to simulate a multicell storm with several overshooting tops (OTs) extending up to 2.5 km above the tropopause. Our goals are to determine the amount of water vapor and ice added to the stratosphere by this storm complex, identify how varying grid spacing affects the amount of hydration, and investigate the processes leading to irreversible mixing into the stratosphere. We find that, relative to the base state, an additional 33.7 kilotons of ice and 4.2 kilotons of water vapor are present in the stratosphere at the end of the simulation. For hydration similar to the 100‐m baseline simulation, 300‐m horizontal and 100‐m vertical grid spacing are necessary. Finally, the mechanism leading to hydration from individual OTs is shown to occur only after the initial overshoots begin to collapse. A region of positive buoyancy develops where previously overshooting cloud has descended, which is followed by a secondary upward movement of moistened air disconnected from the initial updraft. This process, not direct detrainment and sublimation of ice from the initial overshoot itself, triggers irreversible mixing in each case we investigate.