Stratospheric hydration and ice microphysics of a convective overshoot observed during the TPEx campaign over Sweden

Abstract

This study examines the cloud microphysical properties and trace gas signatures associated with a convective overshooting event observed during the TPEx (TropoPause composition gradients and mixing Experiment) campaign in June 2024 over Sweden. While recent studies have predominantly focused on convective overshootings in sub(tropical) air masses, this particular event took place during a cold air outbreak characterized by low tropopause altitudes of 9 km. The measurements by the microphysical cloud spectrometer reveal that ice particles were transported into the lower stratosphere, with detections extending up to 1.5 km above the tropopause. At this altitude, a pronounced stratospheric ozone concentration of approximately 600 ppbv and a notable tropospheric water vapor concentration of up to 60 ppmv (+30 ppmv) were recorded, the latter being twice as high as background levels at the same height. This substantial injection of tropospheric air and ice particles was linked to gravity wave breaking, and subsequently irreversible mixing near the overshooting top. Forward trajectories indicate that the impact of the overshoot on the lower stratosphere, especially regarding the observed hydration, is relatively short-lived with a life time of several days (at 330 K) to weeks (at 345 K).