Observational Constraints on the Quantity of Tropospheric Air Irreversibly Transported to the Lower Stratosphere via Deep-Tropopause-Penetrating Convection Over the United States in Summer

There is increasing observational evidence of the convective penetration of the lower stratosphere at mid-latitudes, particularly in the summertime North American and Asian Monsoon regions. It is theorized that gravity wave breaking and turbulent mixing associated with this tropopause-penetrating convection leads to the irreversible deposition of both air and water as ice in the stratosphere that then rapidly sublimates in the undersaturated environment. As such this mechanism provides a means of delivering water vapor to the stratosphere that bypasses the strict thermal control of tropical cold-point tropopause temperatures. Tropopause-penetrating convection may also efficiently deliver boundary-layer air directly to the overworld stratosphere, and vertically mix air within the lower stratosphere with further implications for the chemical composition of this region. In the present analysis we utilize a mix of simultaneously obtained in situ measurements of long-lived trace species to constrain the quantity of tropospheric air that is transported via convection. The trace species considered include CO, CH₄, CO₂ and O₃, and were acquired in encounters with convectively sourced plumes during a suite of airborne missions targeting the composition of the upper troposphere and lower stratosphere over the US in summer. A simple mixing model is used to constrain the convective contribution to each of the trace species. Preliminary results from an outflow plume sampled during during NASA’s Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission do not show evidence for a large influx of tropospheric air, even where water vapor is significantly elevated. Instead, the data show clear evidence for the “down-mixing” of older stratospheric air into the plume regions, potentially consistent with gravity wave breaking, which is associated with the vertical overturning of isentropes.