Sensitivities of Cross-Tropopause Transport in Midlatitude Overshooting Convection to the Lower Stratosphere Environment

Tropopause-overshooting convection in the midlatitudes rapidly transports lower troposphere air and cloud material to the upper troposphere and lower stratosphere (UTLS), with notable events resulting in the formation of above-anvil cirrus plumes (AACPs). However, there is limited understanding of how transport driven by overshooting convection and AACP properties are modified by variations in UTLS environments, especially the extent of stratosphere-to-troposphere (i.e., downward) transport. Here, AACP development and UTLS transport sensitivities to lower stratosphere stability and the UTLS wind environment are evaluated. The Bryan Cloud Model is used to simulate midlatitude supercell convection and evaluate resulting UTLS composition changes within two common midlatitude thermodynamic environments: a single tropopause and a double tropopause, and two UTLS wind environments: one with a stronger stratospheric wind profile supportive of frequent gravity wave breaking and AACP development and the other with a weaker stratospheric wind profile to suppress gravity wave breaking and discourage AACP formation. Multiple passive tracers and water vapor concentrations are used to evaluate the exchange of air between the troposphere and stratosphere and transport confined to each layer. It is found that greater troposphere-to-stratosphere transport (TST) and stratospheric hydration occurs in storms with AACPs, while stratosphere-to-troposphere transport (STT) is greater in storms without AACPs. This STT is facilitated by a mechanical oscillation induced by the overshoot. AACP-producing storms have increased downward transport of stratospheric overworld air to the lowermost stratosphere, which is enhanced within a double tropopause environment. More expansive AACPs, deeper overshoots, and greater TST also occurs in double tropopause environments.

Automated summary

This study evaluates how transport driven by tropopause-overshooting convection and properties of above-anvil cirrus plumes (AACPs) are modified by variations in the upper troposphere and lower stratosphere (UTLS) environments. The research finds that storms with AACPs have greater troposphere-to-stratosphere transport (TST) and stratospheric hydration, while storms without AACPs have greater stratosphere-to-troposphere transport (STT). These findings suggest that the development of AACPs and UTLS transport are influenced by the stability and wind environment in the UTLS.