Mechanisms of Hydrating the Lower Stratosphere by Overshooting Convection over the Southern Slope of the Himalayas

Overshooting convection efficiently transports water vapor to the upper troposphere and lower stratosphere (UTLS) within minutes to hours. While the tropics are a key region for such transport, the Asian summer monsoon, particularly over the southern slopes of the Himalayas, has emerged as another critical pathway due to its intense convective activities. However, the fine-scale mechanisms governing stratospheric hydration in this region are still not well understood. This study performs a high-resolution Weather Research and Forecasting (WRF) model simulation of an overshooting convective system in this region and provides an in-depth investigation into the mechanisms through which overshooting convection hydrates the lower stratosphere. The findings reveal that, in addition to direct water vapor injection and gravity wave breaking, above-anvil cirrus plumes (AACPs) develop atop the deep convective system as a result of intense convective updrafts penetrating the tropopause, and strong wind shear promotes the formation and horizontal spreading of the AACPs. These AACPs significantly enhance ice content in the lower stratosphere, resulting in substantial moistening of this region. The combined effects of AACPs and gravity waves play a critical role in amplifying this moistening effect. This highlights the AACPs as an indicator of enhanced stratospheric hydration. These transport mechanisms are essential for improving our understanding of stratospheric water vapor distribution and variability, with profound implications for advancing global climate models and predicting future climate change.