The impact of deep overshooting convection on the water vapour and trace gas distribution in the TTL and lower stratosphere

Abstract:

Overshooting convection penetrating the tropical tropopause layer (TTL) and the lower stratosphere has a significant impact on the redistribution of water vapour and further trace gases. This is of importance for the stratospheric water vapour budget, which plays a central role in radiative and chemical processes. Modelling studies and in situ measurements show the hydration potential of convective overshooting partly by direct injection of ice particles into the stratosphere and subsequent sublimation. However, processes leading to dehydration of the TTL may also impact the stratospheric humidity by limiting the amount of water vapour carried aloft. While the large scale drives some of the dehydrating processes, others are of convective origin, for example gravity waves and cooling associated with overshooting turrets. Furthermore, downdrafts may transport dry and ozone rich air masses from the stratosphere into the TTL. Improving our understanding of overshooting convection and its influence on TTL water vapour will ultimately place better constraints on the budget of water vapour in the stratosphere.

In this study we use three-dimensional cloud resolving (WRF-ARW) simulations of a deep convective thunderstorm (Hector) to study the redistribution of water vapour and trace gases in the upper TTL/lower stratosphere. Passive tracers are initialised to investigate the transport of air masses. The simulations focus on an Hector event that has been probed by aircraft during the SCOUT-O3 field campaign. Observations were performed in and around overshoots that even penetrated the stratosphere. These observations as well as the model simulations show downward transport and mixing of air masses from the stratosphere, though less strong and more localised in the simulation. Furthermore, the simulations shows a layering of hydrated and dehydrated air masses post-convection in the upper TTL and lower stratosphere. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.