Mechanisms Responsible For Stratosphere-To-Troposphere Transport Around A Mesoscale Convective System Anvil

Recent observational studies have shown that stratospheric air rich in ozone (O-3) is capable of being transported into the upper troposphere in association with tropopause-penetrating convection (anvil wrapping). This finding challenges the current understanding of upper tropospheric sources of O-3, which is traditionally thought to come from thunderstorm outflows where lightning-generated nitrogen oxides facilitate O-3 formation. Since tropospheric O-3 is an important greenhouse gas and the frequency and strength of tropopause-penetrating storms may change in a changing climate, it is important to understand the mechanisms driving this transport process so that it can be better represented in chemistry-climate models. Simulations of a mesoscale convective system (MCS) around which this transport process was observed are performed using the Weather Research and Forecasting model coupled with Chemistry. The Weather Research and Forecasting model coupled with Chemistry model adequately simulates anvil wrapping of ozone-rich air. Possible mechanisms that influence the transport, including small-scale static and dynamic instabilities and MCS-induced mesoscale circulations, are evaluated. Model results suggest that anvil wrapping is a two-step transport process (1) compensating subsidence surrounding the MCS, which is driven by mass conservation as the MCS transports tropospheric air into the upper troposphere and lower stratosphere, followed by (2) differential advection beneath the core of the MCS upper-tropospheric outflow jet which wraps high O-3 air around and under the MCS cloud anvil. Static and dynamic instabilities are not a leading contributor to this transport process. Continued fine-scale modeling of these events is needed to fully represent the stratosphere-to-troposphere transport process.