Tropospheric-stratospheric exchange of chemical species induced by midlatitude convective storms

Intense convective storms play an important role in transport of chemical species from the surface to the upper troposphere, and are also likely important for exchange across the tropopause. The scarcity of high resolution chemical concentration data in the upper troposphere and lower stratosphere makes it difficult to address this issue solely through observations. Numerical modeling can, therefore, play a crucial role in examining this transport. In this study, midlatitude convective storms are simulated using a three-dimensional nonhydrostatic cloud model. Using idealized, but realistic, initial conditions for both the storm and tracer concentrations, the tracer transport due to convection is analyzed at various levels. These numerical simulations are compared to observations, when available.

It is generally accepted that midlatitude storms overshoot into the stratosphere, but how much air in the overshooting towers actually mixes into the stratosphere is unclear. Due to vertical shear of the horizontal velocity, the simulated trace constituent perturbations above the main anvil travel along a trajectory at an angle to the main storm trajectory. Because three-dimensional models are computationally expensive, previous investigators have confined their domains to two dimensions, thereby artificially constraining these perturbations to the plane of the storm, or they have limited the three-dimensional domain so that such high level perturbations travel out the lateral boundaries. This off-anvil-axis transport may affect the radiative cooling of the parcels on the longer time scales associated with mixing. This paper will summarize our understanding of the role of convection in transporting trace species into the upper troposphere and across the tropopause at midlatitudes.