Stratosphere-troposphere exchange in a midlatitude mesoscale convective complex 2. Numerical simulations

Mixing across the tropopause due to intense convective events may significantly influence the atmospheric chemical balance. Stratosphere-troposphere exchange acts as an important natural source of O₃ in the troposphere, and a source of H₂O, HCs, CFCs, HCFCs, and reactive nitrogen in the stratosphere. The redistribution of atmospheric trace gases produces secondary radiative, dynamical and climate effects, influencing lower stratospheric temperatures and the tropopause height. During the 1989 North Dakota Thunderstorm Project, a severe storm which evolved into a mesoscale convective complex (MCC) on June 28-29 showed the unusual feature of an anvil formed well within the stratosphere and produced strong vertical mixing of atmospheric trace gases including H₂O, CO, O₃ and NO_y as discussed by Poulida et al. [this issue] in Part 1 of this paper. In this paper the two-dimensional NASA Goddard Cumulus Ensemble (GCE) model was employed to simulate this convective storm using observed initial and boundary conditions. The sensitivity to the domain size, initial and boundary conditions, stability, and time resolution are evaluated. Synoptic-scale moisture convergence, simulated by moist boundary inflow, influences significantly the storm intensity, spatial structure, and trace gas transport, and produces a storm that reintensifies after the initial decay, mimicking the observed behavior of the MCC. The deformation of the tropopause documented with aircraft observations was qualitatively reproduced along with transport of stratospheric ozone downward into the troposphere, and the transport of trace species from the boundary layer upward into the stratosphere. If the chemistry and dynamics of this storm are typical of the roughly 100 MCCs occurring annually over midlatiludes, then this mechanism plays an important role in CO, NO_y, and O₃ budgets and could be the dominant source of H₂O in the lower stratosphere and upper troposphere over midlatitudes.