Convectively Induced Ozone Loss

Abstract

Tropopause-penetrating convection in the midlatitudes can induce ozone loss in the lower stratosphere through its unique ability to transport boundary layer air and moisture directly into the lower stratosphere, bypassing the normal slow ascent process of the Brewer-Dobson circulation. This rapid injection of boundary layer air can significantly perturb the chemical composition of the lower stratosphere leading to catalytic ozone loss. This tropopause-penetrating convection is a consistent and frequent summer seasonal feature of the North American Monsoon upper-level anticyclone region. The climate of the central U.S. experiences the deepest convection through the combined influence of the Great Plains Low-Level Jet, which transports water vapor from the Gulf of Mexico northward, and frequent mid-level capping inversions that allow for tropopause-penetrating convection to develop. This convection presents the potential for consistent transport of water into the normally dry upper troposphere and lower stratosphere (UTLS). Water vapor enhancements associated with convection have been observed in both in situ and satellite measurements. These water vapor enhancements can increase the probability of sulfate aerosol catalyzed heterogeneous reactions that convert reservoir chlorine (HCl and $\text{ClONO}_2$) to free radical chlorine (Cl and ClO) that leads to catalytic ozone loss. Finally, entrapment of convectively influenced lower stratospheric air within the circulation of the North American Monsoon upper-level anticyclone determines the lifetime and geographic distribution of any potential chemical perturbations. Convectively induced ozone loss is therefore the result of the coupling of climate and chemistry. In this thesis I model the key dynamical and chemical factors of 1) tropopause-penetrating convection over the central U.S., 2) the chemical response of the lower stratosphere to convective perturbation, and 3) the influence of the North American Monsoon anticyclone on the seasonal, geographic, and altitude dependence of tropopause-penetrating convection to quantify and constrain their contribution to the overall mechanism of convectively induced ozone loss. I show that deep convection over the central U.S. is capable of transporting water vapor to the altitudes indicated by in situ and satellite observation, that the photochemistry of the midlatitude lower stratosphere is highly sensitive to convective perturbation and may become more so in the future, and that the circulation of the North American Monsoon anticyclone results in significant entrapment such that non-local tropopause-penetrating convection is likely to have an important effect on the lower stratospheric ozone of the entire region.