Upper Tropospheric Ozone Enhancement during the North American Monsoon evaluated using the Weather Research and Forecasting model with Chemistry (WRF-Chem)

by J. Wong

A thesis to be submitted to the
Faculty of the Graduate School of the
University of the Colorado in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Department of Atmospheric and Oceanic Sciences
2013

Abstract

Upper tropospheric ozone has significant impacts on the total atmospheric chemistry and radiative budget. Previous studies noted an upper tropospheric ozone enhancement above southern United States during the North American Monsoon (NAM). This recurring phenomenon has been observed by the satellite-borne Tropospheric Emission Spectrometer (TES) and IONS-06 ozonesondes. Using the Weather Research and Forecasting model with Chemistry (WRF-Chem), we attempt to simulate the ozone enhancement and understand the underlying structure, chemical pathways, and sensitivity to emissions.

Using a modified lightning parameterization based on the Price and Rind scheme, a July-August simulation is performed using WRF-Chem. Validation shows that flash rate is over-predicted by a factor of 10, which subsequently cases an overestimation of O₃. Despite the amplified ozone enhancement, boundary layer and stratospheric tracers do not show substantial differences within and outside the anticyclone, contrary to what has been predicted in other studies. On the other hand, lightning tracers, lateral boundary tracers, tracer-tracer correlations, and chemical pathways can be used to distinguish between the anticyclone and the surrounding area.

Sensitivity and control simulations are also performed to investigate how different emission sources contribute to the ozone enhancement. It is shown that lightning emission enhances NO_x super linearly, which leads to NO_x-titration in the upper troposphere. On the other hand, anthropogenic emission is shown to modify the ozone concentration and chemistry profiles via sensitivity to VOC, and biogenic emission affects ozone via sensitivity to NO_x. As a result of the changes in ozone mixing ratio profiles, convective tendency is also affected, which forms a nonlinear feedback between chemistry and convective transport in response to perturbation in emissions.