Aerosol Effects on Mesoscale Convective Systems: Can Urban Regions Affect Severe Weather?

Aerosols affect the climate system via directly reflecting or absorbing incoming or outgoing short or long-wave radiation, or by affecting cloud albedo and precipitation processes. In this dissertation, I focus on the role of aerosols as cloud condensation nuclei and their impact on precipitation patterns and cloud lifetimes (i.e., the second indirect effect). I hypothesize that anthropogenic and natural aerosols originating from the Great Plains region have the ability to affect severe weather through the magnitude, composition and source of aerosol emissions. These influences are addressed using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem 3.6), including the development of a new ice nucleation parameterization for dust aerosol. The first chapter examines the impact of the magnitude of urban aerosol emissions on a squall line in the Central Great Plains. Changes in urban emissions and the resulting aerosol loading drive changes in cloud microphysics, which alter the Mesoscale convective system (MCS) propagation and strength via cold pool strength, and trigger large-scale changes in storm morphology and precipitation patterns. These results show that urban emissions can play an important role in mesoscale weather systems. The second chapter investigates the role of aerosol composition on the precipitation patterns and intensity resulting from the same squall line. By changing the prescribed default hygroscopicity values to updated values from laboratory studies, model assumptions about individual component hygroscopicity are tested for anthropogenic aerosols such as ammonium, sulfate, nitrate, and organic species. The sensitivity simulations yield changes in the distribution of high-intensity precipitation events, indicating that aerosol composition plays an important role in predicting high intensity events. Finally, the third chapter investigates the role of dust on ice nucleation and deep convection. One of the major limitations of WRF-Chem is that it does not include the effects of heterogeneous ice nucleation in cold clouds. We implement the Phillips et al (2008) and DeMott et.al (2010) parameterization in the Morrison microphysics scheme of WRF-Chem with interactive dust aerosols to improve these model processes. Including dust as ice-nucleating particles (INPs) affects important microphysical characteristics, where ice crystals and snowflakes tend to increase effective radii and decrease number concentrations. These changes in cloud properties signify that including dust as IN in WRF-Chem can aid in decreasing uncertainty surrounding the aerosol-cloud interactions effects on climate. Overall, this work highlights the role of aerosol composition and magnitude on cloud microphysical processes. These processes are substantial enough, even during severe weather events, to influence the placement and intensity of simulated precipitation, and an improved representation of these aerosol-cloud interactions will likely improve the predictability of precipitation in this region.PHDAtmospheric, Oceanic & Space ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138619/1/slekaw_1.pd