Urbanization impacts on severe weather dynamical processes and climatology

Rainfall changes are a complex manifestation of multi-scale processes that are influenced by both natural and anthropogenic activities. This dissertation focuses on understanding the relationship between rainfall climatology and urbanization. Even though there is a long-term increase in the rainfall amounts under a global warming background, the land use / land cover change can also be an additional factor contributing to regional climate change. This research study investigates the urbanization effect on rainfall at regional scales: (i) Multidecadal large scale: by investigating the eastern US urban rainfall climatology from 1958-2008; (ii) Decadal mesoscale summer-time thunderstorm climatology over central Indiana from 2000- 2009, and high resolution model studies for representative thunderstorms. For the multi-decadal rainfall climatology over eastern US, we examined the relationship between rainfall characteristics and urbanization by analyzing data from 4593 surface stations over the last 50 years (1958-2008), Parameter-elevation Regressions on Independent Slopes Model (PRISM) data in last two decades, North America Regional Reanalysis (NARR) winds, and a proxy for urbanization through gridded human population data. Results indicate that the summer monthly rainfall amount shows an increasing trend under the influence of urbanization changes. The frequency of heavy rainfall events shows a preferential positive bias towards urbanized regions. Most notably, consistent with case studies for individual cities, the frequency of rainfall amounts downwind of urban-rural boundaries shows a climatologically increasing trend. Analysis of heavy (top 2 percentile) and extreme (top 0.5 percentile) rainfall events indicates decreasing trends of heavy (top 10 percentile) rainfall event frequency and possible increasing trends of extreme rainfall event frequency over urban areas. Spatially the urbanization impact on rainfall was more pronounced in the North and the Central US with an increase in rainfall amounts, while the southern region showed mixed results. For the mesoscale thunderstorm climatology, we used both subjective and objective methods to analyze the summertime thunderstorm behavior over Indianapolis from 2000 to 2009. Results indicate that the intensity of thunderstorm is relatively lower over urban center and higher over upwind and downwind direction. Observations indicate that thunderstorm often split into smaller cells over the urban region and reintensify as larger, merged cells downwind. This splitting or morphology change in the thunderstorm characteristics was noted for nearly 30% of the daytime storms, with a bias for daytime cold fronts. Mesoscale model studies suggest that the urban-rural heterogeneity aids the formation of a mesoscale convergence zone which can alter the thunderstorm characteristics. Overall, this study highlights the important role of land use /land cover and urbanization for understanding the mesoscale rainfall changes as part of regional climate change