Fine-Scale Event-Based Modeling of Design Storms in the Urban Environment

“Design storms” can be used to develop parameters for maintaining infrastructure resilience, as they typically represent a worst-case scenario. For flooding events, these storms can be based on previous, high-impact cloudburst storms that burden society and overwhelm ecosystems. For urban environments, design storms are particularly important because of the high concentration of people, infrastructure, and commerce in a compact area. It is equally important to understand how design storms could evolve under different climate scenarios. As the mean global temperature increases, atmospheric water vapor content is expected to increase consistent with the Clausius-Clapeyron relationship. Increased available water vapor may convert to increased precipitable water and result in higher rain rates and rainfall totals.

This study uses a pseudo-global warming approach to simulate how design storms could change under a warmer climate. Two case studies are investigated: one occurring in Baltimore, Maryland on July 30-31, 2016, and the other occurring in Portland, Oregon on October 31, 2015. The Weather Research and Forecasting (WRF) model is used to simulate these storm events for both historical and projected scenarios. A vertical warming profile derived from an ensemble of CMIP5 RCP 8.5 scenarios is added to the background environment. Comparisons of the historical case studies to the pseudo-global warming simulations are used to quantify changes in precipitation rates, amounts, and the spatial distributions of rainfall in warmer environments. These two case studies are intended to inform design specification of urban infrastructure that considers changes to extreme precipitation.