A Comparison of Downscaling Methods to Create Future Precipitation Intensity-Duration-Frequency Curves for Resilient Transportation within North Carolina

North Carolina (NC) Executive Order No. 80 recognizes climate change and that NC must adapt to changing natural hazards to protect the people, natural environment, and the state’s economy. A notable problem in recent years is the impact of extreme rainfall on state owned infrastructure. NC Department of Transportation (DOT) manages more than 80,000 miles of highway including 15,000 culverts and bridges over water and has a unique challenge to evaluate the impact of climate change to protect its many assets, human health and ecosystems, as well as enhance North Carolina’s economy. A particular challenge is incorporating climate change projections into infrastructure design, especially plausible changes in extreme precipitation as the climate warms.

To help NC DOT address climate change and extreme rainfall for design, future precipitation Intensity-Duration-Frequency (IDF) curves were calculated for different downscaling methods. An apples-to-apples comparison was applied by considering only overlapping downscaled GCMs for two popular statistical downscaling methods and multiple regional climate models, a.k.a. dynamical downscaling. The methodology to produce future IDF curves applied a regionalization approach that considers extreme rainfall aggregated across NC climate divisions to filter the climate change signal from the noise. Regional scale factors (RSFs) were calculated by comparing end-of-century (2071-2100) IDF estimates relative to a historical period (1976-2005) for each downscaled GCM under a high greenhouse gas emission scenario, RCP8.5. Here we will highlight some of the lessons learned about rainfall extremes from the different downscaling methods when updating IDF curves, including differences between RSFs and scientifically defensible recommendations provided to NC DOT when applying future IDF curves for design.