Incorporation of the Effects of Future Anthropogenically-Forced Climate Change in Intensity-Duration-Frequency Design Values

Dr. Kenneth Kunkel | North Carolina State University / NC Institute for Climate Studies (NCICS)

RC-2517

Objective

The overriding objective of this project is to develop a framework for incorporating the potential impact of future climate change into the Intensity-Duration-Frequency (IDF) values of heavy precipitation. The project will leverage previous work on heavy precipitation by the research team, as well as the expertise of the developers of IDF values used in design regulations and guidance by the Department of Defense (DoD) as well as by local, state, and other Federal agencies. The underlying basis for making changes to IDF values is the robust, physically-based projection of global warming with increasing greenhouse gas concentrations. Global warming in turn will increase atmospheric water vapor concentrations with an equally high level of confidence, producing the potential for more intense precipitation. Actual changes in IDF values will result from changes in atmospheric capacity (water vapor concentrations) and opportunity (the number and intensity of heavy precipitation-producing storm systems). This project will evaluate these two components to determine the potential impact for a wide range of frequencies and durations used by civil engineers, and provide a means for adjusting and delivering the IDF values and uncertainty estimates, similar to the National Oceanic and Atmospheric Administration (NOAA) Atlas 14.

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Technical Approach

The approach involves developing IDF adjustment factors for changes in the climate system. The adjustment factors will have associated uncertainties, primarily from (1) uncertainties in the future pathway of greenhouse gas emissions and (2) variations among climate models in the sensitivity of the climate system to greenhouse gas concentration changes. In addition to meteorological considerations, the lifetime of projects designed using IDF values is an essential consideration because the IDF values may change substantially during that time and will be different for projects with different lifetimes. This is an additional variable that needs to be incorporated into IDF adjustment mechanisms.

The research will be organized around the following scientific hypotheses: 

Hypothesis 1: Historically-observed and anthropogenically-forced future changes in IDF values used in the engineering community arise primarily from two principal meteorological sources: (1) changes in atmospheric water vapor concentration (capacity) and (2) changes in the frequency and intensity of the weather systems that cause heavy precipitation (opportunity).

Hypothesis 2: As event duration increases, the impact of synoptic climatology (storm track and strength) will become increasingly important on future changes in IDF values.

Hypothesis 3: Regional variations in IDF value changes will arise primarily from regional gradients in weather system changes.

The primary source of information for determining future changes will be the climate simulations available from the Coupled-Model Intercomparison Project Phase 5 (CMIP5). These simulations will be analyzed for future changes in water vapor concentration and weather system occurrence, making validation comparisons with historical observations. The analysis of weather systems will focus on extratropical cyclones and tropical cyclones, which together account for about 90% of the total number of extreme events in the contiguous United States. The North American Monsoon system will also be analyzed because of its importance in the Southwest United States.

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Benefits

Use of the existing IDF curves is likely to lead to under-design of runoff control structures, and associated increased flood damages, in most regions because of the high likelihood of increases in the intensity of heavy rainfall. The adjusted IDF curves to be produced in this project will incorporate the best scientific knowledge available to properly evaluate this future risk and make appropriate decisions about design of structures. The long-term benefit will be increased resilience to flooding and avoidance of this threat to military readiness. (Anticipated Project Completion - 2020)

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Points of Contact

Principal Investigator

Dr. Kenneth Kunkel

North Carolina State University / NC Institute for Climate Studies (NCICS)

Phone: 828-257-3137

Fax: 828-257-3002

Program Manager

Resource Conservation and Resiliency

SERDP and ESTCP

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