Assessing the potential for changes in extreme precipitation events across the Colorado Front Range

Extreme rainfall events present a number of challenges and questions related to public safety and risk management, and are thus of great interest to both the general public and water resources managers. Uncertainties related to flash flood risk, river dam design, and reservoir operations are strongly related to assessments of the potential for heavy rainfall events over a given region. As many of the environmental factors that drive precipitation generation are predicted to change in future climates, understanding and communicating possible changes in extreme precipitation events becomes an important issue in preparing for new or changing hazards.

This study examines warm-season extreme precipitation events in the western US using climate model projections of past and future periods. Precipitation events are first assessed at the regional scale, using the North American Regional Climate Change Assessment Program (NARCCAP) model dataset. Extreme precipitation events are defined using percentile-based thresholds of warm-season rainfall in a region of highly-variable terrain centered on the Front Range of the Colorado Rocky Mountains. The most extreme of these events are then simulated at high resolution (1-km) using the Weather Research and Forecasting (WRF) model to assess the influence of regional climate-scale environmental changes on storm-scale processes affecting the generation of precipitation. Statistical analysis of potential changes in future precipitation extremes reveals significant spread across the ensemble of regional climate model (NARCCAP) solutions. Comparison of historical (1979 – 2003) and future (2038 – 2070) simulations from individual NARCCAP models are thus performed on a model-by-model basis, with the goal of illustrating the potential for changes in extreme precipitation across a representative spectrum of climate model solutions.

Using the WRF model to simulate the events at storm-scale resolution (1-km gridspacing) allows for examination of convective-scale parameters most relevant to precipitation processes. Analysis focuses on changes in overall precipitation amount, intensity, spatial distribution, maximum terrain elevation at which extreme precipitation is found, as well as a variety of land-surface and hydrologic indicators. The ability to resolve fine-scale precipitation features further allows for increased detail in surface accumulation and runoff fields, potentially forming a more realistic picture of flood risk in complex terrain.

Questions regarding changes in future extreme precipitation press strongly at the interface of the research and operational communities, and the potential for changing risks and hazards affect society at large. It is thus increasingly important that research in this area proceeds in a collaborative manner, with an emphasis on clear communication of findings between the research and operational communities, as well as across multiple disciplines and areas of expertise. This study is one such effort, and results from these model experiments will offer an additional way to better inform the needs of water resources managers in the western US.