Extreme precipitation events across the Colorado Front Range in future climates

Extreme precipitation events are of great importance to both the general public and water resources managers, particularly in the complex terrain of the western US. Safety issues pertaining to flash flooding, river dam design, and reservoir operations depend strongly on accurate 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 the potential for changes in extreme precipitation events thus becomes an important issue for future planning and adaptation.

In this study, warm-season extreme precipitation events in the western US are analyzed using two approaches. First, precipitation events are assessed at the regional scale in both the past and future using the North American Regional Climate Change Assessment Program (NARCCAP) model simulations. 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. Storm environments and event statistics are analyzed to understand the predicted regional-scale changes and their linkages to shifts in precipitation event frequency and intensities. 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, using synoptic signals in the kinematic and thermodynamic fields (e.g., wind shear, moisture convergence, CAPE) to assess environmental changes as well as inform the selection of cases for higher-resolution simulations.

Using the WRF model to simulate the events at storm-scale resolution (1-km gridspacing versus 50-km gridspacing as in the NARCCAP simulations) allows for examination of convective-scale parameters and relevant precipitation processes. Analysis focuses on changes in storms with terrain elevation, particularly with respect to the intensity and distribution of hail vs. rain, the height of the freezing level, and the elevation of various precipitation thresholds. Changes in overall precipitation amount, intensity, spatial distribution, maximum terrain elevation at which extreme precipitation is found, as well as a variety of severe weather indicators are also assessed. The ability to resolve such fine-scale features allows for a more complete explanation of how changes at the storm-scale may relate to systematic changes in the larger-scale environment, and extends the findings of previous efforts to assess future potential of extreme events using large-scale environmental conditions as event indicators. Results are ultimately intended to be applied to the needs of water resources managers in the western US.