Tuesday, 24 January 2017
The Front Range of the Colorado Rocky Mountains is susceptible to downslope windstorms. These storms can impact cities, including Boulder, Colorado, which sustained estimated damage of $20 million during the January 17, 1982, windstorm. Since 1996, 180 windstorms with sustained winds above 40 mph or gusts above 58 mph have occurred in the Boulder County area and vicinity. This project formulated a baseline of windstorm indicators and evaluated how future climate change would affect the frequency of downslope windstorms. Several key indicators are present during windstorm events, as based on prior studies and used by operational NWS forecasters, listed here in order of importance. The most important is the presence of a 700 mb level wind component normal to the Front Range at a velocity exceeding 15 m/s; two others are sufficiently low shear, and a stable layer just above mountain-top level (within the 700 mb to 500 mb layer). Past trends in these indicators were evaluated using the NCEP/NCAR Reanalysis 1 model. The NCEP/NCAR reanalysis (1948-2016) yields a downward trend in the 700 mb wind indicator. We compared this baseline to projections of these windstorm indicators from two climate models, the Geophysical Fluid Dynamics Laboratory (GFDL) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and the Community Earth System Model (CESM). Although the GFDL projection shows no trend in indicators, the CESM shows downward trends for both the 700 mb wind indicator and the stability indicator between two decadal slices, 2026-2035 and 2071-2080.
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