Present and Future Climate Sensitivity Studies of Downslope Winds in Boulder, Colorado

Downslope wind events in Boulder, Colorado can produce hurricane-force winds that damage property and disrupt travel, such as during the Marshall fire on 31 December 2022. Researchers and forecasters are aware of the difficulties forecast models experience when simulating downslope wind events along the Colorado Front Range. Yet, it is unknown how downslope wind characteristics (i.e., frequency and magnitude) in Boulder will change under future climate scenarios. With increasing urban growth, it is imperative that we understand how downslope winds will change in the future to assess the risks of their associated hazards.

In this study, we test various Weather Research and Forecasting (WRF) boundary layer schemes and compare model output to wind speed and gust observations at the NCAR Mesa Lab during downslope wind events in Boulder. We examine the infamous 17 January 1982 downslope wind event with a peak gust of 137 mph (61 m s-1) and a recent 2020 downslope wind event with a peak gust of 79 mph (35 m s-1). An ensemble of WRF model simulations is constructed by varying the boundary layer parameterization to examine the sensitivity of simulated wind speeds and gusts to these parameterizations. We observe that the ensemble mean wind speeds are consistently below measured values at the NCAR Mesa Lab, but some neighboring grid cells do provide a better representation of the wind speeds.

To investigate downslope wind events in a future climate, two approaches are applied: (1) a uniform perturbation of +10°C and -10°C is applied to initial conditions from the North American Regional Reanalysis (NARR) for each event, and (2) NARR initial conditions are perturbed based on long-term trends derived from the Community Earth System Model Large Ensemble (CESM2-LENS) and High Resolution CESM1.3 to provide a more robust representation of how the 1982 and 2020 downslope wind events would vary if they occurred in a future base climate state.