Impact of Upstream Winds and Stability on Rotors

The sensitivity of three-dimensional rotor characteristics to the upstream winds and static stability are investigated through a series of ultra-high-resolution simulations (isotropic resolution of ~50 m) using a large eddy simulation (LES) version of the COAMPS model. This study is motivated by the observations of strong downslope winds and turbulent rotors in the lee of the Front Range that occurred during the recent tragic Marshall fire in Louisville, CO in December 2021. The focus of this investigation is on the impact of upstream environmental variations on the internal structure of rotors and including small-scale intense circulations within rotors known as sub-rotors.

Previous high-resolution numerical simulations conducted indicate that surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel shear instability breaks this vortex sheet into small intense vortices or sub-rotors. In this study, three-dimensional LES simulations are conducted using a reference state based on the upstream conditions of the Front Range prior to the Marshall fire. Experiments will be conducted to quantify the impact of variations in the upstream environment, including the winds and stability near the crest level. These simulations can help to inform the predictability characteristics of downslope windstorm and rotor events.