A modelling study of a nonstationary boundary-layer separation and rotor event

On January 26th 2006, the University of Wyoming King Aircraft documented the occurrence of a wave-induced boundary-layer separation (BLS) event in the lee of the Medicine Bow Mountain range (Wyoming). A distinctive feature of the observed phenomenon is its unsteadiness, as demonstrated by the BLS line moving upstream for about 8 km in approximately half an hour. Mesoscale simulations with the WRF model at a maximum horizontal grid spacing of 400 m reveal the dynamic forcing leading to this rapid evolution.

The upstream motion of the BLS line and of the related rotor appears to depend on the decreasing nonlinearity of the impinging flow, which causes the transition from a flow regime characterized by low-level wave breaking, to another one where trapped lee waves form as a consequence of wave reflection at a self-induced critical level. Diabatic effects related to the diurnal boundary-layer cycle in the cloud-free region downstream of the Medicine Bow Range, which may contribute in suppressing wave breaking in the late afternoon, are also examined. The overall evolution of the phenomenon displays striking analogies with documented unsteady Bora events, observed in the Northern Adriatic Sea.

The sensitivity of simulation results to initial and boundary conditions, model resolution and boundary-layer parameterization is briefly discussed.