Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part II: Numerical Modeling

Abstract

Mountain waves and rotors in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part paper. Part I by French et al. delivers a detailed observational account of two rotor events: one displays characteristics of a hydraulic jump and the other displays characteristics of a classic leewave rotor. In Part II, presented here, results of high-resolution numerical simulations are conveyed and physical processes involved in the formation and dynamical evolution of these two rotor events are examined.The simulation results reveal that the origin of the observed rotors lies in boundary layer separation, induced by wave perturbations whose amplitudes reach maxima at or near the mountain top. An undular hydraulic jump that gave rise to a rotor in one of these events was found to be triggered by midtropospheric wave breaking and an ensuing strong downslope windstorm. Lee waves spawning rotors developed under conditions favoring wave energy trapping at low levels in different phases of these two events. The upstream shift of the boundary layer separation zone, documented to occur over a relatively short period of time in both events, is shown to be the manifestation of a transition in flow regimes, from downslope windstorms to trapped lee waves, in response to a rapid change in the upstream environment, related to the passage of a short-wave synoptic disturbance aloft.The model results also suggest that the secondary obstacles surrounding the Medicine Bow Mountains play a role in the dynamics of wave and rotor events by promoting lee-wave resonance in the complex terrain of southeastern Wyoming.

Dynamics of rotor formation in uniformly stratified two‐dimensional flow over a mountain

“…In this case, the key wave‐ducting mechanism is related to the presence of a sharp density discontinuity collocated with the jet. These findings are consistent with recent real‐case observations of rotor formation in conjunction with a hydrostatic wave‐breaking event (French et al ; Grubišić et al ).…”

Turbulence in breaking mountain waves and atmospheric rotors estimated from airborne in situ and Doppler radar measurements

“…Prior to T‐REX, in January and February 2006, the NASA Winter Orographic Clouds Experiment (NASA06) was conducted over the Medicine Bow Mountains (MBM) in southeastern Wyoming, deploying UWKA and WCR in a moist midlatitude wintertime environment. Recently, French et al () and Grubišić et al () provided a detailed analysis of two NASA06 events exhibiting large‐amplitude mountain waves. In their studies, they revealed the presence of large atmospheric rotors and the key role of mid‐tropospheric gravity‐wave breaking in steering the flow dynamics on both days.…”

Profiling Radar Observations and Numerical Simulations of a Downslope Wind Storm and Rotor on the Lee of the Medicine Bow Mountains in Wyoming

“…Under weak wind shear the rotor produces moderate or severe turbulence. Under stronger shear the rotor is associated with trapped lee waves, in which case the turbulence may be severe [4,20,22,51,52]. Both the WRF simulations and WCR observations suggest that the case presented here was predominantly of the second type.…”

“…While the MBM may be rather prone to downslope wind storms [21,22,31], even under marginally strong wind at mountaintop level, they do occur elsewhere (as mentioned in the Introduction), including in more densely populated areas. Most severe wind events in the Boulder, Colorado area, for instance, are associated with downslope wind storms [53].…”