Wave-Induced Boundary Layer Separation in the Lee of the Medicine Bow Mountains. Part I: Observations

Abstract

wo cases of mountain waves, rotors, and the associated turbulence in the lee of the Medicine Bow Mountains in southeastern Wyoming are investigated in a two-part study using aircraft observations and numerical simulations. In Part I, observations from in situ instruments and high-resolution cloud radar on board the University of Wyoming King Air aircraft are presented and analyzed. Measurements from the radar compose the first direct observations of wave-induced boundary layer separation.The data from these two events show some striking similarities but also significant differences. In both cases, rotors were observed; yet one looks like a classical lee-wave rotor, while the other resembles an atmospheric hydraulic jump with midtropospheric gravity wave breaking aloft. High-resolution (30 ? 30 m2) dual-Doppler syntheses of the two-dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation, the scale and structure of the attendant rotors, and downslope windstorms. In the stronger of the two events, near-surface winds upwind of the boundary layer separation reached 35 m s?1, and vertical winds were in excess of 10 m s?1. Moderate to strong turbulence was observed within and downstream of these regions. In both cases, the rotor extended horizontally 5?10 km and vertically 2?2.5 km. Horizontal vorticity within the rotor zone reached 0.2 s?1. Several subrotors from 500 to 1000 m in diameter were identified inside the main rotor in one of the cases.Part II presents a modeling study and investigates the kinematic structure and the dynamic evolution of these two events.