Orographic Mountain Waves

Lee waves, rotor zones, and hydraulic jumps in stratified flow over topography

60
80
20 m/s
10
Streamlines
Rotor zone
Wave crest (lenticular cloud)
Hydraulic jump
Fr = 0.0 — Subcritical
The Froude number Fr = U/(Nh) determines flow regime. Fr<1: wave-like (subcritical); Fr>1: blocked/hydraulic jump. Vertically propagating waves (scorer parameter l²>0) create lenticular clouds at crests. Rotor zones beneath wave troughs are zones of turbulent reversed flow — dangerous for aircraft.