The Froude Number as a Predictor of Mountain Lee Wave Phenomenon

Abstract

We present a physical and mathematical basis to motivate the use of the Froude number as a predictor of the presence and the vertical extent of mountain lee wave.  Froud numbers were computed from upwind soundings on days when soaring flights were conducted in the Tehachapi-Owens Valley area of southern California; the qualitv of the wave ranged from "poor" to world record setting.  For comparison, the Scorer parameter profile was computed for these days.  The presence of wind shear is implicity accounted for in our approach, since shear conditions are necessary to excite various oscillatory modes in the atmosphere.  Preliminary results indicate that the Froude number may well differentiate betwecn strong and weak wave, as well as indicate the vertical extent of lift.  The Froude number compares favorably with the Scorer parameter, which could not, a priori, distinguish between strong and weak wave days.  More data are needed from wave flights flown under controlled conditions to confirm or refute our hvpotbesis.