DIAGNOSING AND FORECASTING AIRCRAFT TURBULENCE WITH STEEPENING MOUNTAIN WAVES

摘要

If aviation forecasters are to understand why pilots complain of rough rides in some weather situations over mountains, they should understand how mountain waves produce turbulence. Unfortunately, the extensive research in breaking mountain waves hasbarely reached operational meteorology. This paper summarizes the pertinent theory so that a forecaster can recognize the atmospheric conditions favorable for mountain wave breaking. The theory describes two primary parameters needed for mountain wave analysis. First is a local non-dimensional amplitude number (an inverse Froude number). When this number is greater than one, the wave is nonlinear which indicates wave breaking. Second is the wave drag which estimates the wave energy available for turbulence. These two parameters depend on the vertical distribution of stability and wind, which a forecaster can assess on atmospheric soundings, and the height of the mountain. A new term, "breaking wave drag" is defined as the wave drag of turbulent waves and is a useful metric for diagnosing aircraft turbulence in mountain waves. Breaking wave drag can be computed from the stability and wind vertical profiles when the mountain height is known. Certain atmospheric conditions favor two nonlinear enhancements of mountain waves, hydraulic jump-like phenomena and wave re flection I resonance. In order to forecast breaking wave drag over large areas, the MWAVE algorithm was developed to apply the mountain wave equations to model soundings over high terrain.