A simple model for the enhanced frequency spectrum of vertical velocity based on tilting of atmospheric layers by lee waves


  • Kenneth S. Gage,

  • Gregory D. Nastrom


This paper is concerned with the interpretation of the enhanced frequency spectrum of vertical velocity that has been observed at many locations under strong-wind conditions in the vicinity of rough terrain. These enhanced frequency spectra are considerably more energetic than the vertical velocity spectra observed under low-wind conditions, and their shape resembles the shape of the oblique spectra observed by the same radars. In this paper we present the composite spectra of horizontal and vertical velocity observed over a range of wind conditions by the Platteville, Colorado, radar, located in the lee of the Colorado Rockies. The vertical velocity spectra clearly show the increase in magnitude and change in spectral slope that occurs as the troposphere becomes increasingly disturbed. We compare the observations with a simple model that relates the magnitude of the vertical velocity spectrum to the horizontal velocity spectrum and the effective tilt of atmospheric layers. Our results show that it is possible to simulate the observed vertical velocity spectrum with effective tilts in the range of 2.5–10°. Recent observations of vertical velocity spectra from the Flatland radar, located in flat terrain in central Illinois, do not exhibit this behavior, which leads us to conclude that lee waves are responsible for the enhancement in the Colorado spectra.