Saturated Gravity Wave Spectrum in the Polar Summer Lower Thermosphere Observed by Foil Chaff during Campaign “Sodium 88”

Abstract

Ten vertical profiles of horizontal winds have been measured at high resolution (25 m) in the range from 85 to 103 km during the sodium campaign in summer 1988 at Andenes, northern Norway (69°N). The purposes of this study are to examine the hypothesis that the vertical wavenumber spectrum is controlled by saturation processes and to identify the mechanisms that contribute to saturation of the atmospheric motion spectrum within the observed height range. An analysis of vertical wavenumber spectra and vertical wind shears of the ten horizontal wind measurements is presented. Our results indicate a rather good agreement between the observed and predicted velocity spectra in both amplitude and slope for vertical wavelengths up to about 12.8 km. (This wavelength is given by the height interval covered by the experiment.) Very large wind shears, as high as 40?90 m s?1 km?1, are observed to be present at various heights, and at certain heights the shears even exceed 100 m s?1 km?1. Such strong wind shears yield Richardson numbers close to or smaller than 0.25, suggesting that the saturated gravity wave spectrum is associated with the shear instabilities.