Seasonal variability of the observed vertical wave number spectra of wind and temperature and the effects of prewhitening

Variance power spectra of wind and temperature fluctuations as functions of vertical wave number from several hundred balloon ascents over the very fl at terrain of central Illinois are analyzed. Special attention is given to the effects of prewhitening the data prior to spectral analysis, and it is shown that prewhitening should be used when spectral slopes are more negati ve than about -3. The conclusions of a previous study that did not use prew hitening are reexamined using prewhitening with an expanded set of balloon ascents. The present results confirm the past conclusions that the vertical wave number spectra are inconsistent with the major predictions of saturat ed gravity wave models in several important respects. The expanded data set used here permits the study of changes from summer to winter, and it is sh own that spectral slopes are steeper in winter than in summer and that both spectral slope and amplitude have the most significant correlation with pa rameters of the large-scale flow during winter.