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Keywords:

  • jet stream;
  • band-pass filters

Abstract

We present an analysis of eight years' data from the UK MST radar at Aberystwyth, Wales, to determine the occurrence and properties of long-period, quasi-monochromatic oscillations in the wind vector identified with inertia-gravity waves. Wave perturbations were first isolated by filtering out background wind variation in the vertical, then the continuity of the radar measurements was exploited to apply band-pass filters in time to select quasi-monochromatic oscillations with periods 4–8 hours or 12–24 hours. By searching for altitude regions where the wind vector rotates systematically with height, and fitting ellipses to such sections, the properties of inertia-gravity waves can be derived. To guard against contamination by short-intrinsic-period mountain waves, cases were discarded when the vertical velocity measured by the radar exceeded 15 cm s−1. Results show that waves in the stratosphere are dominated by upward energy propagation (clockwise rotation) and those in the troposphere by downward propagation, consistent with the dominant source for inertia-gravity waves being at tropopause level. Long-period waves (>12 hours) are observed 70% of the time in the lower stratosphere, roughly twice as often as the shorter-period waves, and 10–20% of the time in the troposphere. There is a winter maximum in occurrence in the troposphere and for the short-period stratospheric waves; however, the dominant long-period waves show little seasonal dependence. Likewise, they show only a weak correlation with jet-stream velocity or direction, unlike the other categories which are preferentially associated with strong jet streams and propagate along the jet. Inertia-gravity waves observed by the radar have typical amplitudes 1–2 m s−1 and vertical wavelength 2 km. The intrinsic wave period, estimated for non-jet-stream conditions, is similar for the two band-passed series, pointing to the importance of Doppler shifting in generating the observed oscillations. Copyright © 2007 Royal Meteorological Society