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Ann. Geophys., 16, 1367-1379, 1998
www.ann-geophys.net/16/1367/1998/
© European Geosciences Union 1998
Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar
N. J. Mitchell and V. St. C. Howells
Department of Physics, The University of Wales, Aberystwyth, Ceredigion, SY23 3BZ, UK (E-mail:Nick.Mitchell@aber.ac.uk)
Abstract. The EISCAT VHF radar (69.4°N, 19.1°E) has
been used to record vertical winds at mesopause heights on a total of 31 days
between June 1990 and January 1993. The data reveal a motion field dominated by
quasi-monochromatic gravity waves with representative apparent periods of ~30–40
min, amplitudes of up to ~2.5 m s–1 and large vertical wavelength. In
some instances waves appear to be ducted. Vertical profiles of the
vertical-velocity variance display a variety of forms, with little indication of
systematic wave growth with height. Daily mean variance profiles evaluated for
consecutive days of recording show that the general shape of the variance
profiles persists over several days. The mean variance evaluated over a 10 km
height range has values from 1.2 m2s–2 to 6.5 m2s–2
and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial
maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km
has a slope (spectral index) of –1.36 ± 0.2, consistent with observations at
lower heights but disagreeing with the predictions of a number of saturation
theories advanced to explain gravity-wave spectra. The spectral slopes evaluated
for individual days have a range of values, and steeper slopes are observed in
summer than in winter. The spectra also appear to be generally steeper on days
with lower mean vertical-velocity variance.
Key words. Meteorology and atmospheric dynamics (middle
atmosphere dynamics; waves and tides)
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