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Ann. Geophys., 15, 1158-1164, 1997
www.ann-geophys.net/15/1158/1997/
© European Geosciences Union 1997
Zonal mean and tidal dynamics from space: an empirical examination of aliasing and sampling
J. M. Forbes1, M. Kilpatrick1, D. Fritts1, A. H. Manson2, and R. A. Vincent3
1University of Colorado, Boulder, Colorado, USA
2University of Saskatchewan, Saskatoon, Canada
3University of Adelaide, Adelaide, S. Australia
Abstract. Interpretations of space-based measurements
of atmospheric parameters in the mesosphere and thermosphere are complicated by
large local-time variations at these altitudes. For this reason, satellite
orbits are often preferred which precess through all local times one or more
times per season. However, the local-time structure of the atmosphere is
inherently non-stationary, which can lead to sampling and aliasing difficulties
when attempting to deconvolve the measurements into zonal mean and tidal
components. In the present study, hourly radar measurements of mesopause-region
winds are used to form a mock data base which can be used to gain insight into
implications of the aforementioned problems; the use of actual measurements
introduces a realistic element of geophysical temporal variability. Assuming
zonal symmetry (i.e., migrating tides superimposed on a zonal mean circulation),
the radar measurements are sampled from the satellite perspective for orbital
inclinations of 57° and 70°, and compared to the ground or true
perspective. These comparisons provide realistic estimates of the errors to be
expected when attempting to derive mean and tidal components from space-based
measurements. For both diurnal and semidiurnal components, and the quoted
satellite inclinations, acceptable errors (3–4m/srms) are obtained for data
covering 24h local time (i.e., ascending plus descending nodes); the
corresponding errors for single-node data (12h local-time coverage) are of order
8–11m/s, and therefore may not represent reliable estimates of the actual tidal
components. There exist certain caveats in connection with the latter conclusion
which are discussed.
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