Physics
Scientific paper
Jan 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001jastp..63..181m&link_type=abstract
Journal of Atmospheric and Solar-Terrestrial Physics, Volume 63, Issue 2-3, p. 181-191.
Physics
3
Scientific paper
Because of the relatively wide transmitter antenna beamwidths at medium frequency (MF), measurements are expected to represent an integration over space as well as time. For historical (and practical) reasons, the normal record lengths at Saskatoon have been /~5 min (4.5-4.8 min). This is approximately the Brunt-Väisälä (B-V) period, equivalent to 1 cycle of the shortest expected gravity wave (GW) period, so some GW energy may be lost by averaging. In addition the GW oscillations may add ``noise'' to the measured wind or even prevent a successful analysis, which assumes stationary statistics. On the other hand, if the spatial averaging has removed those waves contributing to high-frequency fluctuations in wind, then reducing the record length increases the noise level rather than giving us more information regarding these GW. This paper describes two multiple record length experiments performed with the Saskatoon MF radar. Conclusions are that in the fall, below say 80 km, there is an inverse relationship between wind variance and record length, as would be expected from elementary statistics if the variance were strictly analysis noise. Presumably, this is because GW amplitudes are negligible here. In the summer, short period GW contribute significantly to the variance from 60 to 100 km. If we assume a form for the (unknown) GW spectrum, e.g. /-5/3 log-log slope (VanZandt, 1982), then measurements with different record lengths can be used to separate the analysis noise and GW parts of this variance. Finally, the small amount of 90 s record wind data examined does not reveal obvious coherent features at short periods which are not also seen in 5 min records. On the other hand, their spectra do enhance some features which are less obvious in those of 5 min data, and interestingly, no apparent change is seen in spectral slope at the B-V frequency.
Manson A.
Meek Christopher
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