MF radar spaced antenna experiment: wind variance vs. record length

Because of the relatively wide transmitter antenna beamwidths at medium fre quency (MF), measurements are expected to represent an integration over spa ce as well as time. For historical (and practical) reasons, the normal reco rd lengths at Saskatoon have been similar to5 min (4.5-4.8 min). This is ap proximately the Brunt-Vaisala (B-V) period, equivalent to 1 cycle of the sh ortest 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 statis tics. On the other hand, if the spatial averaging has removed those waves c ontributing to high-frequency fluctuations in wind, then reducing the recor d length increases the noise level rather than giving us more information r egarding these GW. This paper describes two multiple record length experime nts performed with the Saskatoon MF radar. Conclusions are that in the fall , below say 80 km, there is an inverse relationship between wind variance a nd record length, as would be expected from elementary statistics if the va riance were strictly analysis noise. Presumably, this is because GW amplitu des are negligible here. In the summer, short period GW contribute signific antly to the variance from 60 to 100 km. If we assume a form for the (unkno wn) GW spectrum, e.g. -5/3 log-log slope (VanZandt, 1982), then measurement s 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 win d 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 fre quency. (C) 2001 Elsevier Science Ltd. All rights reserved.