The following plots show amplitudes of Maui winds calculated using a height weighting function approximating the OH volume emission profile (mean height 89.3 km, plus/minus 6.7 km). Waves/tides with short vertical wavelengths will be strongly attenuated by this weighting function. Two different time-frequency representations are shown:

(1) Spectrogram produced using complex demodulation and a low-pass filter with time constant of approximately 5-days. This produces a time-frequency distribution with constant bandwidth and time resolution.

(2) S-transform magnitudes produced using the algorithm described by Stockwell et al., 1996. This produces a time-frequency distribution with constant fractional bandwidth (constant "Q").

It is interesting to note that the bandwidth of the S-transform representation increases with frequency. As such, the bandwidth of the S-transform in the vicinity of the 8-hour oscillation is larger than that of the spectrogram produced using complex demodulation. As a consequence, the 8-hour component is not readily detected in the S-transform, whereas it is clearly seen in the spectrogram.

These spectrograms were produced using complex demodulation and have approximately 5-day resolution. The amplitude is shown in decibels relative to 1 m/s peak amplitude. Thus, 20 dB corresponds to a component with 10 m/s amplitude.

S-transform magnitudes

The following plots are the same as the preceeding except that the range of amplitudes plotted is 0-30 m/s.