SPECTRAL AND BISPECTRAL CHARACTERISTICS OF WIND VARIABILITY AT DARWIN, AUSTRALIA OBSERVED BY A VHF RADAR WIND PROFILER

A year of wind data measured by a 50 MHz wind profiler at Darwin, Australia are analysed. The behaviour of vertical motions in the precipitation-free boundary layer is shown to be different from the free troposphere. However, these differences are not reflected in the spectral characteristics of the horizontal wind fields. There are no significant differences between the horizontal-velocity spectra above and within the mixed layer. The power spectra of the horizontal winds show little seasonal differences for the monsoon, dry and transition seasons as well as for the data set overall. The bispectral characteristics are also similar, with the strongest interactions between the lowest frequencies, and a weak self-interaction peak at low frequencies. A cross-peak in the zonal wind above 5 km is also present for frequency sums corresponding to a period of around 10 days. There are no detectable nonlinear interactions at high frequencies. The wind field is not strongly polarized except at the very lowest frequencies (i.e. seasonal changes). Thus there appears to be universality of the spectral characteristics observed despite the expected variations in the forcing in different seasons and stability regimes. These observations have interesting implications for internal-wave models and two-dimensional turbulence models of mesoscale variability.