Time spectra and cross‐spectra of kinetic energy in the planetary boundary layer

Temporal spectra and cross‐spectra of the three turbulent wind components for meso‐synoptic scales at 250 m vertical intervals in the planetary boundary layer are presented. The data were derived from hourly simultaneous balloon ascents from a 5‐point grid at Hay, New South Wales, Australia (lat. 34 ½S), during the Wangara expedition. The spectra indicate a meso‐scale gap for u (eastward component of wind) and v (northward component). The computed w ‐spectra suggest that no marked gap is present. For periods less than 1 day the u‐ and v ‐spectra, with a correction applied for high frequency noise, fall off approximately as n −2·6 , where n is the frequency, and this result appears to be invariant with height within and just above the planetary boundary layer. The corrected w ‐spectra exhibit n −1 dependency in this frequency range. The uv, uw , and vw cospectra also show only small contributions on the meso‐scale (periods less than a day) compared to the synoptic scale, indicating that meso‐scale motions may not have to be explicitly modelled in atmospheric general circulation models. An inertial peak is found for u and v but the w ‐spectra show a diurnal peak. The phase angles showed that u and v were in approximate quadrature at the inertial frequency, demonstrating the existence of the inertial cycle. Tests for horizontally two‐dimensional isotropy indicated that the turbulence did not obey this simplification.