Mixed Layer Spectra from Aircraft Measurements

Abstract

Vertical velocity and temperature spectra from the 1978 Phoenix convective boundary layer aircraft flights are discussed. Both the vertical velocity spectra and the temperature spectra are normalized using procedures which are based on mixed layer similarity. The method of Kaimal et al. is used to normalize the vertical velocity spectra while an adaptation of the method of Corrsin (1951) is used to normalize the temperature spectra. The normalized vertical velocity spectra for heights ranging from 0.2zi to 0.8zi collapse onto a single curve as found by Kaimal et al. The normalized temperature spectra collapse onto a family of curves which vary with height. The maximum amplitude of the vertical velocity spectra occurs at a wavelength of 1.5zi, as found by Kaimal et al. and others. The maximum amplitude of the temperature spectra increase and shift to longer wavelengths with height. The wavelengths of the spectral peaks for w and temperature are the same in the middle of the convective boundary layer. The success of the normalization of temperature spectra by mixed layer scaling decreases rapidly as the wavelengths exceed five times the boundary layer depth. For wavelengths shorter than this, normalization of temperature spectra is most successful in the lower half of the convective boundary layer. In the upper half of the convective boundary layer, the normalization of temperature spectra is not as successful as the normalization of vertical velocity spectra because of the entrainment effects which are not accounted for in mixed layer scaling.