The detection and measurement of turbulent structures in the atmospheric surface layer

Abstract

Turbulence data from the planetary boundary layer (PBL) indicate the presence of deterministic turbulent structures. These structures often show up as asymmetric ramp patterns in measurements of the turbulent fluctuations of a scalar quantity in the atmospheric surface layer (ASL). The sign of the slope of the sharp upstream edge of such a triangular pattern depends on the thermal stability conditions of the ASL.The turbulent structures in the ASL have been tracked by a detection method which searches for rapid and strong fluctuations in a signal -the VITA (variable interval time averaging) technique. This detection method has previously been employed in laboratory boundary layers. The VITA detection method performs well in the ASL and reveals the presence of vertically coherent turbulent structures, which look similar to those in laboratory shear flows. At the moment that a sharp temperature interface appears, the horizontal alongwind velocity shows a sharp increase, along with a sudden decrease of vertical velocity, independent of the thermal stability conditions of the ASL. The fluctuating static pressure reveals a maximum at that moment. The vertical turbulent transports show a twin-peak character around the time that the sharp jumps in the temperature and the velocity signals appear.

Flux decomposition into coherent structures

“…These microfront events contribute significantly to the total turbulent flux as demonstrated in the studies of Chen and Blackwelder (1978), Antonia and Chambers (1978), Finnigan (1979), Raupach (1981), Schols (1984), Kikuchi and Chiba (1985)) and Bergstrom and Hogstrom (1989). Maitani and Ohtaki (1987) find the momentum transport in near neutral conditions to be concentrated in the downward sweeps of momentum while the upward transport of weaker momentum is more spread out.…”

Conditional analysis of temperature and humidity microfronts and ejection/sweep motions within and above a deciduous forest

“…Cantwell (1981) and Hussain (1983) concluded that the existence of organized flow patterns is fundamental to the understanding and numerical modeling of boundary-layer phenomena. In the unstable atmospheric surface layer, characteristic temperature ramp patterns have been reported and believed to be indicative of organized motions (Antonia et al, 1979;Schols, 1984). In a time series, a scalar ramp consists of a slow rise followed by a sharp drop and gradual recovery.…”

“…In the VITA (Variable Interval Time Average) technique, short-term signal variance is compared with longer term variance to identify intervals when the short-to long-term variance ratio exceeds a preset threshold (Blackwelder and Kaplan, 1976;Chen and Blackwelder, 1978;and Schols, 1984). With quadrant analysis, one divides the plane of longitudinal and vertical velocity (u', w') into four categories and examines the relative contribution to momentum transfer by the motions within each quadrant (Grass, 1971;Raupach, 1981;and Shaw et al, 1983).…”

An objective method for determining principal time scales of coherent eddy structures using orthonormal wavelets

“…Most methods proposed to identify coherent structure properties have subjective components 2,3,5,14,20,27,36,37,39,40 . For example, an arbitrary threshold value is set for the wavelet coecients when selecting organized events, or the size of a certain window employed in the analysis must be speci®ed.…”