PNNL

Abstract

Intermittent turbulent temperature fluxes are characterized in terms of a flux interval, defined as the time interval over which flux values are computed, and a flux threshold value. Fluxes whose magnitudes exceed the threshold value are defined as flux events while the times when the threshold is not exceeded are defined as gaps. Turbulent temperature flux data from five sites in Kansas, Utah, and Washington are examined within this descriptive framework. The turbulent fraction fturb, the ratio of the time occupied by turbulent events to the total sampling time, is shown to depend linearly on the average flux for the sampling period over a range of flux intervals, threshold values, and sampling times. As the average flux for a sampling period decreases toward zero, the median magnitude of the fluxes during the gaps also decreases but the median fluxes during the turbulent events become nearly independent of the sampling period average. The variations of median flux values with gradient or Bulk Richardson numbers show considerable scatter and significant intermittency (e.g., fturb