Similarity in VITA-detected events in a nearly neutral atmospheric boundary layer

Abstract

Horizontal velocity data recorded in the atmospheric boundary layer at Boulder, Colorado and Cabauw, the Netherlands under near-neutral conditions, are analysed here by the Variable Interval Time Average (VITA) method. It is found that event frequency shows similarity with VITA threshold and averaging time over a significant range of parameter values, enabling us to devise an objective method for (i) deciding on the choice of these hitherto arbitrarily selected parameters, and (ii) determining values for a well-defined 'characteristic' frequency. The conditional averages obtained from the events in the similarity region, when suitably normalized, are found to be extraordinarily similar to those obtained from low Reynolds number laboratory flows, indicating the existence of similar coherent events at widely varying Reynolds numbers. The characteristic frequency shows little variation with height beyond about 20 m, and a scaling that is poor with height and friction velocity but not inconsistent with outer variables.