A SELECTIVE ANALYSIS OF THE SCALE OF ATMOSPHERIC TURBULENCE

Abstract

A preliminary investigation of the scale problem in atmospheric turbulence has been carried out. An analytic filter of limited resolving power is applied to simultaneous data from four heated-thermocouple anemometers spaced at varying distances either parallel or normal to the mean wind. Sequences of filtered data are thus obtained for three frequency intervals: 1/2 to 1/4, 1/4 to 1/8, and 1/8 to 1/16 cycles per second. Statistical testing of the filtered sequences shows that they are in general normally distributed, have a mean of zero, and possess homogeneous variances. Estimates of the scale or mean eddy size for each frequency band are obtained by two methods: the correlation technique used in wind-tunnel investigations and a mean-difference technique. Results from both methods are in close agreement and indicate eddy dimensions in the range from about 1 to 16 meters for the seven cases studied. At the highest frequencies, there is approximate equality between the transverse and longitudinal scales and the variation in magnitude with the time of day appears small. At the lower frequencies, the nighttime data show slight increases in the scale for both orientations; during the day, considerable increase in the longitudinal scale is noted with decreasing frequency whereas the transverse scale shows a proportionally smaller increase. The limited data available suggest that the scale is proportional to a power of the mean wind speed in the range from 0.5 to 1.0.