Measurements of lagrangian and eulerian properties of turbulence at a height of 2,500 ft

Abstract

AbstractDuring the summer of 1962, super‐pressured, constant‐volume balloons (tetroons) were flown past wind‐vane instruments carried on a barrage‐balloon cable at Cardington, with the primary purpose of comparing the values of vertical velocity spectral density, S (n), obtained from the two sets of data. Based upon 25 comparisons at a mean height of 2,400 ft, on the average the maximum in the nS (n) spectrum derived from the radar‐positioned tetroons occurs at about one‐third the frequency of that derived from the wind vanes, i.e., on average the ratio, β, of Lagrangian to Eulerian scale is about three. There is a significant correlation of — 0·73 between turbulence intensity and log β, with the regression line indicating a β value of one at a turbulence intensity of 0·35 and a value of seven at a turbulence intensity of 0·05. Both factors in the turbulence intensity (mean wind speed and root‐mean‐square vertical velocity) contribute to this significant correlation, with the mean wind speed of greater influence. A comparison of five pairs of morning and afternoon tetroon flights for which lapse‐rate data are available shows that in all five cases β increases as the atmospheric stability increases.At frequencies between 0·1 and 1·0 cycles per minute, three theodolite‐positioned flights suggest that the tetroon vertical velocity spectral density is proportional to between n−5/3 and n−2, where n is frequency. Since the latter value would be in agreement with the theoretical expectation for Lagrangian‐type data, this result is consistent with the assumption that, basically, the tetroons are following the vertical air motion.Based upon 34 cases, there is evidence that surface frictional effects extend to at least 2,500 ft, with the tetroons moving across the surface isobars toward low pressure with a modal angle of 10–20 degrees and the tetroon speed being subgeostrophic to the extent of 1–2 m sec−1.