Characteristics of vertical velocity fluctuations observed on a 430‐m tower

During the summer of 1963, sonic anemometers were mounted at 46, 137, 229 and 320 metres on a 430 metre tower to measure turbulent fluctuations in the vertical velocity component. Mean horizontal wind speeds and temperatures were measured at 12 levels on the tower. Characteristic behaviour of power spectra for four typical stability conditions are described. Evidence of an inertial subrange is found at all levels regardless of stability. Analysis of the rate of dissipation, ϵ, indicates that there is no simple relationship which can describe its behaviour with height for all conditions. Its behaviour is shown to change with thermal stratification; ϵ remains relatively constant with height during well‐established convection but decreases rapidly above 46 m when the temperature gradient is stable. It is also shown that ϵ is roughly proportional to the product of the third power of the standard deviation of the vertical velocity component and the wave number of its logarithmic spectral peak. The data from the sonic anemometers provide some additional information on the structure of convection during very unstable lapse conditions. Analyses of coherence between successive levels indicate the presence of a second generation of convective elements above the plume convection generally observed near the ground. The transition layer is estimated to be in the vicinity of 100 m. The scale of turbulence for the top three levels shows an initial increase with increasing instability followed by a decrease. It is conjectured that this decrease is a consequence of the fact that the second‐generation convective elements move faster than the wind speeds indicated at the level of instrumentation.