Turbulence spectra in stable and convective layers in the free atmosphere

The form of the spectrum of turbulence is studied in an evolving atmospheric system consisting of a growing convective layer surmounted by an inversion layer. Aircraft measurements of the one-dimensional spectrum functions for longitudinal and vertical velocity fluctuations were made at a hieight of 1000 feet above a desert dry lake. The spectra of both velocity components exhibit the inertial subrange –5/5 power law within the convective layer throughout the wavelength bandwidth of the aircraft system, 6.9 to 276 meters. Within the bounding stable layer, for wavelengths greater than 69 meters, the spectra of both velocity components appear to follow a power law approaching – 3, the theoretical prediction for the buoyant subrange according to Shur and Lumley.

Examination of the time series of spectral density at specific frequencies indicates approximate isotropy for these data, although occasional large deviations from isotropy are found, particularly at long wavelengths within the stable layer. The turbulence kinetic energy budget is estimated and the results support the physical models behind the buoyant and inertial subrange concepts. The bounding stable layer was found to be a region into which energy was imported from below and where energy was suppressed by the buoyancy forces associated with a downward flux of heat into the convective layer. In the region near the top of the convective layer, buoyant production of turbulence energy was found to equal the dissipation rate, as is assumed in the inertial subrange theory.