The Budgets of Turbulent Kinetic Energy and Temperature Variance in the Atmospheric Surface Layer

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  • 1 Air Force Cambridge Research Laboratories, Bedford, Mass
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Abstract

Measurements of the shear production, buoyant production, turbulent transport (flux divergence) and dissipation terms in the budget of turbulent kinetic energy, and production and turbulent transport terms in the temperature variance budget are presented. Direct observations of the surface stress and heat flux over a horizontally uniform site enable presentation of the data in terms of surface layer similarity theory.

The dissipation term, obtained from differentiated hot-wire anemometer signals, agrees with estimates made from the inertial subrange levels of longitudinal velocity spectra with a value of 0.5 for the spectral constant. Under stable conditions dissipation essentially balances shear production, while turbulent transport and buoyant production are of secondary importance. Under unstable conditions, dissipation slightly exceeds the total production, and energy is also lost at a substantial rate due to upward export by the turbulence.

The large imbalance among the measured terms in the energy budget under unstable conditions is discussed. The cause of the imbalance cannot at this point be determined with certainty, but an interesting possibility is that pressure transport is significant under very unstable conditions.

The production rate of temperature variance exceeds its rate of vertical transport by an order of magnitude. Estimates of the universal temperature spectral constant were made with the assumption that temperature variance dissipation and production rates are equal; the average value, 0.8, falls within the range reported by other workers.

Abstract

Measurements of the shear production, buoyant production, turbulent transport (flux divergence) and dissipation terms in the budget of turbulent kinetic energy, and production and turbulent transport terms in the temperature variance budget are presented. Direct observations of the surface stress and heat flux over a horizontally uniform site enable presentation of the data in terms of surface layer similarity theory.

The dissipation term, obtained from differentiated hot-wire anemometer signals, agrees with estimates made from the inertial subrange levels of longitudinal velocity spectra with a value of 0.5 for the spectral constant. Under stable conditions dissipation essentially balances shear production, while turbulent transport and buoyant production are of secondary importance. Under unstable conditions, dissipation slightly exceeds the total production, and energy is also lost at a substantial rate due to upward export by the turbulence.

The large imbalance among the measured terms in the energy budget under unstable conditions is discussed. The cause of the imbalance cannot at this point be determined with certainty, but an interesting possibility is that pressure transport is significant under very unstable conditions.

The production rate of temperature variance exceeds its rate of vertical transport by an order of magnitude. Estimates of the universal temperature spectral constant were made with the assumption that temperature variance dissipation and production rates are equal; the average value, 0.8, falls within the range reported by other workers.

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