Turbulent energy dissipation rates and exchange processes above a non‐homogeneous surface

Abstract

AbstractEnergy dissipation rates and the eddy fluxes of sensible heat, momentum, and total kinetic energy have been measured above a non‐homogeneous surface. Calculations were made from simultaneous observations of fluctuations in air temperature and the orthogonal components of wind velocity at heights of 16 m and 40 m. In all thermal stratifications, dissipation rates given by the Kolmogoroff structure function are in close agreement with direct measurements of the energy extracted from the mean flow. During stable stratification, energy losses due to work done against gravity and to the divergence of the vertical flux of turbulent kinetic energy are nearly an order of magnitude smaller than the dissipation. In unstable stratification, energy losses due to the flux divergence are of the same order as the dissipation and are approximately three times larger than the energy produced by buoyancy. Estimates of the vertical heat flux from the Obukhov structure function compare favourably with directly‐measured fluxes if the constant in the structure function is assigned a value of 0.7 in lapse conditions and a value of 1.6 in inversion conditions. The measurements also show that the ratio of the diffusivities of heat and momentum at a height of 16 m is significantly greater than unity in unstable stratification and significantly less than unity in stable stratification. Finally, the new measurements of dissipation rate are shown to be consistent with values obtained in previous studies.