Buoyancy effects on the turbulent transport processes in the lower atmosphere

Abstract

AbstractBuoyancy effects on the turbulent transport processes in the atmospheric boundary layer above the surface layer have been investigated. Direct measurement of heat flux and wind and temperature profiles was made by sonic anemometers and the continuous ascending and descending operation of a kitoon, to determine the eddy diffusivity of heat and the local Richardson number. the present results, along with those of a parallel study on the stratified turbulence structure in a laboratory experiment were examined by comparison with previous results in the surface layer. Under stable conditions significantly different stability dependence in the surface layer and the layer above it is recognized; in the layer above the surface layer the eddy diffusivity of heat. KH, along with the ratio of the eddy diffusivities of heat and momentum, KH/KM, decrease remarkably with the gradient Richardson number, Ri, reaching a value of KH/KHO = 0.01 at Ri = 1 and under strongly stable conditions the ratio KH/KHO varies as (Ri)−2, while in the surface layer the ratio KH/KM remains constant or decreases slightly. This contrasting behaviour in the surface layer and the layer above is presumed due to the modification of the turbulence structure and so the turbulent transport processes by the presence of the ground, as indicated by Gibson & Launder (1978). Under unstable conditions the ground effect is not so significant that results of the present observation along with the laboratory experiment may be applied to the entire region of the atmosphere. the ratios of KH/KHO and KH/KM increase as the stratification shifts from neutral to weakly unstable conditions. the ratio KH/KM takes a peak value of about 3 at Ri ∼ −0.2 and then falls gradually with increasing instability and attains the value of unity at Ri ∼ −10.0. This fact together with the increase of Km/KMO with increasing [Ri] results in a constant value of the ratio KH/KHO equal to about 6.0.