The flux‐gradient relation for turbulent heat transfer in the lower atmosphere

Data obtained during February and December 1962 at a carefully‐chosen micrometeorological site at Kerang in Northern Victoria are used to examine the flux‐gradient relation for sensible heat transfer. The analysis is carried out for 70 half‐hour runs specially selected for steady state lapse conditions to determine the variation with the stability parameter, z/L , of the Monin‐Obukhov ‘universal function,’ ø H . In the range 0.02 < | z/L | < 0.6 it is found that the value of the index ϵ in ø H α | z/L | ϵ is − 0.33 ± 0.02 in excellent agreement with the ‘free convection’ regime of Priestley and the −4/3 power form of the temperature profile. The corresponding value of H* is 1.32 ± 0.06. For | z/L | > 1, ϵ is found to be −0.38 ± 0.10 in contrast to Townsend's suggestion of −1 for this region based on laboratory evidence. Regression analysis for the region | z/L | > 0.2 yields a value of ϵ of −0.50 ± 0.02. The suggestion is offered that the motions observed by Townsend may occur in the atmosphere but as transient phenomena in which the applicability of the conventional similarity principles is to be questioned. The formula H = 24Δθ 3/2 based on Priestley's regime with sensible heat H in mw cm −‐2 , and potential temperature difference Δθ between 1 and 4 metres in ° provides a practical basis for the evaluation of the heat flux from profile measurements.