The Counter-Gradient Heat Flux in the Lower Atmosphere and in the Laboratory

Observations in the lower atmosphere and of laboratory convection which demonstrate the existence of upward heat flux, w′θ′¯ > 0, with vanishing or counter potential temperature gradient, ∂θ¯/∂z ≥ 0, are reviewed. Data of Telford and Warner (1964) are utilized to explain the magnitude of the counter gradient within the framework of the thermal variance equation. The diffusion or triple-correlation term, which can allow a counter-gradient flux to exist, is interpreted qualitatively. For theoretical studies of the planetary boundary layer which employ an eddy coefficient for sensible heat, the modified definitionis suggested, with γc ≈ 6.5 x 10-6C cm-1 in clear air.