Characteristics of Heat Flux in Atmospheric Surface Layer

The turbulent data over three different sites are utilized to analyze the observational behavior of heat flux (H) with respect to the stability parameter (ζ) in the atmospheric surface layer. In the stable conditions, the magnitude of H is observed to be small in weakly stable conditions which increases with increasing ζ till it reaches a peak at ζ =ζc and then decreases further with increasing stability. The observed behavior of H in stable conditions appears to be physically consistent and is in agreement with the studies reported in the literature. In unstable conditions, the variation of H with the ζ is bounded by a curve. This curve first shows increasing behavior with - ζ until it attains a peak at ζ ~ -0.12 and then decreases further with increasing instability.

A mathematical analysis based on Monin-Obukhov similarity (MOS) is carried out to validate the observed behavior of H with stability parameter within the framework of commonly used linear and non-linear similarity functions. The mathematical analysis reveals that the MOS theory is able to capture the observed behavior of H with ζ in stable conditions if the linear similarity functions are incorporated. However, in the case of non-linear similarity functions, the relationship between heat flux and stability parameter appears to breakdown for sufficiently large value of ζ. In unstable conditions, H is expressed as a function of ζ and temperature gradient and the behavior is analyzed by considering vertical surface layer potential temperature gradient as (i) a constant and (ii) a power-law function of heat flux. The analysis reveals that the nature of H with ζ in unstable conditions is consistent with that obtained theoretically from MOS equations by considering the vertical temperature gradient as a power-law function of heat flux having the exponent larger than 2/3.