Effects of surface source/sink distributions on the flux–gradient similarity in the unstable surface layer

Abstract

Based on the micrometeorological measurements at a heterogeneous farmland in the North China Plain, this study focused on the effects of surface source/sink distributions on the flux–gradient similarity theory in the unstable surface layer. Firstly, the quality of the micrometeorological measurements was evaluated by the analysis of the surface energy balance closure and the integral turbulence characteristics. In general, a 22 % deficit of energy balance was found at this site, with the sum of sensible and latent heat being smaller than the available energy. The normalized standard deviations of turbulent quantities behaved in accordance with Monin–Obukhov similarity theory. However, slight departures from the classical formulations might be caused by the surface heterogeneity. Then, the applicability of flux–gradient similarity over the heterogeneous surface was examined. The observed normalized wind gradients agreed with the classical universal function established over homogeneous surface. However, due to the effects of surface source/sink distributions, the observed normalized humidity and temperature gradients deviated from the classical universal functions. Our study shows that the classical universal functions, when adjusted by a coefficient considering the effects of surface heterogeneity, can be utilized to estimate fluxes via gradient method even though over the heterogeneous surface. This adjustment coefficient was found to decrease linearly from unity with the increase of the absolute value of the vertical flux divergence.