On the effect of the near-surface layer energy closure degree on land surface process simulations

Abstract

Many surface-layer observations have shown that the sum of sensible and latent heat fluxes was lower than the available energy when using the eddy covariance method. It means the observed energy was unclosed. The unclosed degree was usually 20% and even more in particular cases. The land surface models were developed based on the concept of energy balance between the atmosphere and land surface, and the parameterization scheme of the boundary layer in the land surface models were often developed from the observed turbulent heat fluxes. Thus, energy closure degree was bound to affect land surface models. Based on the data observed at SACOL site in spring, 2007, following the law of energy conservation, the residual part of the energy from its imbalance was distributed into the turbulent heat fluxes through the Bowen ratio so as to modify the direct measurement and make energy closed. Then two turbulent boundary layer parameterization schemes were developed as energy closure and unclosure schemes from the observed and modified turbulent heat fluxes. The impacts of energy unclosure and closure on land surface process simulations were investigated by using the land surface model SHAW with numerical simulation and comparing the simulations with the observation. The results suggest that the energy closure remarkably affect the land surface models. For taking using of the energy unclosure turbulent boundary layer parameterization schemes the model obviously overestimated upward longwave radiation and soil temperature. And in the case of using the schemes by modifying the measured turbulent heat flux to make the energy closed, the model is able to better simulate upward longwave radiation and soil temperature with any physical and bio-chemical properties of soil that are kept unchanged.