On the factors influencing surface-layer energy closure and their seasonal variability over the semi-arid Loess Plateau of Northwest China

摘要

The energy observed in the surface layer, when using eddy-covariancetechniques to measure turbulent fluxes, is not balanced. Important progresshas been made in recent years in identifying potential reasons for this lackof closure in the energy balance, but the problem is not yet resolved. Inthis paper, long-term data that include output of tower, radiation, surfaceturbulence flux and soil measurement collected from September 2006 to August2010 in the Semi-Arid Climate Change and Environment Observatory, LanzhouUniversity, in the semi-arid Loess Plateau of Northwest China, were analysed,focusing on the seasonal characteristics of the surface energy and thefactors that have impact on the energy balance closure (EBC). The analysisshows that (1) the long-term observations are successful; the interactionbetween the land and the atmosphere in semi-arid climates can be representedby the turbulent transport of energy. In addition, even though the residualis obvious, this suggests that the factors that impact the EBC are stable,and their seasonal variations are identical. The analysis also shows that(2) four factors have obvious impact on the EBC: the diverse schemes forsurface soil heat flux, the flux contribution from the target source area,the low-frequency part of the turbulence spectra, and the strength ofatmospheric turbulence motion. The impact of these four factors on the EBCare similar in all seasons. Lastly, the results indicate that (3)atmospheric turbulence intensity is a very important factor in terms of itsimpact on the EBC. The relative turbulence intensity, RI_w, characterisesthe strength of atmospheric turbulence motion, and is found to exert anoticeable impact on the EBC; in all seasons, the EBC is increased when therelative turbulence intensity is enlarged.