半干旱草原下垫面能量平衡特征及土壤热通量对能量闭合率的影响

土壤热通量在半干旱草原下垫面能量平衡研究中极为重要,土壤热通量估计不够准确是导致地表能量不平衡的一个重要原因。利用2008年6—9月锡林郭勒草原主生长期地表辐射、通量和土壤温度梯度观测资料,研究中纬度半干旱草原下垫面地表能量平衡特征。首先,在分析能量平衡各分量月平均日变化特征的基础上,通过对土壤热流量板观测的5 cm深度土壤热通量(G)的相位前移,研究了土壤热通量相位滞后对地表能量平衡产生的影响;其次,利用谐波分析方法,通过计算地表土壤热通量(Gs),分析了地表到热流量板之间的土壤热量储存对地表能量平衡的影响。结果表明:(1)将土壤热通量相位前移30 min,湍流通量与可利用能量(Rn-G)线性回归的斜率从0.835增加到0.842,地表能量闭合率提高了0.7%,但仍有15.8%的能量不闭合;(2)考虑了地表到热流量板之间的土壤热量储存之后,湍流通量与可利用能量之间的回归斜率达到0.979,能量不闭合程度仅为2.1%。

Characters of surface energy balance over a semi arid grassland and effects of soil heat flux on energy balance closure.

Soil heat flux is important for surface energy balance(SEB),and inaccurate estimation of soil heat flux often leads to surface energy imbalance.In this paper,by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole,Inner Mongolia,China from June to September 2008,the characters of the SEB for the semi-arid grassland were analyzed.Firstly,monthly averaged diurnal variations of SEB components were revealed.A 30-min forward phase displacement of soil heat flux(G) observed by a fluxplate at the depth of 5-cm below the soil surface was conducted and its effect on the SEB was studied.Secondly,the surface soil heat flux(Gs) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined.The results show that with the 30-min forward phase displacement of observed G,the slope of the ordinary linear regression(OLR) of turbulent fluxes(H+LE) against available energy(Rn-G) increased from 0.835 to 0.842,i.e.,the closure ratio of SEB increased by 0.7%,yet energy imclosure of 15.8% still existed in the SEB.When Gs,instead of G was used in the SEB equation,the slope of corresponding OLR of(H+LE) against(Rn-Gs) reached 0.979,thereby the imclosure ratio of SEB was reduced to only 2.1%.