复杂地形条件下涡度相关法通量测定修正方法分析

Analysis of Correction Method on Eddy Flux Measurement over Complex Terrain

复杂地形条件下涡度相关法通量观测修正方法以及夜间净生态系统CO2交换通量(NEE)质量评价方法,是世界范围内通量观测研究中的重大技术问题。本文以鼎湖山南亚热带针阔叶混交林生态系统涡度相关法碳通量观测数据,探讨典型订正方法对CO2通量估算的影响。主要结论有:1)基于流线坐标系统的二次坐标旋转(DR)、三次坐标旋转(TR)和平面拟合坐标旋转(PF),均使得CO2通量(Fc)绝对值趋于变小,变化幅度依次为:DR<PF<TR。坐标转换对Fc影响程度白天小于夜间,冬季小于夏季,冠层上方小于冠层下方。2)冠层下方和冠层上方CO2通量合适的u*订正阈值分别为0.05、0.2m s-1。3)对于涡度相关通量观测中普遍存在的夜间CO2涡度通量偏低问题,WPL订正没有帮助,坐标转换甚至起到反作用,冠层储存项订正在一定程度上减少夜间偏低程度,但效果不显著;u*订正可以明显减少夜间偏低幅度,但估算结果与箱式法相比依然偏低。夜间涡度通量观测数据最好能结合箱式法观测结果进行校正。

Based on the observation data of the flux measurement in the subtropical coniferious and broad-leafed mixed forest ecosystem in Dinghushan, the influence of the typical correction method on eddy flux estimates of carbon dioxide was analyzed. The main results were as follows ： 1 ） By using all of the three coordinate system transforming methods of Double Rotation （DR） , Triple Rotation （TR） and Planer Fit （PF） , the absolute values of CO2 flux （Fc） after terrain correction tended to be smaller than uncorrected values. Sequence order of changed Fc extend was DR 〈 PF 〈 TR. The influence of coordinate transforming on eddy flux was smaller in daytime than that in nighttime, smaller in winter than that in summer, and smaller above canopy than below canopy. 2） The appropriate corrected u ＇ threshold of Fc below and above canopy were estimated as 0.05ms^-1 and 0.2ms^-1respectively. 3） For the commonly problematic issue of underestimation of nighttime CO2 Eddy flux, WPL correction contributed nothing, while coordinate system transforming even made further underestimation. The correction of the nighttime storage item （Fs） increased slightly, but not significantly. The u^＊ correction of the nighttime Fc increased significantly, but still lower than that measured by chamber method. Nighttime Eddy flux measurement could be better corrected if chamber measurement was available and combined.