Objective threshold determination for nighttime eddy flux filtering
Objective threshold determination for nighttime eddy flux filtering
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DOI：
10.1016/j.agrformet.2004.11.006
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发表时间：
2005-02-28
影响因子：
6.2
通讯作者：
Xu, LK
中科院分区：
文献类型：
--
作者：
Gu, LH;Falge, EM;Xu, LK
我们推荐一种自动统计方法(移动点测试，或MPT)来确定夜间涡流过滤中的摩擦速度(u(*))阈值。我们的目的是使U*阈值的确定具有客观性和可重复性，并保持通量治疗随时间和跨地点的一致性。在发展MPT方法时，我们认识到生态系统呼吸和u*都具有昼夜和季节周期，并且它们之间存在潜在的相关变化，在u*可以用作过滤标准之前必须去除这些变化。MPT使用迭代方法同时确定有效的温度响应函数，该函数用于归一化夜间通量测量，并基于归一化通量识别u*阈值。测试表明，MPT在各种场景和植被类型下都能很好地工作。我们还建议，为了提高夜间通量过滤器的可靠性，需要对平均CO2浓度分布进行详细的测量，以准确地计算冠层储量的变化。优选地，应该使用夜间流量足迹内不同位置的多个分布，以便可以进行容量平均的存储改变。此外，应努力尽量减少夏季夜间的测量差距，因为持续时间短，平静条件频繁，这极大地限制了可靠数据的数量。我们强调，MPT方法并不是夜间流量问题的最终解决方案。要克服在准确测量夜间通量方面的挑战，还需要不断的理论和实验研究。(C)2004爱思唯尔B.V.保留所有权利。
We recommend an automated statistical method (Moving Point Test, or MPT) to determine the friction velocity (u(*)) thresholds in nighttime eddy flux filtering. Our intention is to make the determination of the u* thresholds objective and reproducible and to keep flux treatment consistent over time and across sites. In developing the MPT method, we recognize that both ecosystem respiration and u* exhibit diurnal and seasonal cycles and there are potential correlative changes between them, which must be removed before u* can be used as a filter criterion. MPT uses an iterative approach to simultaneously determine a valid temperature response function, which is used to normalize nighttime flux measurements, and identify u* thresholds based on the normalized fluxes. Tests show that MPT works well for a variety of scenarios and vegetation types. We also recommend that in order to increase the reliability of nighttime flux filters, a detailed measurement of mean CO2 concentration profiles need to be employed to calculate canopy storage changes accurately. Preferably, multiple profiles at different locations within the nighttime flux footprint should be used so that volume-averaged storage changes can be made. In addition, efforts should be made to minimize measurement gaps in summer nights as much as possible because of the short-time duration and frequent calm conditions, which greatly limit the amount of reliable data. We emphasize that the MPT method is not meant to be a final solution to the nighttime flux issue. Continuous theoretical and experimental researches are still needed to overcome the challenges in measuring nighttime fluxes accurately. (C) 2004 Elsevier B.V. All rights reserved.