Biases of CO2 storage in eddy flux measurements in a forest pertinent to vertical configurations of a profile system and CO2 density averaging
Biases of CO2 storage in eddy flux measurements in a forest pertinent to vertical configurations of a profile system and CO2 density averaging
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森林中涡流测量中 CO2 存储的偏差与剖面系统的垂直配置和 CO2 密度平均相关
DOI：
10.1029/2006jd008243
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发表时间：
2007
影响因子：
--
通讯作者：
L. Gu
中科院分区：
文献类型：
--
作者：
Bai Yang;P. Hanson;J. Riggs;S. Pallardy;M. Heuer;K. P. Hosman;T. Meyers;S. Wullschleger;L. Gu
高大森林冠层中30分钟内的CO2储存通常对清晨和夜间的净生态系统交换（NEE）做出重大贡献。当正确测量并考虑二氧化碳储存时，可以大大减少平静夜晚对NEE的低估。使用CO2数据从12层的配置文件，我们证明，较低的冠层（下面的逆温）是一个不成比例的贡献者，总CO2储存。这是因为CO2密度的时间导数（c/ t）通常显示在夜间和从日出到1000小时，在生长和休眠季节，随着高度的降低，平均值和标准差的幅度增加。通过比较子集配置文件的12级基准配置文件的分辨率和配置的影响，在剖析系统上的CO2存储估计的准确性进行评估。它表明，在估计CO2存储的剖析系统的有效性不仅取决于它的采样水平的数量，但更重要的是，由它的垂直配置。为了优化剖面，需要在森林内所有垂直剖面中平衡两个因素的影响，c/ t和层厚度。作为CO2总储存量的主要贡献者，较低的冠层（具有相对较大的平均值和c/ t标准差）在剖面系统中需要比上面的层更高的分辨率。然而，如果上层冠层相对于下层冠层的采样过于稀疏，则剖面系统的性能可能会降低，因为在这种情况下，层厚度的影响超过c/ t的影响。我们还发现，由于冠层结构的复杂程度不同，更多的采样水平是必要的，在我们的网站，以达到同样的精度水平，在北方白杨网站。这些结果表明，为了达到足够的精度在CO2储存测量，采样水平的数量在剖面和它的设计应受场地属性，例如，冠层结构和由此产生的热力学和流动结构。如果对来自单个剖面的CO2密度进行时间平均，然后用于评估CO2储存，以使这种测量更具空间代表性，则与这种平均程序相关的偏差将不可避免。一般来说，在平均CO2密度中使用的较大窗口尺寸产生对CO2储存的较差估计。如果考虑绝对误差，似乎在一段时间内（夜间和清晨相对于上午晚些时候和下午，生长高峰期相对于生长初期），CO2储存越显著，平均程序的影响就越大。«少
CO2 storage in a 30-minute period in a tall forest canopy often makes significant contributions to net ecosystem exchange (NEE) in the early morning and at night. When CO2 storage is properly measured and taken into account, underestimations of NEE on calm nights can be greatly reduced. Using CO2 data from a 12-level profile, we demonstrate that the lower canopy layer (below the thermal inversion) is a disproportional contributor to the total CO2 storage. This is because time derivative of CO2 density ( c/ t) generally shows increasing magnitude of mean and standard deviation with decreasing heights at night and from sunrise to 1000 hr in both growing and dormant seasons. Effects of resolution and configuration in a profiling system on the accuracy of CO2 storage estimation are evaluated by comparing subset profiles to the 12-level benchmark profile. It is demonstrated that the effectiveness of a profiling system in estimating CO2 storage is not only determined by its number of sampling levels but, more importantly, by its vertical configuration. To optimize a profile, one needs to balance the influence of two factors, c/ t and layer thickness, among all vertical sections within a forest. As a key contributor to themore » total CO2 storage, the lower canopy (with relatively large means and standard deviations of c/ t) requires a higher resolution in a profile system than the layers above. However, if the upper canopy is over-sparsely sampled relative to the lower canopy, the performance of a profile system might be degraded since, in such a situation, the influence of layer thickness dominates over that of c/ t. We also find that, because of different level of complexity in canopy structure, more sampling levels are necessary at our site in order to achieve the same level of accuracy as at a boreal aspen site. These results suggest that, in order to achieve an adequate accuracy in CO2 storage measurements, the number of sampling levels in a profile and its design should be subject to the site properties, e.g., canopy architecture and the resulted thermodynamic and flow structures. If CO2 density from a single profile is averaged in time and then used in assessing CO2 storage to make this measurement more spatially representative, biases associated with this averaging procedure become inevitable. Generally, larger window sizes used in averaging CO2 density generate poorer estimates of CO2 storage. If absolute errors are concerned, it appears that the more significant the CO2 storage is during a period (nighttime and early morning hours versus late morning and afternoon, peak growing season versus early growing season), the larger effects the averaging procedure has.« less