闪烁仪法测算森林生态系统显热通量不确定性的研究

Sensible heat flux (H) is one of the driving factors of surface turbulent motion and energy exchange. Therefore, it is particularly important to measure sensible heat flux accurately at the regional scale. However, due to the spatial heterogeneity of vegetation above the underlying surface, the flatness of the surface, hydrothermal regime, and different weather conditions, it is difficult to solve the problem of the technology of flux measurement and the space representation of the data at the kilometer scale. The scintillometer have been developed into an effective and universal equipment for measuring heat fluxes at the regional-scale based on the turbulence effect of light in the atmosphere since the 1980s. The parameter directly obtained by the scintillometer is the structure parameter of the refractive index of air based on the changes of light intensity fluctuation. However, scintillometer data do not provide a direct measure of sensible heat flux, and additional data is required, such as the structure parameter of temperature, universal function for the temperature structure parameter, zero-plane displacement, surface roughness, wind velocity, air temperature and the other meteorological data to provide flux values. These additional parameters increase the uncertainty sources of flux because the difference between the actual feature of turbulent motion and the applicable conditions of turbulence theory. Most previous studies often focused on the constant flux layers that are above the rough sub-layers and homogeneous flat surfaces underlying surfaces with suitable weather conditions. Therefore, the criteria and modified forms of key parameters are invariable. In this study, the cork oak, cedar-black and locust will be the objects in the hilly area of northern China. On the basis of key research on the threshold and modified forms of saturation with different turbulence intensity, modified forms of Bowen ratio with different drying-and-wetting conditions, universal function for the temperature structure parameter under different atmospheric stability, the dominant sources of uncertainty will be determined. The above study is significant to reveal influence mechanism of uncertainty and explore influence degree of uncertainty with quantitative analysis. The study can provide theoretical basis and technical support for accurately measuring sensible heat fluxes of forest ecosystem with scintillometer method, and can also provide work foundation for further study on role of forest ecosystem in energy balance and climate change.

显然通量是地表物质和能量交换的驱动因子，闪烁仪法为区域（公里尺度）显热通量测算提供了重要思路。根据光在湍流中传播的特征原理，该方法的测算过程需结合微气象等相关参数，而由于实际湍流运动特征与湍流理论适用条件的差异，在实际测算显热通量的过程中存在诸多不确定性。目前有关不确定性研究仅限于湍流运动发展较好、干湿条件相对适中的气象条件以及地势平坦、植被分布均匀的下垫面条件下，导致关键参数修正形式单一。本研究以华北南部低丘山地人工林为研究对象，重点分析不同湍流强度下光饱和效应阈值及修正系数、不同干湿条件下波文比系数订正形式，计算出不同大气稳定度下结构参数湍流相似理论普适函数，比较确定不同湍流特征条件下不确定性的主导影响因子，以全面揭示不确定性的影响机制、定量分析影响程度，为闪烁仪法准确测算森林生态系统水热通量提供理论依据及技术支撑，为深入研究森林生态系统在能量平衡和调节气候变化中的作用提供工作基础。

显然通量是地表物质和能量交换的驱动因子，闪烁仪法为公里尺度显热通量测算提供了重要思路。根据光在湍流中传播的特征原理，该方法的测算过程需结合微气象等相关参数，而由于实际湍流运动特征与湍流理论适用条件的差异，在实际测算显热通量的过程中存在诸多不确定性。根据项目任务书，本研究以华北南部低丘山地人工林为研究对象，重点分析不同湍流强度下光饱和效应阈值及修正系数、不同干湿条件下波文比系数订正形式，计算出不同大气稳定度下结构参数湍流相似理论普适函数。具体工作有：(1)观测区人工林冠层湍流谱在惯性副区具有明显的耗散趋势，基本符合-5/3规律。冠层结构影响湍流强度。春季风速小于3m·s-1时和夏季风速小于2 m·s-1时，湍流发展最旺盛。无因次风速方差在水平方向比垂直方向上拟合效果好，符合1/3次幂相似规律。总体输送系数随稳定度的增加而逐渐减小。(2)用0.15m和0.075m的大孔径闪烁仪进行饱和信号界限的确定研究，通过比值法和拟合法得出“弱饱和”界限为0.0993，“强饱和”界限为0.3592；大孔径闪烁仪发生的信号饱和现象绝大多数属于“弱饱和”现象，可以通过后期订正提高数据观测质量。(3)在考虑湍流内尺度的前提下，采用数学修正方法计算得出饱和修正系数，并对观测结果进行修正分析。修正处理后，观测结果精度提高显著，并且饱和现象越明显，饱和修正效果越显著。(4)大气层结稳定时要注意分析观测信息的空间代表性；在风向近似垂直于光径路线时，得到的通量信息最全面完整。湍流活动较弱，源区面积较大；湍流活动旺盛，源区面积较小。(5)近中性条件下，机械湍流对MOST适用性有影响，而在对流不稳定时影响较小或可忽略。热力湍流越强，MOST适用性越好，反之则越差。湍流结构的实际情况可作为MOST适用性的判断方法，(6)受到波文比的影响，当环境为半干旱条件或干旱环境时，LAS的观测结果具有较高的相对误差。