基于大孔径闪烁仪(LAS)测定农田显热通量的不确定性分析

显热通量是反映地-气间能量交换强度的重要指标.随着涡度相关(EC)技术的快速发展,点尺度的显热通量测量已经变得比较容易和准确可靠,像元尺度(公里尺度)的显热通量的地面测量成为连接遥感反演和地面验证尺度扩展的一个重要环节.大孔径闪烁仪(LAS)是目前唯一一种可直接用于像元尺度上显热通量测量的仪器.为了研究该仪器在使用过程的问题,比较了用2种方法同时在禹城农田上观测的显热通量,重点分析了两者之间产生误差或不确定性的原因.初步研究表明:①无论白天还是夜间,用大孔径闪烁仪(LAS)测量像元尺度上的显热通量是可行的,与涡度相关方法得到的结果总体相关性比较好,日变化趋势非常一致.②LAS与EC对比观测表明:白天显热通量较小时,HLAS比HEC偏大,白天显热通量较大时,HLAS比HEC小;当显热通量由小变大时,其平均绝对误差越来越大,相对误差越来越小.③由于下垫面的非均匀性,风速风向变化将会改变通量贡献区(footprint),从而造成2种结果出现偏差.另外,波文比、零平面位移和地表粗糙度等的不确定性都可能影响LAS的观测结果.

The Uncertainty of Sensible Heat Flux Measurement over Cropland Using Large Aperture Scintillometer (LAS)

Sensible heat flux is an important index for reflecting exchange intensity at surface-atmosphere interface. With the fast development of Eddy Covariance (EC) technique, the local sensible heat flux measurement becomes easier and more reliable. To study regional or global exchanges, the pixel-size flux measurement becomes an important and key step for up-scaling. Large Aperture Scintillometer (LAS) is the best instrument for measuring pixel-size sensible heat flux. To evaluate its accuracy and uncertainty, in this paper, the sensible heat fluxes over cropland measured by LAS and EC methods were compared, especially the reasons for errors and uncertainties were analyzed. It shows that: ①No matter in daytime or nighttime, the changing trends of the sensible heat flexes by two methods are the same. The relationship between them is very good, especially during strong exchange periods. ②There are still differences between two sensible heat fluxes. The mean abstract difference is less than 50 W/m². Generally, when daytime flux is weak (about less than 70 W/m²), the flux measured by LAS (H_LAS) is larger than that by EC (H_EC), and when flux is strong, H_LAS is smaller than H_EC. ③Due to the heterogeneous surface, the LAS footprint will be affected by the wind speed and wind direction. Advection may induce the uncertainty for LAS sensible heat flux. In addition, based on all the variables and parameters for LAS equations, the possible sources of the uncertainty, including the uncertainty of EC method and its effect on Bowen ratio, the inaccuracy and changeable parameters (e.g. zero plane displacement and surface roughness etc.) were analyzed.