面积平均通量与光闪烁方法

Area Averaged Fluxes and Scintillometry

本文是对近20年新发展的双波段光闪烁方法的一个综述。陆面过程模式常常是基于局地或斑块尺度上的观测发展的,其与大气模式较大网格尺度的不匹配,显然会对后者的效能带来很大影响。如何扩展野外实验站点的代表性尺度已成为当前更好地了解陆面过程,进而改善陆面过程模式与大气模式网格尺度匹配问题的关键。可用的面积平均通量观测方法,包括以涡动相关方法为主的多点微气象观测、飞机观测、卫星和地面遥感等5种。其中,"光闪烁方法"是当前最为可行的、可以大到10 km尺度的感热通量和潜热通量观测方法,特别是,它可以应用于复杂下垫面包括山谷地区和城市等。光闪烁方法的理论涉及电磁波传输和大气湍流。文章从折射指数、结构参数、湍流谱等基本概念开始,对由对数光强方差计算折射指数的结构参数等基本公式的推导,由光程权重函数、空间谱权重函数、光强的时间序列谱等对闪烁仪主要工作尺度的了解,由折射指数结构参数计算温度、湿度的结构参数的方法,以及利用近地层相似理论计算感热通量和潜热通量等光闪烁方法的理论、公式和计算步骤等做了较系统的阐述。进而,在介绍双波段闪烁仪通量足迹函数之后,对光闪烁方法与涡动相关方法从特征、优势与缺点三方面做了比较;指出结合使用涡动相关和光闪烁两种方法,可以更好地进行面积平均通量分析,进而用于模式的发展和检验,以及更好的流域尺度的能量和水循环研究。最后,对光闪烁方法的应用,从较均匀下垫面、复杂下垫面、城市地区、遥感模式的地面"真值"及在大气模式中的应用等几方面做了介绍;特别是,结合黑河流域阿柔和大满两站2020年部分资料的分析,彰显了双波段闪烁仪对较大尺度感热、潜热通量观测的明显优势。但是,有关方法特别是观测水汽通量的微波闪烁仪的研制究竟为时尚短;相关硬件、软件、资料处理方法等,许多地方都还需要研究改进。相对于涡动相关方法,光闪烁方法的理论和数据处理都更为复杂;有关台站和资料分析人员,需要更好的物理和微气象学基础。国内闪烁仪在城市地区的应用,至今仍开展较少;青藏高原和内地一些湖面蒸发研究中的难点,也有望在应用光闪烁方法的过程中有所突破。基于光闪烁法等多种面积平均通量观测,对陆面过程研究做尺度扩展,并借以推动中—大尺度大气模式的发展,更是我们殷切期盼的。

This is a review of the Optical-Microwave Scintillometer(OMS)system newly developed in last two decades,which can measure area averaged sensible and latent heat fluxes over a scale of 1-10 km,especially over heterogeneous surfaces such as cross a valley or over urban areas. Among the methods of area averaged flux measurements,such as eddy-covariance based multi-point observations,air craft observations,satellite,and surface remote sensing etc.,scintillometry is probably the most feasible technique in getting areal fluxes up to 10 kilometers. The basic theory of scintillometry includes electromagnetic wave propagation,atmosphere turbulence,and micrometeorology,which are more sophisticated than that of the popular eddy covariance(EC)system.Based on the introduction of concepts such as refractive index,structure parameter and turbulence spectra etc.,basic scintillometer theories and equations are presented briefly,including:(1)calculation of structure parameter of refractive index via the variance of received light log-intensities;(2)the understanding of the working scale of scintillometry via the light-path weighting function,the spatial spectral weighting function,and the temporal spectral characteristics;(3)the derivation of structure parameters of temperature and humidity via the structure parameters of refractivity;(4)the calculation of fluxes by using the typical functions of Monin-Obukhov similarity;(5)the footprint analysis of scintillometry. A comparison between scintillometry and EC are presented in three aspects:‘Characteristics’,‘Advantages’ and‘Weakness’. It is clear that a combined use of EC& Scintillometry can provide better area averaged fluxes,and,refined flux aggregation schemes. Then,the application of scintillometry is introduced for rather homogeneous surfaces,complicated surfaces,urban areas,and,the‘ground truth’for remote sensing and the application of areal averaged fluxes in atmospheric models.The example utilizations of the OMS systems in the Arou alpine-meadow station and the Zhangye oasis station,of the Heihe River basin,clearly show the advantages of scintillometry over the EC in the measurements of larger scale evapotranspiration. Nowadays there are hundreds flux stations operating over various climate regions and surface states of the world. Upscaling of the spatial representativeness of these stations becomes a key point to better understanding the land surface processes,and improving the spatial matching between land surface models and meso/large scale atmospheric models. Comparatively,the time of development of scintillometry,particularly the microwave scintillometers in measuring water vapor fluxes,are still short. Further improvements of relevant hardware,data sampling,and data processing software etc. are still needed.