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地球科学进展, 2018, 33(9): 898-913
doi: 10.11867/j.issn.1001-8166.2018.09.0898
涡动相关能量闭合问题的研究进展
Progress in the Energy Closure of Eddy Covariance Systems
周彦昭1,2,, 李新3,4,*,
1.中国科学院西北生态环境资源研究院 甘肃省遥感重点实验室,甘肃 兰州 730000
2.中国科学院大学,北京 100049
3.中国科学院青藏高原地球科学卓越创新中心,北京 100101
4.中国科学院青藏高原研究所,北京 100101
Zhou Yanzhao1,2,, Li Xin3,4,*,
1.Key Laboratory of Remote Sensing of Gansu Province, Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences, Lanzhou 730000, China
2.University of Chinese Academy of Sciences,Beijing 100049, China
3.CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China
4.Institute of Tibetan Plateau Research,Chinese Academy of Sciences, Beijing 100101, China
 引用本文:
周彦昭, 李新. 涡动相关能量闭合问题的研究进展[J]. 地球科学进展, 2018, 33(9): 898-913, doi:10.11867/j.issn.1001-8166.2018.09.0898
Zhou Yanzhao, Li Xin. Progress in the Energy Closure of Eddy Covariance Systems[J]. Advances in Earth Science, 2018, 33(9): 898-913, doi:10.11867/j.issn.1001-8166.2018.09.0898

摘要:

涡动相关系统(EC)观测的湍流通量总是低于有效能量(净辐射与地表热储量之差),两者的差值可达有效能量的10%~30%。这种EC观测的湍流通量低估的现象,称之为“EC能量闭合问题”,已成为微气象学研究中的一大瓶颈。过去的几十年间大量学者对此问题进行了深入研究,积累了丰富的认知。系统介绍了EC能量闭合问题的成因,总结了学者对于EC能量闭合问题的认识以及基于这些认识提出的可能的修正方案,并详细评述了这些修正方案的优劣。与之前的综述不同,重点介绍了基于大涡模拟(LES)研究EC能量闭合问题的方法和进展。指出目前缺少高精度且时间连续、空间密集(如矩阵式观测)的地表通量观测数据集、LES尚不足以完全表征真实的EC观测、大尺度湍涡影响EC能量闭合的机制尚不清楚等问题。今后应加强大尺度湍涡影响EC能量闭合的机制研究,同时开展空间矩阵式观测,验证LES的结果。

关键词: EC ; 能量闭合 ; 大涡模拟 ; 陆面过程 ; 微气象

Abstract:

The sum of turbulent fluxes measured by the eddy-covariance method is often 10%~30% lower than available energy (i.e., the net radiation minus the ground heat flux). This systematic bias in the EC method is called the "EC energy closure problem" and has been one of the biggest challenges in micrometeorology. In the past decades, lots of studies have focused on this problem and have found extensive knowledge about it. In this paper, we introduced the reasons to the EC energy closure problem and systematically summarized the understandings on this problem with a discussion regarding the merits and limitations of possible correction methods. Compared with previous overview studies, our study focused on the methods and progresses on the EC energy closure problem using the Large Eddy Simulation (LES). The existing problem, e.g., the lack of high quality and high spatial density of "surface true fluxes" observations (e.g., matric observations), the failure of fully emulating the EC using the LES and how to mechanistically understand that large eddies affect turbulence structures and lead to non-closure problem, hinder the further understanding of the EC energy closure problem. Thus, in the future, focus should be put on the mechanistic understanding of how large eddies affect the non-closure problem and some dense field observations to obtain the high quality "surface true fluxes".

Key words: Eddy Covariance ; Energy Closure ; Large Eddy Simulation ; Land Surface Progress ; Micrometeorology.
1 引 言

地气间的物质、能量交换直接影响着大气环流和气候变化,在地球系统科学中占有非常重要的地位。在过去的几十年中,全球开展了大量的陆面过程试验,例如“黑河地区地—气相互作用野外观测实验研究”(Heihe River Basin Field Experiment,HEIFE)[1]、“黑河综合遥感联合试验”(Watershed Allied Telemetry Experimental Research,WATER)[2]和2012—2015年在黑河流域开展的“黑河流域生态—水文过程综合遥感观测联合试验”(Heihe Watershed Allied Telemetry Experimental Research,HiWATER)[3]。在陆面过程试验中,涡动相关(Eddy Covariance,EC)是最常用也是最直接测量近地表与大气间水热以及痕迹气体通量的方法之一。在过去的几十年,该方法受到了大量学者的广泛关注,得到了空前的发展。目前已经建立了包括美洲通量网(AmeriFlux)、亚洲通量网(AsiaFlux)和中国通量网(ChinaFlux)等在内的全球通量观测网络(FluxNET),一些特定研究区域的通量网如城市通量网(Urban Fluxnet)等也逐步建立起来。到目前为止全球通量网的观测站点已经超过900个(https:∥fluxnet.fluxdata.org/sites/site-summary/),且还在不断增加中。基于EC的通量观测数据已经广泛用于遥感通量产品的验证[4]、陆面过程模型的发展与验证[5]以及全球能量和物质循环研究[6]等。

然而,大量的观测试验发现,全球大部分的EC站点观测的感热通量(H)和潜热通量(LE)之和低于有效能量(净辐射Rn与地表热储量G之差),两者的差值可达有效能量的10%~30%[7,8,9,10,11,12]。目前常用能量闭合率(Energy Balance Ratio,EBR)或能量不闭合率(I)来衡量EC的能量闭合情况,即:

EBR=H+LERn-G,     (1)I=Rn-G-H-LERn-G=1-EBR(2)

EBR越高,I越小,则EC能量闭合越好。表1列出了主要的野外EC观测试验中能量闭合情况。从表1中可以看出在均匀下垫面(如荒漠)能量闭合情况较好,但是在其他下垫面,I为10%~30%。这种EC观测的湍流通量低估的现象,称之为“EC能量闭合问题”。为了深入分析该问题的成因,众多学者于2000年夏季在美国加州开展了专门的野外试验即能量平衡试验(Energy Balance Experiment in 2000,EBEX-2000)。虽然该试验取得了一些关于EC能量闭合问题的重要认识[13,22,23],但是对于EC能量闭合问题的成因和解决方法尚无定论[13,24,25]。EC能量闭合问题的出现,使得EC的精度和可靠性备受质疑,同时也增加了基于EC观测通量数据的相关研究的不确定性[6,26]。目前EC能量闭合问题已经成为微气象和地气相互作用研究的瓶颈之一。该问题的存在也阻碍了遥感通量算法的改进、陆面过程模型的改善[5],降低了气候预报的可靠性[26]

表1

主要野外观测试验中能量闭合情况(据参考文献[13]修改)

Table 1

The energy residuals in field observations (modified after reference[13])

在过去的几十年,大量学者对此问题进行了深入探讨和分析[9,24,25,27~30],积累了丰富的关于EC能量闭合问题的认知。在此背景下,系统总结目前学者们对于EC能量闭合问题的认识,评述已存在的修正方案的优劣,对于清晰地认识EC能量闭合问题的根源,探讨未来EC能量闭合修正的可能途径十分重要。

本文首先简单阐述EC观测的基本原理,总结EC能量闭合的可能成因,然后介绍基于LES的EC能量闭合问题的研究方法和进展。最后总结目前对EC能量闭合问题的认知、存在的问题及未来可能的研究方向。需要注意的是,由于夜间通量值较小,本文只关注于白天时段内对EC能量闭合问题的认知。

2 EC观测的基本原理

EC通过计算大气垂直速度涨落(也称脉动)与所输送量涨落的协方差,得到一段时间内的垂直湍流通量,是目前最为直接的通量观测方法之一。下面介绍EC观测的基本原理。

2.1 基于控制体积的EC观测原理

Finnigan等[31]基于物质守恒推导了基于控制体积的EC观测原理,阐述了EC隐含的假设。为了简化,这里只列出了标量的守恒方程。关于基于控制体积的EC观测原理的详细推导,可以参看参考文献[32]。


图1

EC控制体积方法示意图[31]
u,v,w 分别为顺风向速度、侧风向速度和垂直速度; c为测定的标量

Fig.1

The sketch of control volume method[31]
The u, v, w are the horizontal, lateral and vertical velocity,respectively; The c is the interested scalar

任意控制体(图1)中质量守恒方程为:

0hAc¯tdAdz+ 0hAuc¯x+vc¯ydAdz+ Awc¯dA= ASc¯dA,(3)

式中:A为控制体积下垫面面积;h为EC观测高度;u,v,w分别为顺风向速度、侧风向速度和垂直速度;c为标量,可以是空气密度、水汽密度等;Sc为标量c的源汇项。因为地表为控制体积的下边界,Sc即为下垫面的通量(如感热、潜热通量),故公式(3)右边项即为EC要测定的下垫面的平均地表通量值F。公式(3)也可以表示为:

F= ct¯dz+[ w'c'¯]h+ 14[ v'c'¯] +[ w-c-]h+ 14[ v-c-],(4)

式中:[]代表控制体积内平均,[]代表控制体积面平均。等号右边第一项为体积内的存储项,第二、三项分别为垂直和水平湍流通量,最后2项为垂直和水平平流通量。水平湍流和平流通量包括顺风向和侧风向共4个面的通量值。在水平均匀情况下,水平通量近似为0,此时:

F=ct¯dz+[w'c'¯]h+[w-c-]h(5)

当存储项可以忽略时(如低矮植被),同时不存在垂直平流通量 w-=0时,公式(5)可以简化为:

F=[w'c'¯]h(6)

此时EC观测的垂直湍流通量等于地表通量。很显然,从公式(4)到公式(6)过程中,需要满足几个条件/假设:①时间平稳(stationary),即观测时段内地表到观测高度的存储项可忽略;②空间水平均匀(homogeneity),即无水平平流和湍流通量;③垂直平均速度为0(无水平辐合和辐散),即无垂直平流。

理论上,只有满足以上所有假设,EC观测的垂直湍流通量才等于地表通量,近地表的能量才是闭合的,这在非常均匀的下垫面如沙漠地区,已经得到了证实[33,34]。然而在实际条件下,大多数EC站点很难满足上述3个条件,特别是条件③。大尺度地表异质性或边界层顶部的卷夹作用,都可能会引起EC站点处大气的上升或下沉运动,导致EC能量闭合问题[10,11]

2.2 EC能量闭合问题的成因

只有满足特定的条件,EC观测的垂直湍流通量才等于地表通量。实际观测中,任何一个条件不满足,都会导致EC能量闭合问题的出现。除此之外,仪器的测量误差、数据处理误差以及辐射和湍流通量观测footprint的差异都有可能导致EC能量闭合问题。下面分别介绍上述因子对于EC能量闭合问题的影响。

(1) 仪器的测量误差。EC能量闭合问题首先怀疑的是由于仪器的测量误差导致。王介民等[21]分析了净辐射、地表热通量和湍流热通量的观测误差。目前净辐射的观测误差在5%以内,最大可达25 W/m2[13,33]。而经过严格标定超声风速仪和水汽分析仪等测量误差为5%~10%(10~20 W/m2)[21]。相比之下,土壤热通量的测量误差则较大。由于不同土壤导热率差异很大,且随土壤含水量等变化,导致土壤热流板的观测精度一般较低,其观测误差一般在20%~50%(20~50 W/m2)[9,21,35]。另外,热流板埋深到地表土壤热储存,土壤热流板无法观测到,需要用土壤温度和含水量梯度资料加上已知的土壤热参数计算[9,21]。由于浅层土壤的垂直热通量梯度很大,该修正项可能会存在较大的计算误差[24,28,35~37]

(2) 数据处理引入的误差。EC观测湍流热通量需要经过十分复杂的质量控制和修正[38]。如平稳性检测、坐标旋转、谱修正和攻角订正等[39,40]。这些数据质量控制方法和数据处理算法会剔除或过滤掉一些数据(如低频、非稳态数据),导致湍流通量的低估;同时一些数据处理算法潜在的假设(如平稳性)也可能导致湍流通量的低估。

(3) 观测高度和采样空间尺度的影响,即辐射通量与湍流通量的footprint差异的影响[41]。辐射通量的footprint与观测高度有关,其footprint一般在几平方米到几十平方米,而湍流热通量的footprint不仅和观测高度有关,还与风速、风向、大气稳定度以及下垫面的状况有关,其footprint一般在几百平方米左右。在非均匀地表,辐射通量和湍流通量的footprint差异,导致了辐射的观测对象与湍流的观测对象不一致,会加剧EC能量闭合问题。Xu等[7]研究发现EBR与footprint内地表异质性呈负相关(footprint内地表异质性越大,EBR越小)。

(4) 大尺度低频湍涡的影响。众多的数值模拟试验研究[42,43,44,45,46]表明,在水平均匀的情况下,水平平流通量是可以忽略的,但是垂直平流通量是不能忽略的[13,42]。由于湍流有组织结构的时间尺度大于平均时间,垂直速度的时间平均并不等于0,在此时间段内垂直平流通量不为0。在地表异质性情况下,如地表热力(灌溉或者云过境)和动力非均匀(地形或者树木和建筑物),会诱发二次环流,不仅引起垂直平流,而且会引起较大的水平平流。此时单点EC通量观测往往只能触及其局部特征即单点的湍流通量,无法表征地表真实通量[21,42,47]。Panin等[48]发现EC站点周边地表异质性尺度与EBR存在负相关关系;Stoy等[49]通过分析欧洲通量网的EC观测数据发现,EBR与EC周边大尺度范围内的地表异质性呈负相关。上述相关关系间接证实了大尺度湍涡对于EC能量闭合的影响。Inagaki等[47]和Zhou等[42]通过数值模拟直接证实了地表异质性诱发的大尺度湍涡导致了较大的EC能量闭合问题。

随着技术的进步和人们对于EC能量闭合问题认识的不断深入,通过一定的方法可以减小或者消除以上原因对于EC能量闭合的影响,如:

(1) 在过去几十年,观测仪器的精度和可靠性不断提高,数据处理和质量控制算法的精度不断提高。经过严格的仪器标定、维护、数据质量控制和修正,可以有效地控制辐射通量和湍流通量的系统误差和随机误差,其最终结果有较高的精度(约5%)[9,23,50]。同时使用改进的地表热通量的观测方法和计算方法[21],可以将地表热通量的观测误差控制在一定误差范围之内[36,37]。Mauder等[50]发现数据处理引起的误差不足以解释能量闭合,经过严格质量控制和数据修正的EC观测的湍流热通量仍然被低估20%~30%[9]。左洪超等[27]分析野外观测资料后认为,数据质量控制方法和地表热通量的计算方法固然能影响EBR的大小,却无法改变EBR随大气运动状态变化的实质。总之,经过严格的仪器标定、维护、数据质量控制和修正,仪器的测量误差和数据处理引入的误差不足以解释近地表出现的EC能量闭合问题。

(2) 为了减少两者footprint差异对于EC能量闭合的影响,一般选取周边环境相对均匀的地方设立EC站点。此时辐射通量与湍流热通量footprint的差异不足以对能量闭合产生显著的影响[13]。若EC站点周边地表非均匀,此时需要空间多个站点的辐射、湍流通量的观测以及其他辅助观测如地表温度、气温、反照率、发射率和土壤水分等来分析辐射通量和湍流通量的footprint空间差异对于EC能量闭合的影响。遥感技术的发展为深入分析两者footprint差异提供了可能。

(3) 目前有2种可能的方法来消除或者减小大尺度低频湍涡对于EC能量闭合的影响。一种是延长平均时间[31,50,51];一种是空间平均[44~46,52,53]。延长平均时间,EC可以采样到更多的低频湍涡信息,提高能量闭合率[31]。但是由于天气日变化等影响,延长平均时间会增加EC观测数据的非平稳性,增加通量观测的随机误差[43]。目前认为平均时间最长可为1小时,在该平均时间内,EC能量闭合问题依然存在。相比之下,空间平均可能是较为理想的实现EC能量闭合的一种途径。正如前文所述,单点EC通量观测往往只能触及大尺度低频湍涡局部特征即单点的湍流通量[21,47],无法从根本上解决EC能量闭合问题。而多个站点的空间平均测量则可以观测到大尺度湍涡的整体特征如水平和垂直平流,克服了单点EC通量观测的局限。因此在理论上,空间平均可以实现EC能量闭合。然而实际观测中,实现空间平均是十分困难的。一方面是由于水平梯度往往较小,局部的地表异质性会造成局部较高的水平梯度,这对于仪器的空间布设和观测精度要求很高;另一方面,垂直梯度相对较大,其精确测量需要多层的垂直观测。因此要实现精确的平流通量观测需要空间密集的高精度观测,这种代价是极高的,在现实中几乎是无法实现的[53,54,55,56,57]。如Lee[58]尝试使用单点的垂直平流通量修正EC能量闭合,尽管在某些站点上可行,但后续的理论[59]和实验[46]研究证明单点的垂直平流通量无法修正EC能量闭合问题。Mauder等[55]曾尝试使用多个站点数据来测量平流通量,发现观测的平流通量误差较大,难以用于EC能量闭合问题的修正。目前最有前景的方法是使用高精度的数值模拟如大涡模拟(Large Eddy Simulation,LES)来数值模拟平流通量。随着LES的不断发展,这种方法已经逐渐成为可能[42,43,44,45,46,47]。在下一节,本文将详细阐述基于LES的EC能量闭合问题的国内外研究进展。

总之,通过严格的标定、数据质量控制以及合理的站点选取,仪器的测量误差、数据处理误差以及辐射和湍流通量观测footprint的差异不足以解释EC能量闭合问题。目前微气象学者公认的能量闭合问题的主要原因是:除了EC观测的湍流通量外,存在其他方式的通量输送,如地表异质性引起的平流通量输送和大尺度低频运动引起通量输送[25]

3 基于LES的EC能量闭合问题研究进展

目前EC能量闭合问题的研究主要是基于野外EC的观测数据,分析EBR的变化以及其与环境因子间的相关关系。该方法基于实际的观测数据,方法简单,结果可靠,是目前研究EC能量闭合问题最主要的方法。

然而,由于大气环境多变,观测仪器易受环境因子和电磁因素干扰,不可避免地产生噪声。同时,由于大气条件无法控制,难以单独分析某一变量对EBR的影响。为了克服上述缺陷,同时便于EC能量闭合问题的机理研究,一些学者基于LES数据,分析I的变化,验证并发展了I与相关变量的定量关系,探讨了可能的EC能量闭合修正方法[42,43,44,45,46,47]。与第一种方法相比,基于LES的分析方法无需考虑仪器测量误差,同时可以设置不同情境分析不同因子对I的影响,故可以更加深入、方便地研究EC能量闭合问题的机制。基于LES和野外实测数据研究EC能量闭合问题的优缺点见表2

表2

基于LES和野外实测数据研究EC能量闭合问题的优缺点

Table 2

The advantages and disadvantages of the methods based on LES and filed observations

3.1 LES介绍

通过计算机进行数值模拟流体流动等物理现象的技术称为计算流体力学(Computational Fluid Dynamics,CFD)已经成为研究湍流的主要手段之一。根据数值计算采用模型的不同,常用的CFD模拟方法可以分为3类:①直接求解流体控制方程(即Navier-Stokes方程)的直接模拟方法(Direct Numerical Simulation,DNS);②将控制方程进行时间平均化处理的雷诺平均方法(Reynolds Average Navier-Stokes,RANS);③将控制方程进行滤波处理的LES法。

DNS无需对Navier-Stokes方程作任何简化或近似,直接求解湍流中各级尺度涡的运动。因此理论上DNS可以获得湍流场的全部信息,分辨出湍流中所有的空间结构[60]。然而由于湍流是多尺度的不规则运动,高雷诺数湍流(如大气湍流)包含很宽的尺度范围,湍流的直接数值模拟需要天文数字的网格[60]。在目前的计算条件下,DNS只能作为低雷诺数的简单湍流的研究工具,尚无法实现高雷诺数湍流的直接求解[61,62]。为了减少计算量,RANS只计算大尺度平均流动,湍流脉动对平均流动的作用(即雷诺应力)使用模型假设来求解,即RANS只求解雷诺平均后的Navier-Stokes方程。与DNS相比,RANS计算量小,可以直接计算高雷诺数湍流的复杂流动,是目前工业上广泛应用的CFD方法。然而由于雷诺应力主要由大尺度脉动贡献,而后者与流动的边界条件密切相关,因此对于不同类型的流动,RANS模式的形式或系数需要经验性修正,无法到达普适性,计算准确性较差,难以给出可信的预测结果[61,63]

LES的基本原理是把湍流运动分为可解尺度(大尺度)和亚格子尺度(小尺度)。对于受边界条件影响较大的大尺度运动直接数值求解,而对于具有较好普适性的小尺度湍流运动,则通过构造亚格子(subgrid-scale,SGS)模型来模拟其对大尺度运动的影响。LES的计算量介于DNS和RANS之间,能够以相对较小的代价获得湍流大尺度脉动的信息,模拟非定常湍流的瞬时场,获得比RANS分辨率高、准确度高的结果[60]。另外,由于边界条件对小尺度脉动的影响较小,亚格子模型有较好的普适性。正是由于LES具有其他模拟手段所没有的独特模拟能力,LES是目前研究大气湍流最有前景的数值模拟方法[64]。目前LES已经成为研究高精度非均匀下垫面地气交换[61,64~69]和地表能量平衡[42~47,70~72]的热点。

3.2 LES基本原理

3.2.1 控制方程

大涡模拟第一步是通过滤波分离出湍流大尺度运动。对于任意一个瞬时流动变量 φ~(x,t),滤波运算可以表示为:

φ~(x,t)=G(|x-x')φ(x',t)dV',(7)

式中:G(|x-x'|)为滤波函数。常用的滤波函数有物理空间的盒式滤波器、高斯滤波器和谱空间的截断滤波器[60]。经过滤波后的连续方程、动量方程、浓度和温度输运方程分别为:

u~ixi=0, (8)

u~it+ u~iu~jxj=- 1ρp~xi+v2u~ixjxj+ τijxj+ θ~-θ0θ0i3+ f~i, (9)

c~t+ c~u~jxj=D2c~xjxj+ τcjxj+ Sc, (10)

θ~t+ θ~u~jxj=k2θ~xjxj+ τθjxj+ Sθ,(11)

式中: u~i, c~θ~分别为过滤后(即可解尺度)的速度、浓度和温度;Dk分别为质量扩散系数和热扩散系数; ScSθ分别为浓度源和热源; f~i为附加力, θ0为参考温度。τij,τcjτθj分别为亚格力应力通量、质量通量和热通量,是过滤掉的小尺度脉动和可解尺度湍流间的动量、质量和热量输运。要实现大涡数值模拟,必须构造SGS模型,这是实现LES的关键。

3.2.2 SGS模型

在过去的几十年中,大量学者提出了各种各样的亚格子模式[60]。本文只简单介绍湍流模拟中广泛应用的涡粘和涡扩散模式。具体表达式如下:

τij=2KS~ij+13τkkδij,(12)τcj=KSctc~xj,(13)τθj=KPrtθ~xj(14)

其中:

S~ij=12u~ixj+u~jxi,(15)K=Cs2ΔS~,(16)S~=2S~ijS~ij0.5,(17)

式中: S~ij为可解尺度速度变形率张量;K为常用的Smagorinsky亚格子涡粘系数;Cs是模式系数(约为0.18);Δ为网格长度;SctPrt分别为湍流Schmidt和Prandtl数,是标量湍流模拟中重要的参数,一般常用0.72。

Smagorinsky模式具有形式简单和使用方便的优点,已经广泛应用于大气与工程湍流模拟。然而大量的试验证明Smagorinsky模式耗散过大,为了克服这一缺点,不得不采用额外的修正或者其他的计算方法。比如大气湍流LES中常用的基于亚格子湍动能模式计算亚格子涡粘系数K,详细的内容,可以参看参考文献[60]。

3.3 基于LES的EC能量闭合问题研究方法

目前基于LES研究EC能量闭合问题有2种方法。一种是将某一高度处的空间湍流通量当做“真值”。通过计算该高度处的EC计算的湍流通量与该高度处的“真值”的差异来分析EC能量闭合问题。另外一种方法是基于控制体积的方法(见1.1节),通过分析控制体积内不同通量组分来分析EC能量闭合问题。本文称这2种方法分别为“控制面”和“控制体”的研究方法。下面分别介绍这2种方法。

3.3.1 基于控制面的EC能量闭合问题研究方法

空间中任一高度面的瞬时垂直通量F可以表示为:

F(x,y,z,t)=wc,(18)

式中:w为垂直速度,c为需要测定的标量(如位温θ)或矢量(水平风速u,v)。根据Reynold分解法则,公式(18)的时间和空间平均的垂直通量为:

F(x,y,z,t)=w-c-+w'c'¯,  (19)[F](z,t)=[w][c]+[w″c″],(20)

式中: w'c'¯为EC测定的某个位置时间平均的垂直湍流通量,[w″c″]为瞬时的空间平均的垂直湍流通量(如航空湍流测量)。其中上划线表示时间平均,方括号([ ])表示空间平均。公式(19)和公式(20)等号右边第一项分别为时间和空间的平均通量。公式(19)的空间平均和公式(20)的时间平均,得:

[F-]=[w-c-]+[w'c'¯], (21)[F]¯=[w][c]¯+[w″c″]¯,(22)

式中:[ F-]等于 [F]¯,称为“通量真值”。

在均匀地表,如果平均时间足够长,时间平均近似为空间平均(遍历定理),此时EC能够采样到足够多湍流有组织结构的样本点, w-近似为0。然而实际上,由于大气的非平稳性(如大气日变化),平均时间通常都是有限的,时间平均并不等于空间平均,遍历定理不成立,此时EC无法采样到足够多的湍流有组织结构样本点, w-不等于0[42,43,44,45,46,47],因此导致了能量不闭合。基于此,均匀条件下的能量闭合问题即为时间平均收敛问题(或遍历假设成立问题)。

另外,在Boussinesq流体以及周期边界下,[w]理论上应该等于0[45]。然而,由于数值计算误差,[w]尽管远小于w″但其并不等于0。为了减小数值误差的影响,一般将公式(22)右边第二项即空间平均的垂直湍流通量( [w″θ″]¯)而不是整个公式(22)作为“通量真值”。基于此,任一点的I为:

I(x,y,z)=-w'θ'¯-[w″θ″]¯[w″θ″]¯,(23)

空间平均的能量不闭合率[I]为:

[I]=-w'θ'¯-w″θ″¯[w″θ″]¯=w-θ-[w″θ″]¯(24)

假设,每个网格点即为虚拟的EC站点,那么[I]表示EC测定的时间平均的湍流通量与“真实通量”之间的平均差异。[I]已成为目前衡量EC能量闭合程度的一个重要指标[42,43,44,45,46,47]

以上为水平均匀地表情况下,I的计算方法。当地表非均匀时,空间平均无法在整个水平地表,而只能在水平均匀的方向来近似Reynolds平均。即上述方法只能用于一维地表异质性情况,无法应用于二维地表异质性情况。

当水平地表存在一维异质性(如x方向存在异质性,y方向为均匀地表)时,使用y方向的空间平均(用下标y来表示,例如[w]y)来近似Reynolds平均。由于地表异质性引起的中尺度或二次环流,y方向空间平均的垂直速度(即[w]y)并不等于0,此时无法简单地使用公式(23)来计算I。若平均垂直速度的值远大于其数值误差,那么数值误差对平均垂直速度的影响很小,可以忽略其影响。此时,可以采用公式(23)来计算I。同时,为了分析中尺度或二次环流对于EC能量闭合的影响,将时间平均项(如 w-c-)分解为时间平均的空间平均(即 w-yc-y)以及相应的扰动项(即 w-c-),得:

F-=[w-]y[c-]y+w-c-+w-[c-]y+c-[w-]y+w'c'¯(25)

同时,y方向和整个地表的时间平均的通量为:

[F-]y= [[w-]y[c-]y]y+ [w-c-]y+ [w'c'¯]y, (26)

[ F-]=[ [w-]y[c-]y]+[ w-c-]+[ w'c'¯]。(27)

需要注意的是,尽管水平平均并不等于Reynolds平均,这里使用[ F-]是因为其可以简单地表征非均匀地表的“代表性通量”。

公式(27)与Mahrt[73]中公式(6)以及Inagaki等[47]论文中公式(8)很相似。其中公式(27)等号右边第一项为热力中尺度环流引起的垂直通量。该项在水平均匀地表等于0。第二项为湍流有组织结构引起的垂直通量;第三项为EC可以测定的垂直湍流通量。湍流有组织引起的垂直通量可以看做是时间平均项的空间相关导致的扩散通量(dispersive fluxes)。注意到在冠层与大气交互研究中,扩散通量近似为0[74,75];然而,由于地表异质性引起的长时间的空间结构(即二次环流),该扩散通量并不等于0。

与公式(24)类似,[I]为:

[I]=-w'θ'¯-F-F-(28)

需要注意的是,在上面I的计算中,需要将亚格子通量考虑进去。

3.3.2 基于控制体积的EC能量闭合问题研究方法

正如3.3.1节所示,上述方法只能用于一维异质性地表情况,无法应用于二维异质性地表情况。本节介绍二维地表异质性情况下,I的计算方法。

Finnigan等[31]首次基于控制体积方法阐述了EC方法的假设和局限性。Eder等[72]首次使用该方法研究了EC能量闭合问题。该方法中地表通量为“地表通量真值”,从而避免了2.3.1节中空间平均的局限性。如图2所示,控制体积内时空平均通量的守恒方程为:

[ HFX¯]=[ w'c'¯]+ 14[ v'c'¯]+[ w-c-]

+ 14[ v-c-]+ ct¯dz,(29)

式中:HFX表征地表通量即“地表通量真值”,v表征垂直于控制体积侧面的速度矢量。 w'c'¯为EC测定的时间平均的垂直湍流热通量。很显然,公式(29)等号右边中,除了第一项,其余项不为0导致了EC能量不闭合。这些项分别为:水平辐散通量(the horizontal flux divergence)、垂直平流通量(the vertical mean advection)、水平平流通量(the horizontal mean advection)以及存储项(the storage flux)。很显然,基于I的定义,此时[I]定义为:

[I]=[HFX¯]-w'c'¯[HFX¯](30)

与控制面的方法类似,I的过程中需要将亚格子通量考虑进去。


图2

控制体积示意图(据参考文献[72]修改)

Fig.2

Scheme describing control volume for the sensible heat flux (modified after reference[72])

3.3.3 2种方法的优缺点

尽管上述2种方法存在一定的差异,实际上两者之间存在一定的关系(图2)。从图2中可以清楚地看出,控制面方法计算的垂直通量只是控制体积顶部面的垂直通量;而只有当水平通量和存储量可以忽略不计时,控制体积顶部面的垂直通量才等于地表真实通量。因此,基于控制面的方法计算方法隐含着水平通量和存储量可忽略的假设。基于此,理论上基于控制面的方法只是一维的,是基于三维控制体积方法的简化结果。表3列出了2种方法的优缺点。

表3

基于控制面和控制体的方法的优缺点

Table 3

The advantages and disadvantages between the control face and volume methods

由于无需计算水平通量,特别是水平平流通量,故基于控制面的方法计算对于网格分辨率的要求要比基于控制体积的方法要低,其计算量也小得多,是过去十几年中使用最多的基于LES研究EC能量闭合问题的方法[42,43,44,45,46,47]

3.4 基于LES的EC能量闭合问题研究进展

Kanda等[46]首次基于LES分析了在水平均匀条件下,干对流边界层白天时EC的能量闭合问题。结果发现:由于湍流有组织结构的存在,单个EC站点会出现系统性低估,在100 m高度处I为5%~25%,且I随风速的升高,快速降低。Steinfeld等[45]在此基础上,使用更高分辨率(10 m)的LES模拟,分析了近地面高度(10 m)处的I,发现在近地面(10 m)处湍流有组织结构引起的I大概在5%以内。此后,Huang等[44]使用量纲分析方法,综合考虑了边界层顶卷夹和地面扩散对于能量闭合的影响,给出了I与地表摩擦速度和对流边界层速度尺度之间的参数化方案。Zhou等[42]基于WRF LES发现,均匀地表上述参数化方案在地表温度日变化时,依然成立。以上这些研究都是给定气象条件下理想情景的LES研究。Schalkwijk等[43]以荷兰Cabauw站点2012年气象观测数据作为边界条件,进行了长达1年的LES模拟。与此前的研究相比,这项研究可以进行更长时间尺度(年尺度)的能量闭合分析,同时分析不同环境因子在不同时间尺度对于能量平衡的影响。陈家宜等[70]首次在国内开展类似的研究并首次考虑了水汽的影响。结果发现:湍流有组织结构是导致近地表EC能量不闭合的主因;近地面层潜热通量与感热通量测算的低估份额相近,但随着观测高度的增加潜热通量低估比感热通量严重。

以上研究都是在水平均匀、不考虑地表异质性的影响,分析湍流有组织结构对于EC能量闭合的影响。Inagaki等[47]首次分析了地表热通量的水平异质性对于能量平衡的影响。其中地表热通量在x方向(顺风向)呈现一维正弦变化。在分析中将总的垂直热通量分解为垂直湍流热通量(即EC可以观测的湍流通量)、中尺度热通量(中尺度环流引起的垂直热通量传输)和剩余项(湍流有组织结构引起的垂直热通量传输),并分析了不同地表热通量差异对于垂直热通量的影响。结果表明:地表热通量差异引起的中尺度环流,会造成额外的通量输送。地表热通量差异越大,中尺度热通量越大,剩余项越小。此后,Zhou等[42]基于WRF LES分析了地表温度空间非均匀条件下,其日变化对于I的影响。结果发现:在均匀地表,I主要是由湍流有组织结构导致的;而在非均匀地表,湍流有组织结构对I的影响很小,I主要是热力异质性引起的二次环流导致的。午后湍流动能(Turbulent Kinetic Energy,TKE)的衰减导致了午后I快速减小;由于地表异质性诱发的二次环流的影响,Iu*u*/w*之间无明显的相关关系,却与TKE和地气温差呈现线性相关关系,即TKE和地气温差越大,I越小。以上研究都仅仅考虑一维地表异质性对于EC能量闭合的影响,并未考虑其他异质性的影响。Eder等[72]首次基于控制体积的分析方法,使用LES分析了真实地表异质性(林地与沙漠)对于EC能量闭合的影响。结果发现:林地与沙漠之间的二次环流导致了林地EC站点的能量不闭合。近期De Roo等[71]基于高分辨率(10 m)的LES分析了理想地表异质性(棋盘状地表异质性)对于EC能量闭合的影响,研究发现平流通量与水平湍流通量存在较强的相关性。与Inagaki等[47]和Zhou等[42]研究中一维地表异质性相比,Eder等[72]和De Roo等[71]研究中均为二维地表异质性,更加接近真实地表。表4列出了基于LES的能量闭合的研究。

表4

基于LES的EC能量闭合的研究

Table 4

The studies about the EC energy closure based on LES

总之,目前基于LES的EC能量闭合数值模拟研究并不多。到目前为止仅有9篇文献开展了相关研究。其中仅有4篇分析地表异质性对于EC能量闭合问题的影响。国内也仅有陈家宜等[70]和Zhou等[42]开展了相关的研究。目前基于LES研究EC能量闭合问题最大的难点在于计算量巨大。例如,理论上LES要较好地模拟EC站点,其空间分辨率至少应该为0.3 m,对应的时间分辨率至少应该为0.1 s(假设平均水平风速为3 m/s)。与此同时,要模拟大气边界层对EC能量闭合的影响,LES的计算域至少应在3~5倍大气边界层高度[76]。假设大气边界层高度为1 km,那么整个计算域至少应该为3 km×3 km ×1 km。此时网格数大约在1010量级。如此数量网格的LES模拟,其计算量和花费是十分惊人的。当前大气边界层的LES研究中,水平分辨率最高为10 m[71]。即便如此,其计算量和花费也是巨大的。

4 目前对EC能量闭合问题的认识及可能的解决方法
4.1 EC能量闭合问题与相关因子的关系

在过去的十几年中,学者对于EC能量闭合问题进行了大量的研究,获得了大量关于EC能量闭合问题的认知。其中既有EBR与EC数据处理方法(滤波方法和平均时间)的关系,又有与环境因子(如水平风速、z/Lzi/L以及地表异质性等)的关系。表5列出了目前已知的与EC能量闭合相关的影响因子。

表5

EC能量闭合相关的影响因子

Table 5

The factors related to the flux imbalance of EC during daytime

4.2 目前EC能量闭合问题可能的修正方法

基于以上对于EC能量闭合问题的认知,不同学者提出了不同的EC能量闭合修正方法(表6)。

表6

EC能量闭合修正公式

Table 6

The correctional equations for the EC flux imbalance

如Lee[58]基于一维的质量守恒方程,提出了一个EC能量闭合的修正方案,具体如下:

NEE= 0hc-tdz+( w'c'¯)h+ w-hc-h-1h0hc-dz,(31)

式中:NEE为标量c的净生态系统交换量,h为EC传感器高度。该公式所需的变量均可由EC站点观测到,实际应用起来十分简单、方便。然而,尽管在一些特定的站点[58,85],上述公式可用于EC能量闭合的诊断或修正。然而,如Finnigan[59]所述,该公式由一维的守恒方程推导而来,在实际三维的流场中适用范围有限。Kanda等[46]基于LES模拟结果也发现单点EC观测的wI之间的相关关系很弱(几乎不相关),公式(31)无法用于修正EC能量闭合问题。

胡隐樵等[86]基于大气线性热力学理论[87],证明了热通量(如感热和潜热通量)由垂直梯度输送通量和垂直速度耦合输送通量2个部分组成,即:

H=-ρcpKθθz+ρcpK0hh·Vdz, (32)

LE=-ρλvKqqz+ρλvKWq0h( h· V)dz,(33)

式中:ρcp分别为空气密度和定压比热,KθKq分别为热量和水汽的湍流输送系数,Ñ为哈尔密顿算子, V为水平风速。KK分别为垂直速度对位温θ和水汽q的耦合系数,其需要由试验来确定[88]。与经典的湍流理论不同,该公式认为垂直梯度输送通量才是湍流输送的来自地表的感热通量,而垂直速度耦合输送通量则是大尺度低频涡旋的耦合湍流通量。然而,需要特别注意的是虽然胡隐樵等[86]认为该公式可能用于EC能量闭合问题修正,但是从公式(32)~(33)以及陈晋北等[88]的做法来讲,其隐含着一个假设:EC观测的湍流通量已经包含了垂直梯度输送通量和垂直速度耦合输送通量。因此本文认为该公式只针对垂直梯度观测修正,并不适用于EC能量闭合问题的修正。这也可能是导致至今未有人将其用于EC能量闭合修正的原因。

基于均匀地表下LES的结果以及此前对I的认识,Huang等[44]给出了I与无量纲变量u*/w*z/zi间的经验关系,如下:

[I]= expa+bu*w*+c1.1+dzzi+f20.5,(34)

式中:a=4.2,b=-16,c=2.1,d=-8.0和f=-0.38为拟合参数。该参数化方法能够较好地解释I与摩擦风速u*z的关系。但该方案最大的不足是无法直接应用于异质性地表。由于EC能量闭合问题主要出现在异质性地表,此时上述参数化方案几乎无用武之地。除此之外,上述参数化方案是基于0.3<z/zi<0.5范围内LES的结果给出的,将其用于近地表(< 0.1 z/zi)的EC观测值得商榷。这可能是实际观测中上述参数化方案失败的原因之一[86]。最后,公式(34)中w*的计算需要地表真实热通量,故实际上公式(34)只能作为诊断方程而不能用作预测方程。

为了考虑地表异质性的影响,Panin等[17,89]直接构建了I与有效地表粗糙长度 z0eff及地表异质性尺度Leff间的关系。该经验参数化方案如下:

EBR=1-[I]=Kz0effLeff+C,(35)

式中:KC为拟合参数。与Huang等[44]相比,该参数化方案最大的优点是显式地考虑了地表异质性对I的影响,因而可直接用于异质性地表。当然,该方案也存在一些缺点。首先,该参数化方案只考虑了动量粗糙长度的影响。其他如热力异质性的影响并未考虑进去;其次,该方法仅考虑地表异质性尺度的影响,忽略大气状况对于EBR或I的影响。这使得拟合参数(即KC)在不同站点甚至同一站点的不同大气状况下都有可能不同。因此上述参数化方案在不同的站点效果差异较大[88]

总之,现有的这些修正方案都只是基于对EC能量闭合问题某一方面的认识,来试图修正EC能量闭合问题。很显然,这不现实,也不可能实现。

然而,在实际中,无论是作为陆面和遥感ET模型的验证数据还是气候模型的输入数,地表通量都应该是闭合的。由于目前没有好的EC能量闭合修正方案,学者只能采用经验性的EC能量闭合修正方法。目前常用的经验性的EC能量闭合修正方法有:能量余项法和波文比法。前一种方法假设EC观测的H是准确的,使用有效能量减去H即为调整后的LE[12];后一种方法假设EC所测定的HLE的比值(波文比)是准确的,使用测定的波文比将有效能量进行分割,即为调整后的HLE[12,25]。Twine等[12]和Foken等[25]均建议使用波文比修正方法。

5 存在的问题

目前EC能量闭合问题的面临以下几个困难:

(1) 从观测上来讲,目前缺少高精度且时间连续、空间密集(如矩阵式观测)的地表通量观测数据集。任何一种修正方案,是否有效,最终都必须使用观测数据来验证。因此,获取区域内高精度且时间连续、空间密集(如矩阵式观测)的地表真实通量观测数据集,对EC能量闭合问题的解决是极其重要的。然而,矩阵式观测耗费巨大,目前还比较少见。HiWATER在黑河中游开展的“非均匀下垫面多尺度地表蒸散发观测试验”是目前为数不多的嵌套EC矩阵观测试验[90],这为EC能量闭合问题的研究提供了重要的观测数据集。

(2) 从模型上来讲,目前LES尚不足以完全表征真实的EC观测。一方面是因为要完全表征真实的EC观测,理论上网格数至少应该在1010量级。如此数量网格的LES模拟,其计算量和花费是十分惊人的。因此,目前基于LES的EC能量闭合研究的垂直高度都远高于实际EC的观测高度,这些研究结果能否直接应用于近地表的EC观测,目前尚不清楚,需要高精度且时间连续、空间密集(如矩阵式观测)的地表通量观测数据集的检验。另一方面,大量的观测表明,在异质性地表M-O相似理论是不成立的。然而,目前在异质性地表尚不存在比M-O相似理论更好的理论。因此,当前研究大气的LES中一般采用M-O相似理论计算近地表(第一层网格)处的地表通量。这使得基于LES的EC能量闭合问题的研究结果存在一定的不确定性。

(3) 从机理上来讲,大尺度湍涡影响EC能量闭合的机制尚不清楚。尽管之前大量的观测和数值模拟研究发现了EBR与环境因子间的相关关系。然而,如何理解这些相关关系?哪些变量或环境因子可以作为独立的变量或因子来解释已知的相关关系?是否存在一个模型或修正方法来解释已知的相关关系?这些问题目前都不清楚。这些问题不解决,EC能量闭合问题的修正是不可能实现的。

6 结论与展望

EC能量闭合问题已成为微气象学以及地气间相互作用研究的一大瓶颈,阻碍了遥感通量算法的改进和陆面过程模型的改善。过去的几十年,大量学者对此问题进行了深入探讨和分析,积累了丰富的认知。本文系统地介绍了EC能量闭合问题的成因,总结了目前学者对于EC能量闭合问题的认识以及基于这些认识提出的可能的修正方案,并指出了目前存在的问题。与之前的综述不同,本文重点介绍了基于LES研究EC能量闭合问题的方法,研究进展及存在的问题。

综合以上分析,本文认为未来应该加强以下几方面的研究:

(1) 大尺度湍涡影响EC能量闭合的机制。大尺度湍涡影响EC能量闭合的机制是定量EBR与环境因子或变量的前提,也是EC能量闭合修正的前提。一种可能的途径是选取合适的环境因子或变量,然后基于高精度数值模拟构建这些环境因子或变量与EBR的定量关系。该方法的难点在于如何选取合适的环境因子,未来需要深入地研究。

(2) 高质量且时间连续,空间密集(如HiWATER矩阵式观测)的地表通量观测数据集。区域内高精度且时间连续、空间密集(如HiWATER矩阵式观测)的地表真实通量观测数据集,是验证和发展EC能量闭合修正方案不可或缺的重要一环。发展高效经济的矩阵式观测的地表通量观测数据集将会有助于EC能量闭合问题修正方案的发展。未来应该进一步系统深入地分析HiWATER矩阵观测数据,检验目前已存在的EC能量闭合方法。

总之,基于LES探讨EC能量闭合问题的产生机制,提出可能的修正方法,然后使用HiWATER矩阵式的地表观测数据,验证该修正方法并分析存在的问题。这是目前看来最有可能解决EC能量闭合问题的一种方法。

The authors have declared that no competing interests exist.

参考文献

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DOI:10.3321/j.issn:1000-0534.1999.03.005 Magsci URL
To improve the parameterization scheme in the grid scale of GCMs over different land surface status,China has organized Land Surface Processes experiments and the study of interaction of surface with atmosphere (LSPI) since 1987.These experiments are all international cooperation,and coordinated by WCRP and IGBP.Lanzhou Institute of Plateau Atmospheric Physics has played a pioneer role in these experimental studies. (a) The progress of HEIFE The first LSPI,the HEIFE (Heihe River Basin Field Experiment),was carried out in 1988 through 1993,scaled 70-90 km,in an arid region of Northwest China on a large Gobi/sandy desert with oasis dispersed along Heihe River and irrigation canals. A concept of "Desert-Oasis System" was put forward in 1993,which declared that in addition to the prevailing wind system,because of topography and surface status,local and regional circulation can be formed in conjunction with diurnal heating cycle in Heihe basin.Sensible heat advected from surrounding desert area with an integrated value as large as half of the net radiation in clear windy afternoon,becoming one of the main energy sources to the oasis vegetation.On the other hand,water vapor transported from oasis to adjacent desert is possibly one of the water sources for the desert plants.Land surface processes over desert and oasis are correlated closely. The derivation of regional surface effective parameters to be used in modeling has been always a major task.Different schemes were used in the area-representative aggregation,based on the observation data from more than 10 surface stations in such complex area scaled.It has been found that some parameters the results from different schemes might have difference of order of magnitude.To use remote sensing from space is indispensable.Many scenes of different satellite data,including NOAA AVHRR,LANDSAT TM,and ERS ATSR etc.,have been analyzed in the 'up-scaling' study.The derivation of statistics of important parameters was successful,such as surface temperature,albedo,vegetation index,roughness length,transportation coefficients,etc.,as well as the surface radiation,sensible heat and evaporation fluxes. Modeling study has also been a major part in HEIFE.1-D surface processes study has been done mainly in desert area,concentrated in the water process near desert surface.Planetary boundary layer modeling has been carried out in analyzing regional wind field and the process in between oasis and desert.Some detailed studies have been done by using 2-D and 3-D mesoscale models,particularly,the Regional Atmospheric Modeling System (RAMS),a non-hydrostatic and compressible 3-D mesoscale model,has been used recently in the LSPI study in Heihe Basin.By using of HEIFE data set,also,some parameterization results retrieved from satellite data in recent years,the boundary layer structure and energy/water exchange processes in this area have been simulated successfully;some specific phenomenon have also been modeled and explained theoretically.However,more detailed simulation is still needed. (b) IMGRASS Another LSPI,the IMGRASS (Inner Mongolia Semi-Arid Grassland Soil-Vegetation-Atmosphere Interaction),has started preparation soon after the HEIFE field campaign,and carried out its field observation on a comparatively homogeneous area in the summer of 1998.IMGRASS is a BAHC-GEWEX type experiment emphasized on the understanding of land surface process in temperate semi-arid grassland,and the validation of satellite remote sensing algorithms.It also attempts to understand the grassland-climate interaction under the human activity stress with the target for sustainable development in the area.Four core stations,with comprehensive observations of soil,vegetation,atmospheric boundary layer,surface fluxes of radiation,heat,water vapor,and trace gases,have been operated in 1998 in the representatives parts of a region around 100 150 km,centered at about (116.3-E,43.5-N).A radio-controlled mini-aircraft equipped with simple meteorological sensors and a camera has also been operated during the field campaign.A preliminary data analysis is in progress. (c) Land surface processes study on the Tibetan Plateau 1998 was also a year for the implementation of other two LSPI projects in China,the GAME-Tibet/TIPEX and the GAME-HUBEX.The latter is an experiment in Huaihe River Basin,East China,within the framework of GAME. GAME-Tibet and TIPEX are cooperative studies of land surface processes on Tibetan Plateau.The former is in the framework of GAME and carried out by China,Japan and some other Asian countries,while the TIPEX (Tibetan Plateau Hydro-meteorological Experiment) is a national key project of China.The overall goal of GAME-Tibet and TIPEX is to clarify the interactions between the land surface and the atmosphere over the Tibetan Plateau in the context of the Asian monsoon system. Two scales of experiments have been implemented:(1) A Plateau-scale (2 000脳1 500 km) experiment using the north-south and east-west networks of operational meteorological and hydrological stations (including 12 new automatic weather stations);(2) A mesoscale (100-150 km) experiment with two or three dimensional intensive LSP observations at about five sites.Process models and methods for applying them over large spatial scales can then be developed.Besides,GAME-Tibet also pays great attention to the development and validation of satellite based observing methods.Proper ground truth measurements have been arranged. The intensive observation has been done successfully during May though September 1998.A large amount of data has been collected,which is the best data set so far for the study of plateau hydrometeorology.An international scientific workshop on the preliminary study of GAME-Tibet has been held in Xi'an in January 1999.Preliminary Results of GAME-Tibet are in following aspects: a Dramatic climate change after the monsoon setup. b Strong insolation on the Tibetan plateau;surface energy balance (?) c PBL structure and the rapid convection development. d Retrieving of surface characteristic parameters. e Soil moisture study;using of SSM/I,TMI. f Fine data from X-band 3-d Doppler radar for cloud and precipitation study;ground truth for TRMM.
[王介民. 陆面过程实验和地气相互作用研究——从HEIFE到IMGRASS和GAME-Tibet/TIPEX[J]. 高原气象, 1999, 18(3): 280-294.]
DOI:10.3321/j.issn:1000-0534.1999.03.005 Magsci URL
To improve the parameterization scheme in the grid scale of GCMs over different land surface status,China has organized Land Surface Processes experiments and the study of interaction of surface with atmosphere (LSPI) since 1987.These experiments are all international cooperation,and coordinated by WCRP and IGBP.Lanzhou Institute of Plateau Atmospheric Physics has played a pioneer role in these experimental studies. (a) The progress of HEIFE The first LSPI,the HEIFE (Heihe River Basin Field Experiment),was carried out in 1988 through 1993,scaled 70-90 km,in an arid region of Northwest China on a large Gobi/sandy desert with oasis dispersed along Heihe River and irrigation canals. A concept of "Desert-Oasis System" was put forward in 1993,which declared that in addition to the prevailing wind system,because of topography and surface status,local and regional circulation can be formed in conjunction with diurnal heating cycle in Heihe basin.Sensible heat advected from surrounding desert area with an integrated value as large as half of the net radiation in clear windy afternoon,becoming one of the main energy sources to the oasis vegetation.On the other hand,water vapor transported from oasis to adjacent desert is possibly one of the water sources for the desert plants.Land surface processes over desert and oasis are correlated closely. The derivation of regional surface effective parameters to be used in modeling has been always a major task.Different schemes were used in the area-representative aggregation,based on the observation data from more than 10 surface stations in such complex area scaled.It has been found that some parameters the results from different schemes might have difference of order of magnitude.To use remote sensing from space is indispensable.Many scenes of different satellite data,including NOAA AVHRR,LANDSAT TM,and ERS ATSR etc.,have been analyzed in the 'up-scaling' study.The derivation of statistics of important parameters was successful,such as surface temperature,albedo,vegetation index,roughness length,transportation coefficients,etc.,as well as the surface radiation,sensible heat and evaporation fluxes. Modeling study has also been a major part in HEIFE.1-D surface processes study has been done mainly in desert area,concentrated in the water process near desert surface.Planetary boundary layer modeling has been carried out in analyzing regional wind field and the process in between oasis and desert.Some detailed studies have been done by using 2-D and 3-D mesoscale models,particularly,the Regional Atmospheric Modeling System (RAMS),a non-hydrostatic and compressible 3-D mesoscale model,has been used recently in the LSPI study in Heihe Basin.By using of HEIFE data set,also,some parameterization results retrieved from satellite data in recent years,the boundary layer structure and energy/water exchange processes in this area have been simulated successfully;some specific phenomenon have also been modeled and explained theoretically.However,more detailed simulation is still needed. (b) IMGRASS Another LSPI,the IMGRASS (Inner Mongolia Semi-Arid Grassland Soil-Vegetation-Atmosphere Interaction),has started preparation soon after the HEIFE field campaign,and carried out its field observation on a comparatively homogeneous area in the summer of 1998.IMGRASS is a BAHC-GEWEX type experiment emphasized on the understanding of land surface process in temperate semi-arid grassland,and the validation of satellite remote sensing algorithms.It also attempts to understand the grassland-climate interaction under the human activity stress with the target for sustainable development in the area.Four core stations,with comprehensive observations of soil,vegetation,atmospheric boundary layer,surface fluxes of radiation,heat,water vapor,and trace gases,have been operated in 1998 in the representatives parts of a region around 100 150 km,centered at about (116.3-E,43.5-N).A radio-controlled mini-aircraft equipped with simple meteorological sensors and a camera has also been operated during the field campaign.A preliminary data analysis is in progress. (c) Land surface processes study on the Tibetan Plateau 1998 was also a year for the implementation of other two LSPI projects in China,the GAME-Tibet/TIPEX and the GAME-HUBEX.The latter is an experiment in Huaihe River Basin,East China,within the framework of GAME. GAME-Tibet and TIPEX are cooperative studies of land surface processes on Tibetan Plateau.The former is in the framework of GAME and carried out by China,Japan and some other Asian countries,while the TIPEX (Tibetan Plateau Hydro-meteorological Experiment) is a national key project of China.The overall goal of GAME-Tibet and TIPEX is to clarify the interactions between the land surface and the atmosphere over the Tibetan Plateau in the context of the Asian monsoon system. Two scales of experiments have been implemented:(1) A Plateau-scale (2 000脳1 500 km) experiment using the north-south and east-west networks of operational meteorological and hydrological stations (including 12 new automatic weather stations);(2) A mesoscale (100-150 km) experiment with two or three dimensional intensive LSP observations at about five sites.Process models and methods for applying them over large spatial scales can then be developed.Besides,GAME-Tibet also pays great attention to the development and validation of satellite based observing methods.Proper ground truth measurements have been arranged. 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There is a growing consensus that land surface models (LSMs) that simulate terrestrial biosphere exchanges of matter and energy must be better constrained with data to quantify and address their uncertainties. FLUXNET, an international network of sites that measure the land surface exchanges of carbon, water and energy using the eddy covariance technique, is a prime source of data for model improvement. Here we outline a multi-stage process for "fusing" (i.e. linking) LSMs with FLUXNET data to generate better models with quantifiable uncertainty. First, we describe FLUXNET data availability, and its random and systematic biases. We then introduce methods for assessing LSM model runs against FLUXNET observations in temporal and spatial domains. These assessments are a prelude to more formal model-data fusion (MDF). MDF links model to data, based on error weightings. In theory, MDF produces optimal analyses of the modelled system, but there are practical problems. We first discuss how to set model errors and initial conditions. In both cases incorrect assumptions will affect the outcome of the MDF. We then review the problem of equifinality, whereby multiple combinations of parameters can produce similar model output. Fusing multiple independent and orthogonal data provides a means to limit equifinality. We then show how parameter probability density functions (PDFs) from MDF can be used to interpret model validity, and to propagate errors into model outputs. Posterior parameter distributions are a useful way to assess the success of MDF, combined with a determination of whether model residuals are Gaussian. If the MDF scheme provides evidence for temporal variation in parameters, then that is indicative of a critical missing dynamic process. A comparison of parameter PDFs generated with the same model from multiple FLUXNET sites can provide insights into the concept and validity of plant functional types (PFT) ??? we would expect similar parameter estimates among sites sharing a single PFT. We conclude by identifying five major model-data fusion challenges for the FLUXNET and LSM communities: (1) to determine appropriate use of current data and to explore the information gained in using longer time series; (2) to avoid confounding effects of missing process representation on parameter estimation; (3) to assimilate more data types, including those from earth observation; (4) to fully quantify uncertainties arising from data bias, model structure, and initial conditions problems; and (5) to carefully test current model concepts (e.g. PFTs) and guide development of new concepts.
[本文引用: 2]
[6]
Jung M, Reichstein M, Ciais P, et al.Recent decline in the global land evapotranspiration trend due to limited moisture supply[J]. Nature, 2010, 467(7 318): 951-954.
DOI:10.1038/nature09396 PMID:20935626 URL
More than half of the solar energy absorbed by land surfaces is currently used to evaporate water1. Climate change is expected to intensify the hydrological cycle2 and to alter evapotranspiration, with implications for ecosystem services and feedback to regional and global climate. Evapotranspiration changes may already be under way, but direct observational constraints are lacking at the global scale. Until such evidence is available, changes in the water cycle on land--a key diagnostic criterion of the effects of climate change and variability--remain uncertain. Here we provide a data-driven estimate of global land evapotranspiration from 1982 to 2008, compiled using a global monitoring network3, meteorological and remote-sensing observations, and a machine-learning algorithm4. In addition, we have assessed evapotranspiration variations over the same time period using an ensemble of process-based land-surface models. Our results suggest that global annual evapotranspiration increased on average by 7.165±651.065millimetres per year per decade from 1982 to 1997. After that, coincident with the last major El Ni09o event in 1998, the global evapotranspiration increase seems to have ceased until 2008. This change was driven primarily by moisture limitation in the Southern Hemisphere, particularly Africa and Australia. In these regions, microwave satellite observations indicate that soil moisture decreased from 1998 to 2008. Hence, increasing soil-moisture limitations on evapotranspiration largely explain the recent decline of the global land-evapotranspiration trend. Whether the changing behaviour of evapotranspiration is representative of natural climate variability or reflects a more permanent reorganization of the land water cycle is a key question for earth system science
[本文引用: 2]
[7]
Xu Ziwei, Ma Yanfei, Liu Shaomin, et al.Assessment of the energy balance closure under advective conditions and its impact using remote sensing data[J]. Journal of Applied Meteorology and Climatology, 2017, 56(1): 127-140.
[本文引用: 2]
[8]
Eder F, Schmidt M, Damian T, et al.Mesoscale eddies affect near-surface turbulent exchange: Evidence from lidar and tower measurements[J]. Journal of Applied Meteorology and Climatology, 2015, 54(1): 189-206.
[本文引用: 1]
[9]
Foken T, Mauder M, Liebethal C, et al.Energy balance closure for the LITFASS-2003 experiment[J]. Theoretical and Applied Climatology, 2010, 101(1/2): 149-160.
DOI:10.1007/s00704-009-0216-8 URL
In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft).
[本文引用: 6]
[10]
Wilson K, Goldstein A, Falge E, et al.Energy balance closure at FLUXNET sites[J]. Agricultural and Forest Meteorology, 2002, 113(1/4): 223-243.
DOI:10.1016/S0168-1923(02)00109-0 URL
A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO 2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated with the eddy covariance technique or from errors in calculating the available energy terms. Although it was not entirely possible to critically evaluate all the possible sources of the imbalance, circumstantial evidence suggested a link between the imbalance and CO 2 fluxes. For a given value of photosynthetically active radiation, the magnitude of CO 2 uptake was less when the energy imbalance was greater. Similarly, respiration (estimated by nocturnal CO 2 release to the atmosphere) was significantly less when the energy imbalance was greater.
[本文引用: 2]
[11]
Aubinet M, Grelle A, Ibrom A, et al.Estimates of the annual net carbon and water exchange of forests: The EUROFLUX methodology[J]. Advances in Ecological Research, 2000, 30: 113-175.
DOI:10.1016/S0065-2504(08)60018-5 URL
The chapter has described the measurement system and the procedure followed for the computation of the fluxes and the procedure of flux summation, including data gap filling strategy, night flux corrections and error estimation. It begins with the introduction of estimates of the annual net carbon and water exchange of forests using the EUROFLUX methodology. The chapter then provides us with the theory and moves on to discuss the eddy covariance system and its sonic anemometer, temperature fluctuation measurements, infrared gas analyser, air transport system, and tower instrumentation. Additional measurements are also given in the chapter. Data acquisition and its computation and correction is discussed next in the chapter by giving its general procedure, half-hourly means (co-)variances and uncorrected fluxes, intercomparison of software, and correction for frequency response losses. The chapter has also discussed about quality control and four criteria are investigated here for the same. Spatial representativeness of measured fluxes and summation procedure are reviewed. The chapter then moves on to the discussion of data gap filling through interpolation and parameterization and neural networks. Corrections to night-time data and error estimation are also explored in the chapter. Finally, the chapter closes with conclusions.
[本文引用: 2]
[12]
Twine T E, Kustas W, Norman J, et al.Correcting eddy-covariance flux underestimates over a grassland[J]. Agricultural and Forest Meteorology, 2000, 103(3): 279-300.
DOI:10.1016/S0168-1923(00)00123-4 URL
Independent measurements of the major energy balance flux components are not often consistent with the principle of conservation of energy. This is referred to as a lack of closure of the surface energy balance. Most results in the literature have shown the sum of sensible and latent heat fluxes measured by eddy covariance to be less than the difference between net radiation and soil heat fluxes. This under-measurement of sensible and latent heat fluxes by eddy-covariance instruments has occurred in numerous field experiments and among many different manufacturers of instruments. Four eddy-covariance systems consisting of the same models of instruments were set up side-by-side during the Southern Great Plains 1997 Hydrology Experiment and all systems under-measured fluxes by similar amounts. One of these eddy-covariance systems was collocated with three other types of eddy-covariance systems at different sites; all of these systems under-measured the sensible and latent-heat fluxes. The net radiometers and soil heat flux plates used in conjunction with the eddy-covariance systems were calibrated independently and measurements of net radiation and soil heat flux showed little scatter for various sites. The 10% absolute uncertainty in available energy measurements was considerably smaller than the systematic closure problem in the surface energy budget, which varied from 10 to 30%. When available-energy measurement errors are known and modest, eddy-covariance measurements of sensible and latent heat fluxes should be adjusted for closure. Although the preferred method of energy balance closure is to maintain the Bowen atio, the method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of energy. Based on numerous measurements over a sorghum canopy, carbon dioxide fluxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance closure is not achieved.
[本文引用: 5]
[13]
Oncley S P, Foken T, Vogt R, et al.The energy balance experiment EBEX-2000. Part I: Overview and energy balance[J]. Boundary-Layer Meteorology, 2007, 123(1): 1-28.
DOI:10.1007/s10546-007-9161-1 Magsci URL
An overview of the Energy Balance Experiment (EBEX-2000) is given. This experiment studied the ability of state-of-the-art measurements to close the surface energy balance over a surface (a vegetative canopy with large evapotranspiration) where closure has been difficult to obtain. A flood-irrigated cotton field over uniform terrain was used, though aerial imagery and direct flux measurements showed that the surface still was inhomogeneous. All major terms of the surface energy balance were measured at nine sites to characterize the spatial variability across the field. Included in these observations was an estimate of heat storage in the plant canopy. The resultant imbalance still was 10%, which exceeds the estimated measurement error. We speculate that horizontal advection in the layer between the canopy top and our flux measurement height may cause this imbalance, though our estimates of this term using our measurements resulted in values less than what would be required to balance the budget.
[本文引用: 6]
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Lee Xuhui, Black T A.Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour[J]. Boundary-Layer Meteorology, 1993, 64(4): 369-389.
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Foken T, Gerstmann W, Richter S H, et al.Study of the Energy Exchange Processes over Different Types of Surfaces during TARTEX-90’[M]. Offenbach am Main: Forschung und Entwicklung, 1993.
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Panin G N, Tetzlaff G, Raabe A.Inhomogeneity of the land surface and problems in the parameterization of surface fluxes in natural conditions[J]. Theoretical and Applied Climatology, 1998, 60(1/4): 163-178.
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Foken T, Jegede O O,Weisensee U, et al.Results of the LINEX-96/2 Experiment[M]. Offenbach am Main: Forschung und Entwicklung, 1997.
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Beyrich F, Richter S H, Weisensee U, et al.Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment[J]. Theoretical and Applied Climatology, 2002, 73(1): 19-34.
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Zhang Yu, Shihua, Chen Shiqiang, et al.Characteristics of energy budget and microclimate on the edge of oasis summer[J]. Plateau Meteorology, 2005, 24(4): 527-533.
[张宇, 吕世华, 陈世强, . 绿洲边缘夏季小气候特征及地表辐射与能量平衡特征分析[J]. 高原气象, 2005, 24(4): 527-533.]
[21]
Wang Jiemin, Wang Weizhen, Liu Shaomin, et al.The problems of surface energy balance closure—An overview and case study[J]. Advances in Earth Science, 2009, 24(7): 705-713.
Magsci URL
近地层能量闭合问题,即测量到的感热和潜热通量之和一般总小于近地层可利用能量(净辐射与土壤热通量之差),是近20年来困扰地气相互作用实验研究的主要难点之一。对国内外有关研究现状做了综述,与解决此问题日益迫切的要求相适应,问题的实质及解决途径近年来已逐渐明朗。"涡动相关方法"应用在复杂的大气湍流通量观测中的局限,特别是对低频较大尺度湍流通量的低估,仍是关键所在。在理论分析的基础上,结合2008年部分"黑河综合实验"资料对有关计算结果做了具体介绍。以阿柔站6天连续资料为例,仔细计算土壤浅层热储存,在涡动相关资料再处理中加上高低频损失修正等,再参考该站大口径闪烁仪(LAS)观测对感热通量的提高,能量闭合率可达到99%,当然这只是个例。许多复杂情况下,较大尺度的涡旋或湍流有组织结构(TOS)会有更明显影响。近地层能量闭合问题的根本解决必须考虑后者的贡献,提高通量观测的时空代表性。
[王介民, 王维真, 刘绍民, . 近地层能量平衡闭合问题———综述及个例分析[J]. 地球科学进展, 2009, 24(7): 705-713.]
Magsci URL
近地层能量闭合问题,即测量到的感热和潜热通量之和一般总小于近地层可利用能量(净辐射与土壤热通量之差),是近20年来困扰地气相互作用实验研究的主要难点之一。对国内外有关研究现状做了综述,与解决此问题日益迫切的要求相适应,问题的实质及解决途径近年来已逐渐明朗。"涡动相关方法"应用在复杂的大气湍流通量观测中的局限,特别是对低频较大尺度湍流通量的低估,仍是关键所在。在理论分析的基础上,结合2008年部分"黑河综合实验"资料对有关计算结果做了具体介绍。以阿柔站6天连续资料为例,仔细计算土壤浅层热储存,在涡动相关资料再处理中加上高低频损失修正等,再参考该站大口径闪烁仪(LAS)观测对感热通量的提高,能量闭合率可达到99%,当然这只是个例。许多复杂情况下,较大尺度的涡旋或湍流有组织结构(TOS)会有更明显影响。近地层能量闭合问题的根本解决必须考虑后者的贡献,提高通量观测的时空代表性。
[本文引用: 7]
[22]
Kohsiek W, Liebethal C, Foken T, et al.The energy balance experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements[J]. Boundary-Layer Meteorology, 2007, 123(1): 55-75.
DOI:10.1007/s10546-006-9135-8 Magsci
<a name="Abs1"></a>An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30&nbsp;W&nbsp;m<sup>&#8722;2</sup> over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50&nbsp;W&nbsp;m<sup>&#8722;2</sup>; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m<sup>&#8722;2</sup> across the field, rising to 50&nbsp;W&nbsp;m<sup>&#8722;2</sup> or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25&nbsp;W&nbsp;m<sup>&#8722;2</sup>, 5%) per site during the day and 10&nbsp;W&nbsp;m<sup>&#8722;2</sup> at night.
[本文引用: 1]
[23]
Mauder M, Oncley S P, Vogt R, et al.The energy balance experiment EBEX-2000. Part II: Intercomparison of eddy-covariance sensors and post-field data processing methods[J]. Boundary-Layer Meteorology, 2007, 123(1): 29-54.
DOI:10.1007/s10546-006-9139-4 Magsci
<a name="Abs1"></a>The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion&#8212;closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30&nbsp;min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H<sub>2</sub>O/CO<sub>2</sub> gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made.
[本文引用: 2]
[24]
Foken T.The energy balance closure problem: An overview[J]. Ecological Applications, 2008, 18(6): 1 351-1 367.
DOI:10.1890/06-0922.1 PMID:18767615 URL
Abstract This paper gives an overview of 20 years of research on the energy balance closure problem. It will be shown that former assumptions that measuring errors or storage terms are the reason for the unclosed energy balance do not stand up because even turbulent fluxes derived from documented methods and calibrated sensors, net radiation, and ground heat fluxes cannot close the energy balance. Instead, exchange processes on larger scales of the heterogeneous landscape have a significant influence. By including these fluxes, the energy balance can be approximately closed. Therefore, the problem is a scale problem and has important consequences to the measurement and modeling of turbulent fluxes.
[本文引用: 3]
[25]
Foken T, Aubinet M, Finnigan J J, et al. Results of a panel discussion about the energy balance closure correction for trace gases[J]. Bulletin of the American Meteorological Society Results of a panel discussion about the energy balance closure correction for trace gases[J]. Bulletin of the American Meteorological Society, 2011, 92(4): Es13-Es18.
[本文引用: 6]
[26]
Katul G G, Oren R, Manzoni S, et al. Evapotranspiration: A process driving mass transport Evapotranspiration: A process driving mass transport and energy exchange in the soil-plant-atmosphere-climate system[J]. Reviews of Geophysics, 2012, 50: RG3002.
[本文引用: 2]
[27]
Zuo Hongchao, Xiao Xia, Yang Qidong, et al.On the atmospheric movement and the imbalance of observed and calculated energy in the surface layer[J]. Science in China (Series D), 2012, 55(9): 1 518-1 532.
DOI:10.1007/s11783-011-0280-z URL
本文在已有的研究基础上,从理论上概括了近地层大气运动和能量物质的传输特征,以及近地层能量物质传输的理想模型和实际模型.之后利用兰州大学半干旱气候与环境监测站(SACOL)连续四年(2006-09~2010-08)中所有5~10月间观测的涡旋相关资料,详细分析了该站近地层的能量传输特征.研究中引入了相对垂直湍强来表征湍流的强度,RIw=w2/(w2+U),并且还采用了资料质量控制的分级处理手段.研究得到如下结论:(1)近地层能量的传输必须满足物质和能量守恒定律,理论上近地层的能量必须是平衡的或者闭合的,但是实际观测的近地层能量只是能量平衡的某种程度的近似,很难达到平衡.(2)近地层的能量闭合率决定于大气运动的状态,总体上近地层能量闭合率随相对垂直湍强增大而增加.(3)通过资料质量控制分级处理表明,资料质量控制的程度可以影响近地层能量闭合率,但是它不能影响近地层能量闭合率决定于大气运动状态的实质.(4)地表热通量的计算方法可以影响近地层能量闭合率,但也不能影响近地层能量闭合率决定于大气运动状态的实质.
[左洪超, 肖霞, 杨启东, . 论近地层大气运动特征与观测和计算能量不平衡的成因[J]. 中国科学:D辑, 2012, 42(9): 1 370-1 384.]
DOI:10.1007/s11783-011-0280-z URL
本文在已有的研究基础上,从理论上概括了近地层大气运动和能量物质的传输特征,以及近地层能量物质传输的理想模型和实际模型.之后利用兰州大学半干旱气候与环境监测站(SACOL)连续四年(2006-09~2010-08)中所有5~10月间观测的涡旋相关资料,详细分析了该站近地层的能量传输特征.研究中引入了相对垂直湍强来表征湍流的强度,RIw=w2/(w2+U),并且还采用了资料质量控制的分级处理手段.研究得到如下结论:(1)近地层能量的传输必须满足物质和能量守恒定律,理论上近地层的能量必须是平衡的或者闭合的,但是实际观测的近地层能量只是能量平衡的某种程度的近似,很难达到平衡.(2)近地层的能量闭合率决定于大气运动的状态,总体上近地层能量闭合率随相对垂直湍强增大而增加.(3)通过资料质量控制分级处理表明,资料质量控制的程度可以影响近地层能量闭合率,但是它不能影响近地层能量闭合率决定于大气运动状态的实质.(4)地表热通量的计算方法可以影响近地层能量闭合率,但也不能影响近地层能量闭合率决定于大气运动状态的实质.
[本文引用: 2]
[28]
Leuning R, Van Gorsel E, Massman W J, et al.Reflections on the surface energy imbalance problem[J]. Agricultural and Forest Meteorology, 2012, 156: 65-74.
DOI:10.1016/j.agrformet.2011.12.002 URL
The ‘energy imbalance problem’ in micrometeorology arises because at most flux measurement sites the sum of eddy fluxes of sensible and latent heat (H+λE) is less than the available energy (A). Either eddy fluxes are underestimated or A is overestimated. Reasons for the imbalance are: (1) a failure to satisfy the fundamental assumption of one-dimensional transport that is necessary for measurements on a single tower to represent spatially-averaged fluxes to/from the underlying surface, and (2) measurement errors in eddy fluxes, net radiation and changes in energy storage in soils, air and biomass below the measurement height. Radiometer errors are unlikely to overestimate A significantly, but phase lags caused by incorrect estimates of the energy storage terms can explain why H+λE systematically underestimates A at half-hourly time scales. Energy closure is observed at only 8% of flux sites in the La Thuile dataset (http://www.fluxdata.org/DataInfo/default.aspx) with half-hourly averages but this increases to 45% of sites using 24h averages because energy entering the soil, air and biomass in the morning is returned in the afternoon and evening. Unrealistically large and positive horizontal gradients in temperature and humidity are needed for advective flux divergences to explain the energy imbalance at half-hourly time scales. Imbalances between H+λE and A still occur in daily averages but the small residual energy imbalances are explicable by horizontal and vertical advective flux divergences. Systematic underestimates of the vertical heat flux also occur if horizontal u′T′ covariances contaminate the vertical w′T′ signal due to incorrect coordinate rotations. Closure of the energy balance is possible at half-hourly time scales by careful attention to all sources of measurement and data processing errors in the eddy covariance system and by accurate measurement of net radiation and every energy storage term needed to calculate available energy.
[本文引用: 1]
[29]
Wohlfahrt G, Widmoser P.Can an energy balance model provide additional constraints on how to close the energy imbalance?[J]. Agricultural and Forest Meteorology, 2013, 169: 85-91.
DOI:10.1016/j.agrformet.2012.10.006 PMID:3898304 URL
Elucidating the causes for the energy imbalance, i.e. the phenomenon that eddy covariance latent and sensible heat fluxes fall short of available energy, is an outstanding problem in micrometeorology. This paper tests the hypothesis that the full energy balance, through incorporation of additional independent measurements which determine the driving forces of and resistances to energy transfer, provides further insights into the causes of the energy imbalance and additional constraints on energy balance closure options. Eddy covariance and auxiliary data from three different biomes were used to test five contrasting closure scenarios. The main result of our study is that except for nighttime, when fluxes were low and noisy, the full energy balance generally did not contain enough information to allow further insights into the causes of the imbalance and to constrain energy balance closure options. Up to four out of the five tested closure scenarios performed similarly and in up to 53% of all cases all of the tested closure scenarios resulted in plausible energy balance values. Our approach may though provide a sensible consistency check for eddy covariance energy flux measurements.
[30]
Metzger S.Surface-atmosphere exchange in a box: Making the control volume a suitable representation for in-situ observations[J]. Agricultural and Forest Meteorology, 2018, 255: 68-80.
[本文引用: 1]
[31]
Finnigan J J, Clement R, Malhi Y, et al.A re-evaluation of long-term flux measurement techniques—Part I: Averaging and coordinate rotation[J]. Boundary-Layer Meteorology, 2003, 107(1): 1-48.
DOI:10.1023/A:1021554900225 Magsci
<a name="Abs1"></a>Experience of long term flux measurements over tall canopiesduring the last two decades has revealed that the eddy flux of sensible plus latentheat is typically 30% smaller than the available radiant energy flux. This failureto close the energy balance is less common close to the surface over short roughnessbut is still sometimes seen, especially in complex topography. These observationscast doubt on the results obtained from long term flux studies where daily and annualnet ecosystem exchange is usually the small difference between large positive andnegative fluxes over 24 h. In this paper we investigate this problem by examiningsome fundamental assumptions entailed in analysis of surface exchange by the eddyflux method.<div class="AbstractPara"><div class="">In particular, we clarify the form and use of the scalar conservation equation thatunderlies this analysis and we examine the links between averaging period androtation of coordinates in the situation where coordinates are aligned with thewind vector. We show that rotating coordinates so that the x axis is alignedwith the mean wind vector has the effect of high pass filtering the scalar covariance,&macr;wc, such that contributions to the aerodynamic flux from atmosphericmotions with periods longer than the averaging period are lost while those of shorterperiod are distorted.
[本文引用: 4]
[32]
Aubinet M, Vesala T, Papale D.Eddy Covariance—A Practical Guide to Measurement and Data Analysis[M]. New York: Springer Science & Business Media, 2012.
[本文引用: 1]
[33]
Xu Ziwei, Liu Shaomin, Li Xin, et al.Intercomparison of surface energy flux measurement systems used during the HiWATER-MUSOEXE[J]. Journal of Geophysical Research: Atmospheres, 2013, 118: 13 140-13 157.
DOI:10.1002/2013JD020260 URL
[1] Agreement among instruments is very important for the Multi-Scale Observation Experiment on Evapotranspiration over heterogeneous land surfaces of The Heihe Watershed Allied Telemetry Experimental Research (HiWATER-MUSOEXE), particularly in regard to radiation and turbulent flux measurements. Before HiWATER-MUSOEXE was conducted, 20 eddy covariance (EC) system sets, 18 radiometer sets, and seven large aperture scintillometers (LASs) sets were intercompared over the Gobi desert between 14 and 24 May 2012. For radiometers, the four-component radiation measurements exhibited good agreement090000the average root-mean-square error (RMSE) and mean relative error (MRE) for the net radiation were 10.38090009W090009m0908082 and 1.24%, respectively. With regard to the EC systems, the best consistency for sensible heat fluxes was found among CSAT3 sonic anemometers and Li7500A/Li7500/EC150 combinations (average RMSE, 12.30090009W090009m0908082 and MRE, 0908081.36%), followed by Gill sonic anemometers and Li7500A/Li7500 combinations when a proper angle of attack correction method was applied (average RMSE, 16.75090009W m0908082 and MRE, 0908085.52%). The sensible heat flux measured using different LASs agreed well with high correlation coefficients090000the average RMSE and MRE values were 10.26090009W090009m0908082 and 5.48% for boundary layer scintillometer (BLS) 900, 16.32090009W090009m0908082 and 10.47% for BLS450, and 14.38090009W090009m0908082 and 0908083.72% for ZZLAS, respectively. The EC and LAS measurements were compared and agreed well over homogeneous underlying surfaces, which also indicated that the EC and LAS measurements would be comparable in the follow-up experiment. The intercomparison results can be used to determine instrument placement and are very helpful for subsequent data analysis.
[本文引用: 2]
[34]
Mauder M, Jegede O O, Okogbue E C, et al.Surface energy balance measurements at a tropical site in West Africa during the transition from dry to wet season[J]. Theoretical and Applied Climatology, 2007, 89(3/4): 171-183.
[本文引用: 1]
[35]
Xu Ziwei, Liu Shaomin, Xu Tongren, et al.The observation and calculation method of soil heat flux and its impact on the energy balance closure[J]. Advances in Earth Science, 2013, 28(8): 875-889.
Magsci
<p>土壤热通量是地表能量平衡的重要分量,对其测算方法的研究对理解能量平衡过程具有十分重要的意义。利用2010年馆陶站土壤热通量等相关观测数据对多种测算土壤热通量的方法:实测土壤热通量和热储存量的结合方法(PlateCal)、热传导方程校正法(TDEC)、谐波分析法(HM)、平均土壤热电偶法(TCAV)、耦合热传导&mdash;对流法(ITCC)获取的地表土壤热通量进行了对比分析,并且采用最优方法计算馆陶站2008&mdash;2010年的地表土壤热通量,分析了该站土壤热通量日、季节变化特征。主要结论如下:①PlateCal和TDEC法分别为获取土壤热通量的最优观测与计算方法,而HM,TCAV和ITCC法计算结果均不理想;②PlateCal与TDEC法对地表土壤温度均不敏感,而HM法对地表土壤温度则比较敏感,各种地表土壤热通量的观测与计算方法均对土壤湿度敏感;③馆陶站冬小麦、玉米覆盖地表及地表裸露时期的地表土壤热通量均呈现典型的日、季节变化特征,与净辐射变化趋势一致;④考虑热储存后,可将馆陶站2010年各月地表能量闭合率提高4%~11%,对2008&mdash;2010年的年能量平衡闭合率提高3%~5%。地表土壤热通量;土壤热流板;地表能量平衡闭合率</p>
[徐自为, 刘绍民, 徐同仁, . 不同土壤热通量测算方法的比较及其对地表能量平衡闭合影响的研究[J]. 地球科学进展, 2013, 28(8): 875-889.]
Magsci
<p>土壤热通量是地表能量平衡的重要分量,对其测算方法的研究对理解能量平衡过程具有十分重要的意义。利用2010年馆陶站土壤热通量等相关观测数据对多种测算土壤热通量的方法:实测土壤热通量和热储存量的结合方法(PlateCal)、热传导方程校正法(TDEC)、谐波分析法(HM)、平均土壤热电偶法(TCAV)、耦合热传导&mdash;对流法(ITCC)获取的地表土壤热通量进行了对比分析,并且采用最优方法计算馆陶站2008&mdash;2010年的地表土壤热通量,分析了该站土壤热通量日、季节变化特征。主要结论如下:①PlateCal和TDEC法分别为获取土壤热通量的最优观测与计算方法,而HM,TCAV和ITCC法计算结果均不理想;②PlateCal与TDEC法对地表土壤温度均不敏感,而HM法对地表土壤温度则比较敏感,各种地表土壤热通量的观测与计算方法均对土壤湿度敏感;③馆陶站冬小麦、玉米覆盖地表及地表裸露时期的地表土壤热通量均呈现典型的日、季节变化特征,与净辐射变化趋势一致;④考虑热储存后,可将馆陶站2010年各月地表能量闭合率提高4%~11%,对2008&mdash;2010年的年能量平衡闭合率提高3%~5%。地表土壤热通量;土壤热流板;地表能量平衡闭合率</p>
[本文引用: 2]
[36]
Heusinkveld B G, Jacobs A F G,Holtslag A M, et al. Surface energy balance closure in an arid region: Role of soil heat flux[J]. Agricultural and Forest Meteorology, 2004, 122(1/2): 21-37.
DOI:10.1016/j.agrformet.2003.09.005 URL
The large soil heat fluxes in hot desert regions are very important in energy balance studies. Surface energy balance (SEB) observations, however, reveal that there is an imbalance in surface flux measurements and that it is difficult to isolate those flux measurements causing the imbalance errors. In this paper a new approach was tested focusing on a high temporal resolution of soil heat flux measurements at the surface. To determine if improved soil heat flux measurements could be obtained for a sandy desert, a standard soil heat flux sensor was buried at the surface of a playa and covered with less than 1mm of playa soil, as opposed to burial several centimetres deep. It was anticipated that this would permit direct surface soil heat flux measurements. A field campaign was carried out in September and October in 1997 and for the same period in 2000, in a sandy desert belt situated in Nizzana, NW Negev, Israel. The research was designed to examine the separate components of the SEB. The location was considered ideal since only the soil heat flux and sensible heat flux are balanced by the net radiation. This new approach was compared to traditional soil heat flux measurements. The resulting energy balance closure was found to be very good. The results suggest that this method could be used as a reference surface soil heat flux measurement. Further examination of the energy balance closure is needed over a range of land surface types.
[本文引用: 1]
[37]
Meyers T P, Hollinger S E.An assessment of storage terms in the surface energy balance of maize and soybean[J]. Agricultural and Forest Meteorology, 2004, 125(1/2): 105-115.
DOI:10.1016/j.agrformet.2004.03.001 URL
Discrepancies in closure of the surface energy balance is often an issue for many land surface types. The role of canopy storage terms from canopy water content and photosynthesis is usually neglected in the surface energy balance of crops. Data from a research flux tower in central Illinois were used to evaluate these storage terms and their impact on the closure of the surface energy balance. When considered separately, the storage terms are generally a small fraction (<5%) of the net radiation. However, the combination of soil and canopy heat storage and the stored energy in the carbohydrate bonds from photosynthesis are shown to comprise roughly 15% of the total net radiation for maize and 7% for soybean during the morning hours from 06:00 to 12:00聽h when the canopy is fully developed. When all of the storage terms were considered, the slopes of the 1:1 line between net radiation and the partitioned fluxes (latent, sensible, ground, and storage) increased by 10% and the scatter about the 1:1 line decreased for both maize and soybean with the r 2 increasing by 0.05.
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Zhou Yanzhao, Li Dan, Liu Heping, et al.Diurnal variations of the flux imbalance over homogeneous and heterogeneous landscapes[J]. Boundary-Layer Meteorology, 2018, 168(3): 417-442.
DOI:10.1007/s10546-018-0358-2 URL
It is well known that the sum of the turbulent sensible and latent heat fluxes as measured by the eddy-covariance method is systematically lower than the available energy (i.e., the net radiation...
[本文引用: 14]
[43]
Schalkwijk J,Jonker H J J, Siebesma A P. An investigation of the eddy-covariance flux imbalance in a year-long large-eddy simulation of the weather at Cabauw[J]. Boundary-Layer Meteorology, 2016, 160(1): 17-39.
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Huang Jianping, Lee Xuhui, Patton E G.A modelling study of flux imbalance and the influence of entrainment in the convective boundary layer[J]. Boundary-Layer Meteorology, 2008, 127(2): 273-292.
DOI:10.1007/s10546-007-9254-x Magsci
<a name="Abs1"></a>It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared to the available energy. The flux imbalance problem is investigated using the NCAR&#8217;s large-eddy simulation (LES) model imbedded with an online scheme to calculate Reynolds-averaged fluxes. A top&#8211;down and a bottom&#8211;up tracer are implemented into the LES model to quantify the influence of entrainment and bottom&#8211;up diffusion processes on flux imbalance. The results show that the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on <i>u</i> <sub>*</sub>/<i>w</i> <sub>*</sub>, where <i>u</i> <sub>*</sub> and <i>w</i> <sub>*</sub> are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to <i>z</i>/<i>z</i> <sub> <i>i</i> </sub>, where <i>z</i> <sub> <i>i</i> </sub> is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude and its horizontal and vertical distributions. The flux imbalance of heat and the bottom&#8211;up tracer is tightly related to turbulent coherent structures, whereas for the top&#8211;down diffusion, such relations are weak to nonexistent. Our results are broadly consistent with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts of numerical schemes.
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Steinfeld G, Letzel M O, Raasch S, et al.Spatial representativeness of single tower measurements and the imbalance problem with eddy-covariance fluxes: Results of a large-eddy simulation study[J]. Boundary-Layer Meteorology, 2007, 123(1): 77-98.
DOI:10.1007/s10546-006-9133-x Magsci
<a name="Abs1"></a>A large-eddy simulation (LES) study is presented that investigates the spatial variability of temporal eddy covariance fluxes and the systematic underestimation of representative fluxes linked to them. It extends a prior numerical study by performing high resolution simulations that allow for virtual measurements down to 20 m in a convective boundary layer, so that conditions for small tower measurement sites can be analysed. It accounts for different convective regimes as the wind speed and the near-surface heat flux are varied. Moreover, it is the first LES imbalance study that extends to the stable boundary layer. It reveals shortcomings of single site measurements and the necessity of using horizontally-distributed observation networks. The imbalances in the convective case are attributed to a locally non-vanishing mean vertical advection due to turbulent organised structures (TOS). The strength of the TOS and thus the imbalance magnitude depends on height, the horizontal mean wind and the convection type. Contrary to the results of a prior study, TOS cannot generally be responsible for large energy imbalances: at low observation heights (corresponding to small towers and near-surface energy balance stations) the TOS related imbalances are generally about one order of magnitude smaller than those in field experiments. However, TOS may cause large imbalances at large towers not only in the case of cellular convection and low wind speeds, as found in the previous study, but also in the case of roll convection at large wind speeds. <div class="AbstractPara"> <div class="">In the stably stratified boundary layer for all observation heights neither TOS nor significant imbalances are observed.
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Kanda M, Inagaki A, Letzel M O, et al.LES study of the energy imbalance problem with eddy covariance fluxes[J]. Boundary-Layer Meteorology, 2004, 110(3): 381-404.
DOI:10.1023/B:BOUN.0000007225.45548.7a Magsci URL
The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the `turbulent' heat flux at a grid point from the horizontally averaged `total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks. The temporally averaged `turbulent' flux based on a point measurement systematically underestimates the `total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.
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Inagaki A, Letzel M O, Raasch S, et al.Impact of surface heterogeneity on energy imbalance: A study using LES[J]. Journal of the Meteorological Society of Japan, 2006, 84(1): 187-198.
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Stoy P C, Mauder M, Foken T, et al.A data-driven analysis of energy balance closure across FLUXNET research sites: The role of landscape scale heterogeneity[J]. Agricultural and Forest Meteorology, 2013, 171: 137-152.
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Mauder M, Foken T.Impact of post-field data processing on eddy covariance flux estimates and energy balance closure[J]. Meteorologische Zeitschrift, 2006, 15(6): 597-609.
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Sakai R K, Fitzjarrald D R, Moore K E.Importance of low-frequency contributions to eddy fluxes observed over rough surfaces[J]. Journal of Applied Meteorology, 2001, 40(12): 2 178-2 192.
DOI:10.1175/1520-0450(2001)0402.0.CO;2 URL
Eddy covariance flux observations at a deciduous temperate forest site (83 days) and at a boreal forest site (21 days) are analyzed for midday periods (1100-1400 LT). Approximate stationarity of the time series is demonstrated, and the ensemble-averaged roughness sublayer cospectra are presented. Spectral and cospectral forms in the roughness sublayer are more peaked than those found in an inertial sublayer. They exhibit similar forms dependent on (z d)/(h d), where d is the displacement height and h is the canopy height. The inertial-layer spectral forms are recovered when observations are made where this scaled height is approximately 4. For a sample summer at the midlatitude deciduous forest, large eddies with periods from 4 to 30 min contribute about 17% to surface eddy fluxes of heat, water vapor, and carbon dioxide (CO). Much larger contributions can occur in light-wind conditions. This effect, likely caused by the passage of convective boundary layer eddies, is not observed when using many currently popular averaging procedures. Several running-mean periods have been used to assess the effect of the mean removal procedure on flux estimates. Given the assumption that large eddies would have been sampled at the towers had an ensemble measurement been possible, a correction is proposed based primarily on the mean wind speed to adjust fluxes obtained using short averaging intervals. This correction is successful in achieving observational energy-balance closure at two dissimilar forested sites. Cospectral similarity is found for all scalars studied. Daytime fluxes of CO, for example, can be underestimated at standard flux towers by 10%-40%, depending on wind speed.
[本文引用: 1]
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Mauder M, Desjardins R L, Pattey E, et al.An attempt to close the daytime surface energy balance using spatially-averaged flux measurements[J]. Boundary-Layer Meteorology, 2010, 136(2): 175-191.
DOI:10.1007/s10546-010-9497-9 Magsci
Single-tower eddy-covariance measurements represent the complete surface flux of a scalar only under idealized conditions. Therefore, we often find an underestimation of energy fluxes expressed as a lack of energy balance closure at many sites. In this study, a multi-tower approach to measure atmospheric energy fluxes based on spatial averaging is evaluated and possible mechanisms causing a lack of energy balance closure are analysed, focussing on daytime data only. It is shown that the multi-tower technique is also unable to measure the entire flux for our site, likely because the assumption of horizontal homogeneity is violated. Heterogeneity-induced and buoyancy-driven quasi-stationary circulations are probably the dominant processes causing the underestimation of energy fluxes. A dependence of the energy balance residual on stability is found, with residuals close to zero for stable stratification, a maximum under unstable to near-neutral conditions and still relatively large residuals for stronger instability. Assuming the processes transporting energy and CO<sub>2</sub> are similar, the implications on long-term CO<sub>2</sub> flux measurements are analysed. Accordingly, the resulting selective systematic error of cumulative net ecosystem exchange estimates for agricultural regions such as ours can be of the order of more than 100%, since mainly the fluxes during periods of net CO<sub>2</sub> uptake are underestimated while periods of net CO<sub>2</sub> release are much less affected by this bias. Further investigations about this issue are highly warranted.
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Liu Shaomin, Xu Ziwei, Wang Weizhen, et al.A comparison of eddy-covariance and large aperture scintillometer measurements with respect to the energy balance closure problem[J]. Hydrology and Earth System Sciences, 2011, 15(4): 1 291-1 306.
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Aubinet M, Feigenwinter C, Heinesch B, et al.Direct advection measurements do not help to solve the night-time CO2 closure problem: Evidence from three different forests[J]. Agricultural and Forest Meteorology, 2010, 150(5): 655-664.
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Mauder M, Desjardins R L, Pattey E, et al.Measurement of the sensible eddy heat flux based on spatial averaging of continuous ground-based observations[J]. Boundary-Layer Meteorology, 2008, 128(1): 151-172.
DOI:10.1007/s10546-008-9279-9 Magsci
<a name="Abs1"></a>Using the standard eddy-covariance (EC) method to quantify mass and energy exchange at a single location usually results in an underestimation of vertical eddy fluxes at the surface. In order to better understand the reasons for this underestimation, an experimental set-up is presented that is based on spatial averaging of air temperature data from a network of ground-based sensors over agricultural land. For eight days during the 34-day observational period in May and June 2007, additional contributions to the sensible heat flux of more than 50Wm<sup>&#8722;2</sup> were measured in the lower surface layer by applying the spatial EC method as opposed to the standard temporal EC method. Smaller but still significant additional sensible heat fluxes were detected for four more days. The additional energy is probably transported in organised convective structures resulting in a mean vertical wind velocity unequal to zero at the tower location. The results show that convective transport contributes significantly to the surface energy budget for measurement heights as low as 2&#8211;3&nbsp;m. Since these structures may be quasi-stationary, they can hardly be captured by a single-location measurement. The spatial EC set-up presented here is capable of quantifying contributions to the sensible heat flux from structures up to the scale of our spatial sensor network, which covered an area 3.5 ×&nbsp;3.5&nbsp;km. For future experiments aiming at closing the energy balance, the spatial EC method should be employed to measure both the sensible and latent heat fluxes. Experimental determination of the horizontal advection of sensible and latent heat should also be considered, since such transport must occur due to convergence and divergence related to convection.
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Anemometer and CO2concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While...
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DOI:10.1023/A:1002786702909 Magsci
<a name="Abs1"></a>Equations are presented to correct eddy-covariancemeasurements for both fluctuations in density andnon-zero mean advection, induced by convergence ordivergence of flow, and spatial source/sinkinhomogeneity, under steady-state and transientconditions. This correction collapses to theWebb&#x2013;Pearman&#x2013;Leuning expression ifthe mean vertical velocity is zero, and formally addsthe Webb&#x2013;Pearman&#x2013;Leuning expression to the correctionssuggested by Lee for conditions ofnon-zero vertical velocity and source/sink and meanscalar horizontal homogeneity. The equation requiresmeasurement of the mean vertical gradients of thescalar concentration of interest (air temperature,humidity, CO<sub>2</sub>) as well as an accurateestimation of the mean vertical velocity, in additionto the vertical eddy covariance of the scalar. Simplemethods for the approximation of sensor tilt andcomplex terrain flow angle are presented, to allowestimation of non-zero mean vertical velocities. Theequations are applied to data from a maize crop and aforest to give examples of when the correction issignificant. In addition, a term for thethermodynamic expansion energy associated with watervapour flux is derived, which implies that the sonictemperature derived sensible heat flux will accuratelyinclude this contribution.
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It is shown from the mass conservation and the continuity equations that the net ecosystem exchange (NEE) of a scalar constituent with the atmosphere should be
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DOI:10.1175/BAMS-D-14-00114.1 URL
Since the advent of computers midway through the twentieth century, computational resources have increased exponentially. It is likely they will continue to do so, especially when accounting for recent trends in multicore processors. History has shown that such an increase tends to directly lead to weather and climate models that readily exploit the extra resources, improving model quality and resolution. We show that Large-Eddy Simulation (LES) models that utilize modern, accelerated (e.g., by GPU or coprocessor), parallel hardware systems can now provide turbulence-resolving numerical weather forecasts over a region the size of the Netherlands at 100-m resolution. This approach has the potential to speed the development of turbulence-resolving numerical weather prediction models.
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Cui Guixiang, Xu Chunxiao, Zhang Zhaoshun, et al.Progress in large eddy simulation of turbulent flows[J]. Acta Aerodynamica Sinica, 2004, 22(2): 121-129.
DOI:10.3969/j.issn.0258-1825.2004.02.001 Magsci URL
本文简要陈述湍流大涡数值模拟的原理、优点,着重讨论湍流大涡数值模拟方法的关键问题及其可能解决的途径,包括脉动的过滤、亚格子模型、近壁模型和标量湍流的大涡数值模拟中的特殊问题.文章强调大涡数值模拟中亚格子应力的本质是可解尺度湍流和不可解尺度湍流动量间的输运,并以作者最近提出的新型亚格子模型说明发展亚格子模型的正确途径.文章最后提出湍流大涡数值模拟近期需要迫切解决的问题和其他具有挑战性的方向.
[崔桂香, 许春晓, 张兆顺,. 湍流大涡数值模拟进展[J]. 空气动力学学报, 2004, 22(2): 121-129.]
DOI:10.3969/j.issn.0258-1825.2004.02.001 Magsci URL
本文简要陈述湍流大涡数值模拟的原理、优点,着重讨论湍流大涡数值模拟方法的关键问题及其可能解决的途径,包括脉动的过滤、亚格子模型、近壁模型和标量湍流的大涡数值模拟中的特殊问题.文章强调大涡数值模拟中亚格子应力的本质是可解尺度湍流和不可解尺度湍流动量间的输运,并以作者最近提出的新型亚格子模型说明发展亚格子模型的正确途径.文章最后提出湍流大涡数值模拟近期需要迫切解决的问题和其他具有挑战性的方向.
[本文引用: 1]
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Jiang Weimei, Miao Shiguang.30 years review and perspective of the research on the large eddy simulation and atmospheric boundary layer[J]. Advances in Nature Sciences, 2004, 14(1): 13-21.
DOI:10.3321/j.issn:1002-008X.2004.01.003 URL
对大涡模拟(LES)技术应用于大气边界层(PBL)研究30年来的发展历程、应用前景及发展趋势作了简要评述,内容包括:LES网格体积平均方程组,次网格闭合方案(K闭合、湍能(TKE)闭合、二阶闭合、动力学闭合、随机闭合、各向异性TKE闭合和非线性闭合);均匀、非均匀下垫面对流边界层((2BL)的大涡模拟,稳定边界层(SBL)、实际PBL的大涡模拟,森林下垫面流场和建筑物周围流场的大涡模拟,污染扩散的大涡模拟,LES-化学模式,对流层大涡模拟,大涡模拟对模拟域、网格及大涡时间尺度的敏感性,适用于L,ES和中尺度模拟的湍流方案研究等.提出了LES需要进一步研究的问题,如次网格(SGS)闭合方案,物理过程参数化方案,高分辨卫星和Doppler雷达资料的使用,与中尺度模式的连接与嵌套等;以及可能的应用领域,如大气湍流发展及湍涡相互作用,复杂地形流场及城市边界层模拟,污染扩散模拟,LES-大气化学模拟,中尺度气象模拟,数值天气预报及大气环流模式中高分辨边界层方案(大涡机制)的引入.希望能为大气边界层研究与应用中发挥LES模拟效能的成功实施起到一定的推动作用.
[蒋维楣, 苗世光. 大涡模拟与大气边界层研究——30年回顾与展望[J]. 自然科学进展, 2004, 14(1): 13-21.]
DOI:10.3321/j.issn:1002-008X.2004.01.003 URL
对大涡模拟(LES)技术应用于大气边界层(PBL)研究30年来的发展历程、应用前景及发展趋势作了简要评述,内容包括:LES网格体积平均方程组,次网格闭合方案(K闭合、湍能(TKE)闭合、二阶闭合、动力学闭合、随机闭合、各向异性TKE闭合和非线性闭合);均匀、非均匀下垫面对流边界层((2BL)的大涡模拟,稳定边界层(SBL)、实际PBL的大涡模拟,森林下垫面流场和建筑物周围流场的大涡模拟,污染扩散的大涡模拟,LES-化学模式,对流层大涡模拟,大涡模拟对模拟域、网格及大涡时间尺度的敏感性,适用于L,ES和中尺度模拟的湍流方案研究等.提出了LES需要进一步研究的问题,如次网格(SGS)闭合方案,物理过程参数化方案,高分辨卫星和Doppler雷达资料的使用,与中尺度模式的连接与嵌套等;以及可能的应用领域,如大气湍流发展及湍涡相互作用,复杂地形流场及城市边界层模拟,污染扩散模拟,LES-大气化学模拟,中尺度气象模拟,数值天气预报及大气环流模式中高分辨边界层方案(大涡机制)的引入.希望能为大气边界层研究与应用中发挥LES模拟效能的成功实施起到一定的推动作用.
[本文引用: 2]
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Liu Yushi, Miao Shiguang, Zhang Chaolin, et al.Study on micro-atmospheric environment by coupling large eddy simulation with mesoscale model[J]. Journal of Wind Engineering and Industrial Aerodynamics, 2012, 107: 106-117.
DOI:10.1016/j.jweia.2012.03.033 URL
This paper investigates wind field and traffic pollutant dispersion at street level in a local urban area. A coupling method is employed that the time-varying boundary values of velocity and temperature at the local urban area are provided by the Weather Research and Forecasting (WRF) mesoscale model which computes the atmospheric flow over the entire city while the atmospheric environmental flows are computed by a large eddy simulation (LES) code developed by the authors in the local urban area. In order to reduce the LES computation cost, a combined model for buildings in the urban area is proposed that in a central area, where the wind and pollution is most concerned, the buildings are properly resolved in fine grids and non-slip flow condition is prescribed at rigid boundaries whereas in the surrounding region the buildings are treated as drag element and coarse grids are used in computation. The wind field, temperature and carbon monoxide concentration fields are computed from 9 a.m. of October 26, 2009 to 8 a.m. of the next day in a built-up district of Beijing and the results show good agreement with observations. The advantage of proposed approach is that the multi-scale effect of atmospheric motion is considered and temporal evolution of flow structure in local urban area can be simulated by large eddy simulation at moderate computation cost.
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Hong S Y, Dudhia J. Next-generation numerical weather prediction: Bridging parameterization, explicit clouds,large eddies[J]. Bulletin of the American Meteorological Society, 2012, 93(1): ES6-ES9.
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[68]
Huang H-Y, Margulis S A.On the impact of surface heterogeneity on a realistic convective boundary layer[J]. Water Resources Research, 2009, 45(4): W04425. DOI:10.1029/2008WR007175.
DOI:10.1029/2008WR007175 URL
Turbulent interactions of momentum and scalars within the convective boundary layer (CBL) over a heterogeneous surface can have a strong influence on both local flow properties and regional weather and climate. Using realistic heterogeneous surface flux data as well as mean vertical soundings obtained from the Soil-Moisture Atmosphere Coupling Experiment 2002 (SMACEX) as boundary and initial conditions in a large-eddy simulation (LES) model, this research attempts to further our understanding of the impact of surface heterogeneity on a growing CBL in a realistic environment. On the basis of a newly proposed method, a set of realizations precisely replicating the basic statistics (mean and variance) and cross correlation of surface flux boundary conditions (relative to the actual fields at the SMACEX) are used for a series of LES experiments. The impacts of heterogeneity on two aspects of the CBL are discussed in this study: (1) the characteristic vertical profiles of CBL properties and (2) the vertical mixing scales of the CBL. Within the set of conditions examined in this study, the potential temperature is more sensitive to the surface heterogeneity scale than to specific humidity, and the variation of velocity (both horizontal and vertical) clearly reflects the impact of surface organization. The vertical profile of the coefficient of variation calculated from the potential temperature field is introduced to identify the thermal blending height, and a good agreement is obtained compared to the predicted values using formulations from the literature. A commonly used formulation for the momentum blending height was found to inadequately represent the vertical mixing. Another stability-dependent formulation combining both shear and convective characteristics is shown to provide more reasonable results.
[69]
Cui Guixiang, Zhang Zhaoshun, Xu Chunxiao, et al.Research advances in large eddy simulation of urban atmospheric environment[J]. Advances in Mechanics, 2013, 43(3): 295-328.
DOI:10.6052/1000-0992-13-016 Magsci URL
本文回顾城市大气污染扩散的研究实践与进展,介绍城市大气环境流动的特点和用现代计算流体力学手段开展城市大气环境的大涡模拟研究方法,包括:数学模型,控制方程、亚网格湍流模式、定解条件及其数值方法.其中,重点介绍中尺度到微尺度的耦合模型与适用于复杂城市下垫面(满足大涡模拟分辨率要求且计算量较小)的组合模型;并给出实际算例和结果分析,包括准确度的统计估算和湍流特性等.最后,讨论城市大气环境数值模拟方法进一步改进的方向和城市大气环境流动与污染物扩散数值研究应重点关注的几个问题.
[崔桂香, 张兆顺, 许春晓, . 城市大气环境的大涡模拟研究进展[J]. 力学进展, 2013, 43(3): 295-328.]
DOI:10.6052/1000-0992-13-016 Magsci URL
本文回顾城市大气污染扩散的研究实践与进展,介绍城市大气环境流动的特点和用现代计算流体力学手段开展城市大气环境的大涡模拟研究方法,包括:数学模型,控制方程、亚网格湍流模式、定解条件及其数值方法.其中,重点介绍中尺度到微尺度的耦合模型与适用于复杂城市下垫面(满足大涡模拟分辨率要求且计算量较小)的组合模型;并给出实际算例和结果分析,包括准确度的统计估算和湍流特性等.最后,讨论城市大气环境数值模拟方法进一步改进的方向和城市大气环境流动与污染物扩散数值研究应重点关注的几个问题.
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Chen Jiayi, Fan Shaohua, Zhao Chuanfeng, et al.The underestimation of the turbulent fluxes in eddy correlation techniques[J]. Chinese Journal of Atmospheric Sciences, 2006, 30(3): 423-432.
DOI:10.3878/j.issn.1006-9895.2006.03.06 Magsci
针对野外实验所发现的不同观测法测定地表能通量不平衡问题,进行了均匀加热大气边界层的大涡模拟实验.用模拟的湍流风、温度和湿度涨落的时间序列证实,对流边界层低频涡普遍存在,并经常以一簇一簇热泡的形式出现.风速较小时,有限时长的取样不足以捕捉低频涡的贡献,可造成涡旋相关法测量的统计量异常偏低.仿照涡旋相关法的步骤进行数据处理发现,经去除平均或趋势计算的温度和湿度通量偏低程度在边界层下部随观测高度的增高而显著,其中尤以湿度通量为甚.其结果在一定程度上可以解释低风速条件下地表能通量测量的不闭合问题,但是尚不能完全解释诸如青藏高原实验出现的严重不闭合.文中对此作了探讨性的讨论.
[陈家宜, 范邵华, 赵传峰, . 涡旋相关法测定湍流通量偏低的研究[J]. 大气科学, 2006, 30(3): 423-432.]
DOI:10.3878/j.issn.1006-9895.2006.03.06 Magsci
针对野外实验所发现的不同观测法测定地表能通量不平衡问题,进行了均匀加热大气边界层的大涡模拟实验.用模拟的湍流风、温度和湿度涨落的时间序列证实,对流边界层低频涡普遍存在,并经常以一簇一簇热泡的形式出现.风速较小时,有限时长的取样不足以捕捉低频涡的贡献,可造成涡旋相关法测量的统计量异常偏低.仿照涡旋相关法的步骤进行数据处理发现,经去除平均或趋势计算的温度和湿度通量偏低程度在边界层下部随观测高度的增高而显著,其中尤以湿度通量为甚.其结果在一定程度上可以解释低风速条件下地表能通量测量的不闭合问题,但是尚不能完全解释诸如青藏高原实验出现的严重不闭合.文中对此作了探讨性的讨论.
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With the recognition that eddy flux measurements are relatively accurate for a variety of common situations, a number of issues leading to inaccurate flux estimates and/or ambiguous interpretation of flux values are surveyed. These issues include inadvertent conversion of random errors to systematic errors, ambiguous differentiation between turbulence and other motions, and omission of transport by stationary eddies. Correcting for sonic misalignment and flow distortion in the presence of real systematic vertical motions is also problematic. Special emphasis is placed on the need for spatial information, partly to include vertical transport by stationary circulations induced by small-scale surface features. While no categorical solutions to the above problems are offered, promising approaches worthy of more investigation are discussed.
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DOI:10.1007/BF00128057 URL
Most one-dimensional models of flow within vegetation canopies are based on horizontally averaged flow variables. This paper formalizes the horizontal averaging operation. Two averaging schemes are considered: pure horizontal averaging at a single instant, and time averaging followed by horizontal averaging. These schemes produce different forms for the mean and turbulent kinetic energy balances, and especially for the ‘wake production’ term describing the transfer of energy from large-scale motion to wake turbulence by form drag. The differences are primarily due to the appearance, in the covariances produced by the second scheme, of dispersive components arising from the spatial correlation of time-averaged flow variables. The two schemes are shown to coincide if these dispersive fluxes vanish.
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DOI:10.1007/s10546-014-9922-6 Magsci
The modified ogive analysis and the block ensemble average were employed to investigate the impact of the averaging time extension on the energy balance closure over six land-use types. The modified ogive analysis, which requires a steady-state condition, can extend the averaging time up to a few hours and suggests that an averaging time of 30 min is still overall sufficient for eddy-covariance measurements over low vegetation. The block ensemble average, which does not require a steady-state condition, can extend the averaging time to several days. However, it can improve the energy balance closure for some sites during specific periods, when secondary circulations exist in the vicinity of the sensor. These near-surface secondary circulations mainly transport sensible heat, and when near-ground warm air is transported upward, the sensible heat flux observed by the block ensemble average will increase at longer averaging times. These findings suggest an alternative energy balance correction for a ground-based eddy-covariance measurement, in which the attribution of the residual depends on the ratio of sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction is larger than in the energy balance correction that preserves the Bowen ratio.
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Li Zhengquan, Yu Guirui, Wen Xuefa, et al.Energy balance closure at ChinaFLUX sites[J]. Science in China (Series D), 2004, 48(Suppl.1): 51-62.
URL
涡度相关观测网络能够对生态系统的水碳通量及其气候背景资料进行长期观测,能量平衡闭合状况作为通量观测数据评价的一个重要参考指标,备受通量界研究者的重视.本研究运用OLS(OrdinaryLeastSquares)、RMA(ReducedMajorAxis)、EBR(EnergyBalanceRatio)和δ频率分布四种统计方法对湍流通量(显热和潜热)与有效能量(净辐射、土壤热通量、冠层热储量)的关系进行了分析,对ChinaFLUX各站点的能量平衡闭合状况进行了综合评价,给出了能量平衡闭合程度日变化和季节变化趋势.研究结果表明各站点能量平衡都没有完全闭合,但因站点的条件不同,其不闭合程度略有差异;在夜间不闭合的程度比白天更加明显;闭合程度随摩擦风速的增大而有所改善.总体来说,在现有通量观测系统中,显热和潜热湍流通量往往会被低估,而有效能量项则会被高估.最后讨论了通量观测中的采样误差、仪器测量可能产生的系统偏差、其他能量吸收项的作用、高频与低频湍流通量损失以及平流作用对能量平衡闭合状况的影响.
[李正泉, 于贵瑞, 温学发, . 中国通量观测网络(ChinaFLUX)能量平衡闭合状况的评价[J]. 中国科学:D辑, 2004, 34(A02): 46-56.]
URL
涡度相关观测网络能够对生态系统的水碳通量及其气候背景资料进行长期观测,能量平衡闭合状况作为通量观测数据评价的一个重要参考指标,备受通量界研究者的重视.本研究运用OLS(OrdinaryLeastSquares)、RMA(ReducedMajorAxis)、EBR(EnergyBalanceRatio)和δ频率分布四种统计方法对湍流通量(显热和潜热)与有效能量(净辐射、土壤热通量、冠层热储量)的关系进行了分析,对ChinaFLUX各站点的能量平衡闭合状况进行了综合评价,给出了能量平衡闭合程度日变化和季节变化趋势.研究结果表明各站点能量平衡都没有完全闭合,但因站点的条件不同,其不闭合程度略有差异;在夜间不闭合的程度比白天更加明显;闭合程度随摩擦风速的增大而有所改善.总体来说,在现有通量观测系统中,显热和潜热湍流通量往往会被低估,而有效能量项则会被高估.最后讨论了通量观测中的采样误差、仪器测量可能产生的系统偏差、其他能量吸收项的作用、高频与低频湍流通量损失以及平流作用对能量平衡闭合状况的影响.
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Eder F, De Roo F, Kohnert K, et al.Evaluation of two energy balance closure parametrizations[J]. Boundary-Layer Meteorology, 2014, 151(2): 195-219.
DOI:10.1007/s10546-013-9904-0 Magsci URL
A general lack of energy balance closure indicates that tower-based eddy-covariance (EC) measurements underestimate turbulent heat fluxes, which calls for robust correction schemes. Two parametrization approaches that can be found in the literature were tested using data from the Canadian Twin Otter research aircraft and from tower-based measurements of the German Terrestrial Environmental Observatories (TERENO) programme. Our analysis shows that the approach of Huang et al. (Boundary-Layer Meteorol 127:273–292, 2008 ), based on large-eddy simulation, is not applicable to typical near-surface flux measurements because it was developed for heights above the surface layer and over homogeneous terrain. The biggest shortcoming of this parametrization is that the grid resolution of the model was too coarse so that the surface layer, where EC measurements are usually made, is not properly resolved. The empirical approach of Panin and Bernhofer (Izvestiya Atmos Oceanic Phys 44:701–716, 2008 ) considers landscape-level roughness heterogeneities that induce secondary circulations and at least gives a qualitative estimate of the energy balance closure. However, it does not consider any feature of landscape-scale heterogeneity other than surface roughness, such as surface temperature, surface moisture or topography. The failures of both approaches might indicate that the influence of mesoscale structures is not a sufficient explanation for the energy balance closure problem. However, our analysis of different wind-direction sectors shows that the upwind landscape-scale heterogeneity indeed influences the energy balance closure determined from tower flux data. We also analyzed the aircraft measurements with respect to the partitioning of the “missing energy” between sensible and latent heat fluxes and we could confirm the assumption of scalar similarity only for Bowen ratios \(\approx \) 1.
[本文引用: 2]
[86]
Hu Yinqiao, Chen Jinbei, Shihua.From the clasic theory of turbulence to the Nonequilibrium Thermodynamic Theory of atmospheric turbulence[J]. Plateau Meteorology, 2012, 31(1): 1-27.
Magsci
湍流是日常生活中一种普遍的自然现象, 也是经典物理学仍未完全解决的难题。湍流更是大气运动的最基本特征。本文系统地回顾了大气湍流经典理论发展简史, 进一步详细介绍了大气湍流非平衡态热力学理论。大气湍流非平衡态热力学理论在熵平衡方程中引入动力过程, 进而统一推导出大气湍流输送的Fourier定律、 Flick定律和Newton定律, 证明了Dufour效应、 Soret效应、 可逆动力过程与热力不可逆湍流输送过程之间的交叉耦合效应, 以及湍流强度定理。这些定律和定理中得到了观测的事实验证, 同时它们的唯象系数也由观测资料所确定。湍流强度定理揭示, 湍流发展的宏观原因是速度和温度的剪切效应, Reynolds湍流和Rayleigh-B&eacute;nard湍流共存于大气湍流中。热力过程和动力过程间耦合效应现象的发现突破了传统湍流输送理论, 即Fourier定律、 Flick定律和Newton定律的观点一个宏观量的输送通量等价于这个宏观量的梯度湍流输送通量。热力和动力过程间的耦合原理认为, 一个宏观量的输送通量包括这个量的梯度湍流输送通量和速度耦合输送通量两部分。因此, 能量和物质的垂直输送通量除了相应物理量梯度造成的湍流输送外, 还应包括垂直速度耦合效应, 即辐散或辐合运动造成的耦合效应。在一个很宽的尺度范围内, 地表面的空间特征是非均匀的。下垫面非均匀性造成的对流运动将引起大气的辐散或辐合运动。这可能是导致地表能量收支不平衡的重要原因之一。垂直速度对垂直湍流输送的交叉耦合效应为非均匀下垫面大气边界层理论的发展, 并为克服地表能量收支不平衡问题及非均匀下垫面大气边界层参数化遇到的困难提供了可能的线索。&nbsp;
[胡隐樵, 陈晋北, 吕世华. 从湍流经典理论到大气湍流非平衡态热力学理论[J]. 高原气象, 2012, 31(1): 1-27.]
Magsci
湍流是日常生活中一种普遍的自然现象, 也是经典物理学仍未完全解决的难题。湍流更是大气运动的最基本特征。本文系统地回顾了大气湍流经典理论发展简史, 进一步详细介绍了大气湍流非平衡态热力学理论。大气湍流非平衡态热力学理论在熵平衡方程中引入动力过程, 进而统一推导出大气湍流输送的Fourier定律、 Flick定律和Newton定律, 证明了Dufour效应、 Soret效应、 可逆动力过程与热力不可逆湍流输送过程之间的交叉耦合效应, 以及湍流强度定理。这些定律和定理中得到了观测的事实验证, 同时它们的唯象系数也由观测资料所确定。湍流强度定理揭示, 湍流发展的宏观原因是速度和温度的剪切效应, Reynolds湍流和Rayleigh-B&eacute;nard湍流共存于大气湍流中。热力过程和动力过程间耦合效应现象的发现突破了传统湍流输送理论, 即Fourier定律、 Flick定律和Newton定律的观点一个宏观量的输送通量等价于这个宏观量的梯度湍流输送通量。热力和动力过程间的耦合原理认为, 一个宏观量的输送通量包括这个量的梯度湍流输送通量和速度耦合输送通量两部分。因此, 能量和物质的垂直输送通量除了相应物理量梯度造成的湍流输送外, 还应包括垂直速度耦合效应, 即辐散或辐合运动造成的耦合效应。在一个很宽的尺度范围内, 地表面的空间特征是非均匀的。下垫面非均匀性造成的对流运动将引起大气的辐散或辐合运动。这可能是导致地表能量收支不平衡的重要原因之一。垂直速度对垂直湍流输送的交叉耦合效应为非均匀下垫面大气边界层理论的发展, 并为克服地表能量收支不平衡问题及非均匀下垫面大气边界层参数化遇到的困难提供了可能的线索。&nbsp;
[本文引用: 4]
[87]
Hu Yinqiao, Chen Jinbei.Nonequilibrium thermodynamic theory of the atmospheric turbulence[M]∥Lang P R, Lombargo F S, eds. Atmospheric Turbulence, Meteorological Modeling and Aerodynamics. Nova Science Publishers Inc., 2009.
[本文引用: 1]
[88]
Chen Jinbei, Hu Yinqiao, Shihua, et al.Experimental demonstration of the coupling effect of vertical velocity on latent heat flux[J]. Science in China (Series D), 2013, 56(4): 684-692.
[陈晋北, 胡隐樵, 吕世华, . 垂直速度对水汽垂直湍流通量交叉耦合效应的实验验证[J]. 中国科学:D辑, 2013, 43(3): 490-498.]
[本文引用: 3]
[89]
Panin G N, Bernhofer C.Parametrization of turbulent fluxes over inhomogeneous landscapes[J]. Izvestiya Atmospheric and Oceanic Physics, 2008, 44(6): 701-716.
DOI:10.1134/S0001433808060030 Magsci URL
Reasons for the nonclosure of the heat balance in the atmospheric boundary layers over natural land surfaces are analyzed. Results of measuring the heat-balance components over different land surfaces are used. The Cabauw (Netherlands) data (obtained throughout 1996 over a grass surface with intermittent shrubs and single trees) and the data from the Anchor station in Germany (measured over coniferous forest in 2000–2001) are analyzed. In all, the analysis involves about fifty thousand independent values of the heat-balance components measured in the experiments, which should be indicative of the reliability of the results obtained in the paper. The data have shown that the heat balance is not closed and the imbalance is 50–250 W/m 2 . The sum of the latent and sensible heat fluxes λ E + H = STF is found to be systematically smaller than the difference between the net radiation and the heat flux into the ground R n 61 G . It is shown that the main cause of a systematic heat imbalance in the atmospheric boundary layers over inhomogeneous land surfaces is that the methods of surface-flux measurement and estimation are based on the theory that requires the hypothesis of stationarity and horizontal homogeneity. Direct data analysis has shown that the heat imbalance increases with landscape inhomogeneity. In the paper, a parametrization of the heat imbalance is carried out and the coefficient k f ( z 0 ef / L ef ) is introduced as a measure of inhomogeneity. For this, data from the experiments FIFE, KUREX, TARTEX, SADE, etc., are also used. Empirical formulas are presented to refine the results of direct measurements and calculations of surface fluxes over natural (inhomogeneous) land surfaces from profile and standard (using bulk parametrizations) data. These formulas can also be used to determine surface fluxes over inhomogeneous underlying land surfaces in order to take into account so-called subgrid-scale effects in constructing prediction models.
[本文引用: 1]
[90]
Li Xin, Liu Shaomin, Xiao Qing, et al.A multiscale dataset for understanding complex eco-hydrological processes in a heterogeneous oasis system[J]. Scientific Data, 2017, 4: 1-11.
[本文引用: 1]
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