不同下垫面大孔径闪烁仪观测数据处理与分析

大孔径闪烁仪是近年兴起的测量大尺度（500 m~10 km）地表通量的仪器。北京师范大学等单位分别于2002年、2004年在北京昌平小汤山开展了大孔径闪烁仪短期观测实验, 2006年6月又在北京密云建立了长期观测站。利用这些数据, 对大孔径闪烁仪观测数据进行处理与分析, 结果表明:闪烁仪光径高度和风速是影响观测显热通量的关键因子。当地表粗糙元的高度变化相对于光径高度不可忽略时, 零平面位移需要精确确定。波文比在湿润地表需要准确确定, 而气温、气压和动力学粗糙度则为不敏感因子。计算中所需的大气稳定度可用理查孙数判断, 也可借助日出日落时间或净辐射观测值确定。稳定条件下的普适函数目前无统一表达式, 可采用仪器说明书推荐的函数。通过几个站点闪烁仪观测显热通量与涡动相关仪测量值的比较表明:大孔径闪烁仪在均匀和非均匀地表都能得到合理的显热通量观测值。     

国产光学型大孔径闪烁仪的技术性能分析

利用2009—2012年在北京密云、青海阿柔、河南济源、甘肃张掖、河北馆陶和沽源的野外对比试验数据,分析了国产光学型大孔径闪烁仪的观测精度、稳定性和一致性,为国产光学型大孔径闪烁仪的后续生产和改进提供科学依据。分析结果表明:(1)以德国BLS900闪烁仪观测值作为参考值,张掖巴吉滩3套和沽源站5套国产大孔径闪烁仪样机感热通量观测值的平均均方根误差分别为19.91 W·m~(-2)和9.81 W·m~(-2);样机感热通量观测值均值与涡动相关仪观测值比较,均方根误差分别为21.08 W·m~(-2)和12.87 W·m~(-2),表明国产大孔径闪烁仪具有较好的观测精度;(2)国产大孔径闪烁仪在高温、低温、高湿或低湿的恶劣天气条件下以及长时间序列的观测均具有较好的稳定性;(3)样机观测感热通量之间的回归斜率差异较大,为6%,相关系数大于0.98,表明国产大孔径闪烁仪的一致性较好。综上所述,国产光学型大孔径闪烁仪基本达到了国外同类仪器的性能水平。     

北京城市通量足迹及源区分布特征分析

城市下垫面具有不同于其他下垫面的特殊复杂性,通量的源汇分布十分不均,导致直接测量以及定量的描述城市中水汽、二氧化碳等通量变得非常困难。涡动观测系统在城市通量观测中得到广泛应用,但由于涡动协方差观测系统传感器都架设在一定的高度上,其测量结果只反映特定点下垫面或某部分下垫面的物理过程,不能说明观测结果是否具有下垫面属性的代表性,无法反映观测通量的空间变异性。足迹函数(Footprint)的产生就是为了解决这一问题,其又称源权重函数,是描述近地面层表面源或汇的空间分布和仪器观测通量值之间关系的函数。本文采用北京325 m气象塔近1年7层涡动协方差观测数据、超声数据,分析了不同风向、不同稳定度、不同高度下足迹函数所表达的通量贡献区域范围的变化规律。结果表明,在大气稳定条件下时,通量贡献区范围的大小与主风向无显著差异,而不稳定条件下计算结果与主风向无关。在不同高度下所有稳定条件下通量贡献区范围要大于不稳定条件,中性条件则介于两者之间。8 m、16 m高度上的观测结果不能完全代表城市下垫面通量贡献区,47 m以上能够代表城市下垫面通量贡献区,280 m则已经包含了郊区和城区的权重平均。同一大气稳定度条件下,高度越高通量贡献区范围越大,90%通量贡献区范围与观测高度成线性关系,这种线性关系可以预测没有观测高度或者更高处的通量贡献区范围。     

非均匀下垫面湍流通量观测的印痕分析

利用2005年在北京昌平区小汤山开展的非均匀下垫面观测实验的观测数据,分别处理了涡旋相关法和大孔径闪烁仪LAS测得的感热通量,并运用印痕模型对数据进行“源区”分析。分析结果表明:在复杂地表上,涡旋相关的测量值由于混杂了其他下垫面的通量信息,不能真实地反映其观测区域的湍流特征,存在着观测误差。而LAS观测出现的低估问题则与掺混高度和“源区”有关。通过用印痕方法进行数据订正后,以上问题均得到很好的改善,订正结果与LAS观测通量的线性关系良好,两者的相关系数达到0.9。实验结果验证了印痕模型在非均匀下垫面的适用性。     

LAS在西北干旱区荒漠均匀下垫面的观测研究

利用“古浪非均匀近地层观测试验”的仪器平行对比观测部分试验数据,首先分析荒漠均匀下垫面大孔径闪烁仪(Large Aperture Scintillometer,LAS)光径线上等间距架设的4台涡动相关仪(Eddy Covariance System,EC)观测的摩擦速度和感热通量的一致性.然后,基于4种不同下垫面建立稳定度普适函数分析,利用4台EC观测摩擦速度平均值,计算了LAS观测区域的感热通量,进而比较了LAS和EC两者观测值的差异.结果表明,4台EC观测的摩擦速度与感热通量一致性好,摩擦速度的差别＜10％,感热通量的差别在10％左右.4种稳定度普适函数计算的LAS区域感热通量在白天、夜晚与EC观测平均值变化趋势一致,但计算值偏大;TAG (Thiermann and Grassl)稳定度普适函数计算值与EC观测平均值较接近.其主要原因除不同下垫面试验所得稳定度普适函数的差异外,Kipp&Zonen LAS所测感热通量的系统性偏高不可忽视.     

大孔径闪烁仪测量戈壁地区感热通量

利用2008年6月11～30日在金塔开展的"绿洲系统非均匀下垫面能量水分交换和边界层过程观测与理论研究"期间第一阶段戈壁下垫面大孔径闪烁仪(LAS)的观测资料,用混合对流方法和自由对流方法分别计算了戈壁感热通量。结果表明,对于利用LAS资料计算地表感热通量的方法中,混合对流方法相对于自由对流方法更加适用,且混合对流方法中Andreas给出的参数相对于DeBruin的参数更加适用于戈壁下垫面。此外,LAS测得的感热通量相对涡动相关方法的值较大,提高了地表能量闭合度。     

黄土高原影响LAS观测感热通量的物理因素分析

利用2010年1月和6月黄土高原定西和庆阳两站大孔径闪烁仪(LAS)的观测数据,结合涡动相关系统(EC)、辐射观测、梯度塔等观测系统的同步观测资料,分析了不同下垫面LAS和EC观测感热通量的时空差异及其与下垫面净辐射、风向、风速和稳定度等物理量的关系。结果表明,LAS和EC观测感热通量值之间的差异(HLAS-HEC)大小与下垫面的不均匀性有关,相对复杂的下垫面HLAS-HEC较大。净辐射Rn是感热通量的主要驱动因子,HLAS-HEC在生长季大于非生长季,并与Rn成正相关。风向对LAS和EC观测感热通量值有显著影响。对流边界层低频涡旋的存在导致EC通量统计值偏低,HLAS/HEC随着风速的增大而减小,随着稳定度z/L的增大而增大。     

夏季金塔绿洲近地层通量足迹及源区分布特征分析

选取2005年5月24日～6月18日在金塔开展的"绿洲系统能量与水分循环过程的观测实验"中的3层CSAT3的实验数据,应用Schmid的FsAM(The Flux-Source Area Model)模型,分析了不同观测高度的通量贡献源区分布以及观测高度对通量贡献源区分布的影响,同时分析了不同大气层结条件下源区的分布以及稳定度对通量贡献源区分布的影响.结果表明,稳定条件下的通量贡献源区大于不稳定条件下的通量贡献源区,并且随着观测高度的增加通量贡献源区会显著增大.     

怀来地区蒸渗仪测定玉米田蒸散发分析

利用2012年和2013年怀来遥感综合试验站蒸渗仪、涡动相关仪和自动气象站观测资料,分析了土壤蒸发和玉米农田蒸散的日、季节变化,用多元回归分析法研究了气象因子(净辐射、空气温度、空气湿度、风速)、土壤水分和农田蒸散量的关系,并将蒸渗仪蒸散观测值与涡动相关仪蒸散量观测值进行了比较。结果表明,土壤蒸发和玉米农田蒸散日变化曲线较一致,季节性差异明显;怀来地区日蒸散量与净辐射和土壤水分相关性较好,与其他影响因子相关性不明显;蒸渗仪的农田代表性受其观测范围内的作物长势影响显著,涡动相关仪观测的蒸散量与蒸渗仪观测值相关关系较好,蒸渗仪观测值较涡动相关仪观测值高10.5%,这是由于不能同周围农田进行热交换,蒸渗仪内平均土壤温度较农田高了9.5%,导致蒸渗仪对蒸散量的相对高估。     

变分方法估算西北半干旱区地表热通量的适用性研究

利用2009年6-8月兰州大学半干旱气候与环境观测站(SACOL)的地表辐射观测资料,近地层大气温度、湿度和水平风速的三层梯度观测资料,以及热通量的涡动相关观测,比较了变分法估算的地表热通量与涡动相关观测值间差异,评估了不同相似函数对变分法估算地表热通量的影响.结果表明,变分法估算的地表热通量在时间变化趋势上与涡动相关观测值一致,但在中午前后时段还存在数值大小差异.不同相似函数对变分法估算热通量的影响不一样:当层结不稳定时,其对热通量估算的影响较大;当层结不稳定变弱时,影响会显著变小;当层结稳定时,影响非常小,与观测误差相比可以忽略.     

Intercomparison of Sensible Heat Flux from Large Aperture Scintillometer and Eddy Covariance Methods: Field Experiment over a Homogeneous Semi-arid Region

Scintillometers are becoming increasingly popular for the validation of satellite remote sensing sensible heat-flux estimates due to the comparable spatial resolutions. However, it is important to gain confidence in the accuracy of the sensible heat-flux measurements obtained by the scintillometer. Large aperture scintillometer (LAS) and eddy-covariance (EC) measurements were collected over a homogeneous, dry and semi-arid region near Las Cruces, New Mexico, USA, where the homogeneity allowed direct comparison of the two instruments despite their differences in footprint sizes. The differences between the sensible heat-flux measured by both LAS and EC systems fall within the differences between two EC systems. We conclude that the large aperture scintillometer is a reliable system for measuring sensible heat flux in a dry semiarid region.     

Surface Fluxes and Characteristics of Drying Semi-Arid Terrain in West Africa

This study examines the seasonal cycle of the components of the surface energy balance in the Volta basin in West Africa as part of the GLOWA-Volta project. The regional climate is characterized by a strong north–south gradient of mean annual rainfall and the occurrence of pronounced dry and wet seasons within one annual cycle, causing a strong seasonal variation in the natural vegetation cover. The observations are conducted with a combined system, consisting of a Large Aperture Scintillometer (LAS) for areally averaged sensible heat flux, radiometers and sensors for soil heat flux. For comparisons the eddy-covariance (EC) method providing the fluxes of momentum, sensible and latent heat is utilized as well. The measurements of a seasonal cycle in 2002/2003 were gathered including the rapid wet-to-dry transition after the wet season at two locations in Ghana, one in the humid tropical southern region and one in the northern region. A direct comparison and the energy balance closure of the two methods are investigated for daytime and nighttime separately. An attempt is made to understand and explain the differences between the two methods and the closure of energy budget found for these. It is found that the two systems correspond well during daytime. During nighttime the LAS seems to perform more realistically than the EC system. Considering the fact that a LAS system is much easier to use in the climate conditions of the Volta basin, it is concluded that the LAS approach is very suitable in this type of climate conditions. Surface conductances are estimated by rearranging the Penman–Monteith equation and compared to a Jarvis-type model optimised for savannah conditions. It is found that temperature dependence should be included in the conductance formulation in contrast to earlier findings. Based on the findings the gathered dataset can be used for further model studies of the climate and environment of West Africa.     

Estimation of turbulent sensible heat and momentum fluxes over a heterogeneous urban area using a large aperture scintillometer

The accurate determination of surface-layer turbulent fluxes over urban areas is critical to understanding urban boundary layer (UBL) evolution. In this study, a remote-sensing technique using a large aperture scintillometer (LAS) was investigated to estimate surface-layer turbulent fluxes over a highly heterogeneous urban area. The LAS system, with an optical path length of 2.1 km, was deployed in an urban area characterized by a complicated land-use mix (residential houses, water body, bare ground, etc.). The turbulent sensible heat (Q H) and momentum fluxes (τ) were estimated from the scintillation measurements obtained from the LAS system during the cold season. Three-dimensional LAS footprint modeling was introduced to identify the source areas ("footprint") of the estimated turbulent fluxes. The analysis results showed that the LAS-derived turbulent fluxes for the highly heterogeneous urban area revealed reasonable temporal variation during daytime on clear days, in comparison to the land-surface process-resolving numerical modeling. A series of sensitivity tests indicated that the overall uncertainty in the LAS-derived daytime Q H was within 20%-30% in terms of the influence of input parameters and the non-dimensional similarity function for the temperature structure function parameter, while the estimation errors in τ were less sensitive to the factors of influence, except aerodynamic roughness length. The 3D LAS footprint modeling characterized the source areas of the LAS-derived turbulent fluxes in the heterogeneous urban area, revealing that the representative spatial scales of the LAS system deployed with the 2.1 km optical path distance ranged from 0.2 to 2 km2 (a "micro-α scale"), depending on local meteorological conditions.     

Scintillometer-Based Turbulent Fluxes of Sensible and Latent Heat Over a Heterogeneous Land Surface – A Contribution to Litfass-2003

The performance of a combined large aperture scintillometer (LAS) and a millimetre wave scintillometer (MWS) for estimating surface fluxes of sensible and latent heat over natural landscape is investigated, using data gathered during LITFASS-2003. For this purpose the LAS–MWS system was installed in a moderate heterogeneous landscape over a path length of 4.7 km with an effective beam height of 43 m. The derived surface fluxes have been compared with aggregated eddy-covariance (EC) measurements. The fluxes of sensible and latent heat from the LAS–MWS combination, as well as sensible heat fluxes of the single LAS, agreed fairly well with the EC-based fluxes, considering the uncertainties of the similarity stability functions and observed energy imbalance.     

Influence of Mean Rooftop-Level Estimation Method on Sensible Heat Flux Retrieved from a Large-Aperture Scintillometer Over a City Centre

The sensible heat flux (H) is determined using large-aperture scintillometer (LAS) measurements over a city centre for eight different computation scenarios. The scenarios are based on different approaches of the mean rooftop-level \((z_{H})\) estimation for the LAS path. Here, \(z_{H}\) is determined separately for wind directions perpendicular (two zones) and parallel (one zone) to the optical beam to reflect the variation in topography and building height on both sides of the LAS path. Two methods of \(z_{H}\) estimation are analyzed: (1) average building profiles; (2) weighted-average building height within a 250 m radius from points located every 50 m along the optical beam, or the centre of a certain zone (in the case of a wind direction perpendicular to the path). The sensible heat flux is computed separately using the friction velocity determined with the eddy-covariance method and the iterative procedure. The sensitivity of the sensible heat flux and the extent of the scintillometer source area to different computation scenarios are analyzed. Differences reaching up to 7% between heat fluxes computed with different scenarios were found. The mean rooftop-level estimation method has a smaller influence on the sensible heat flux (?4 to 5%) than the area used for the \(z_{H}\) computation (?5 to 7%). For the source-area extent, the discrepancies between respective scenarios reached a similar magnitude. The results demonstrate the value of the approach in which \(z_{H}\) is estimated separately for wind directions parallel and perpendicular to the LAS optical beam.     

The Effective Height of a Two-Wavelength Scintillometer System

A scaling factor, S, is derived to account for the difference in path-weighted measurement heights of a combined system consisting of a large-aperture scintillometer (LAS) and a millimetre-wave scintillometer (MWS), operating at wavelengths of 0.88 μm and about 3 mm respectively, and designed to determine the area-averaged latent heat flux. This work extends an earlier derivation of Z _ef , the effective height for the LAS yielding the area-averaged sensible heat flux. The LAS and MWS have different path-weighting functions, therefore, in general, it is expected that the path-weighted beam heights are different, for paths other than with the beam parallel to the land surface. Their difference will depend on the detail of the experimental set-up. The objective is to introduce a scaling factor that transforms the MWS refractive-index structure parameter measurement to the same path-weighted height of the LAS measurement. By applying S as a prelude to the calculation of the temperature and humidity structure parameters, and the sensible and latent heat fluxes, these quantities are made representative of the same measurement height, thus simplifying the application of two-wavelength scintillometry. The equations presented here enable the analysis of LAS-MWS paths such that they can be selected to optimize S towards its ideal value of unity. For this purpose we provide a new analytical approximation of the LAS path-weighting function. The importance of accounting for S is demonstrated by example applications over varying topography as well as for slanted beams.     

Monitoring the Sensible Heat Flux over Urban Areas using Large Aperture Scintillometry: Case Study of Marseille City During the Escompte Experiment

Abtract Sensible heat flux estimated by Large Aperture Scintillometry (LAS) has been tested against the more traditional eddy covariance technique over Marseille city centre, a reasonably homogeneous surface. Over the 3 week test period fluxes were found to be similar, yet less noisy for the LAS due to the spatial integration. No systematic bias between the estimates was found as a function of wind direction, indicating the homogeneity of the site. Sensitivity analysis of the required aerodynamic parameters shows that careful attention must be paid to the displacement height along the measurement path. Spatial variability of surface sensible heat flux is studied via a second LAS measurement path over the city.