青藏高原典型下垫面地表能量通量的模型估算与验证

青藏高原地区地表能量通量的估算与验证对高原及其周边地区能量和水循环研究具有重要意义,地表能量平衡系统SEBS(Surface Energy Balance System)模型为研究高原非均匀地表区域地表能量通量提供了一种行之有效的方法。基于中国科学院那曲高寒气候环境观测研究站(简称那曲站)、中国科学院纳木错多圈层综合观测研究站(简称纳木错站)和中国科学院珠穆朗玛大气与环境综合观测研究站(简称珠峰站) 2008年辐射资料、大气边界层塔站观测资料,结合MODIS卫星数据,利用SEBS模型估算地表能量通量,并用站点地表能量通量观测资料进行验证。结果表明,模型估算的感热通量和土壤热通量与站点实测值具有较好的一致性,且感热通量和土壤热通量的估算精度明显优于潜热通量;感热通量的估算精度最高,那曲站、纳木错站和珠峰站的均方根误差分别为54. 98,37. 37和27. 10 W·m~(-2);而模型估算的潜热通量验证结果偏差较大和站点实测数据存在"能量不闭合"问题相关。鉴于在地表能量通量观测中广泛存在"能量不闭合"的问题,利用波文比校正方法校正站点实测潜热通量。研究表明波文比校正方法可以明显改善地表通量观测数据"能量不闭合"的问题,那曲站、纳木错站和珠峰站的能量闭合率分别提高了19. 4%,21. 4%和19. 1%;与原始站点实测潜热通量相比,校正后的潜热通量与SEBS模型估算结果一致性较好,3个站点潜热通量的均方根误差分别减少了6. 78,33. 48和29. 30 W·m~(-2)。     

卫星遥感结合数值模式估算金塔绿洲非均匀地表能量通量

针对遥感在面上计算的特点,将模式模拟的气象要素区域分布替代以往使用的单点观测值作为遥感估算蒸散发模型的初始场,引入到SEBS(Surface Energy Balance System)模型中,利用EOS/MODIS遥感资料对金塔绿洲非均匀下垫面地表能量通量进行了估算。对比估算结果与实测值表明,引入数值模拟结果后,绿洲地表感热通量的估算误差平均减小了5.8%,潜热通量的误差平均减小了5.5%,说明数值模拟结果的引入比较成功。通过分析地表通量的区域分布特征表明,数值模拟结果的引入在一定程度上更加细致地刻画了荒漠绿洲能量平衡的分布特征,在戈壁荒漠地区,潜热通量非常小,而绿洲集中的地区及水库附近出现了潜热通量的极大值。     

丘陵区土壤热通量遥感估算模型适应性分析

利用具有丘陵区典型特征的四川省乐至县气象站的土壤热通量和净辐射观测数据,分析了二者的变化特征,并验证了土壤热通量遥感估算模型的适用性。分析结果表明：晴空土壤热通量G与净辐射Rn存在明显日变化,最大值出现13时左右;其比值G／Rn受土壤湿润程度和地表覆盖的影响,地表湿润、覆盖率高,比值小。在干旱时,瞬时比值可高达0．7,而湿润情况,可低至0．05。对目前广泛使用的G／Rn卫星遥感模型估算结果与实测值的对比分析表明,不同的G／Rn卫星遥感估算模型估算结果存在明显的差异,仅依靠植被指数的模型不适合南方丘陵区。本研究认为Bastiaanssen等的模型较适合丘陵区的土壤热通量遥感估算模型。     

黑河实验区非均匀地表能量通量的数值模拟

利用三维非静力RAMS模拟研究了黑河实验区非均匀地表能量通量，模拟结果表明：绿洲地表净辐射通量较沙漠戈壁大；绿洲及沙漠戈壁下垫面上的Bowen比分别为0．4和4．0；夜间绿洲上整晚维持蒸发，并有负感热通量。模拟结果与测站实测结果与卫星反演值的对比研究指出，RAMS对绿洲下垫面潜热通量的模拟和沙漠戈壁下垫面感热通量的模拟与实测值基本一致。卫星遥感反演及数值模拟方法对净辐射的估算与实测较吻合，绿洲地表感热通量的卫星反演值较数值模拟结果更接近于实际，但潜热通量的模拟值则较卫星估算值更接近于实际；沙漠戈壁地表则是感热通量的模拟值较卫星反演值更接近于实际。上述分析为今后结合卫星遥感改进RAMS陆面过程参数化，使使用于模拟研究干旱区非均匀下垫面地气相互作用提供了可靠依据。     

干旱半干旱区非均匀地表区域能量通量的卫星遥感参数化

卫星遥感在估算非均匀地表区域能量通量时有其独到的作用。文中介绍了利用Landsat TM资料估算非均匀地表区域地表能量通量和蒸发(蒸散)量的参数化方案、研究结果和存在的难点问题。并提出了解决问题的可能途径。     

非参数化蒸散发估算方法在黑河流域的适用性分析

陆面蒸散发是水循环过程中的重要组成部分,直接关系到地表的能量和水量平衡。基于哈密顿原理的非参数化蒸散发估算方法能够避免复杂的参数化过程,降低计算过程的不确定性。首先,利用非参数化方法估算了黑河流域不同下垫面的蒸散发,并利用地面观测数据进行了验证,分析了非参数化方法在不同下垫面和不同季节的适用性。对不同下垫面的验证结果表明,在湿润下垫面该方法会低估实际蒸散发,在干旱下垫面会高估实际蒸散发;对不同季节的验证结果表明,夏季蒸散发估算精度明显优于冬季。其次,进一步对非参数化方法进行了敏感性分析:在湿润下垫面,地表净辐射通量对估算结果影响较大;在干旱下垫面,地表净辐射通量和地表温度对非参数化估算方法结果影响较大。最后,利用非参数化方法结合遥感数据和大气驱动数据估算了黑河流域中上游的区域蒸散发,并利用地面观测数据结合足迹模型进行了验证,分析了非参数化方法估算区域蒸散发的适用性,估算结果能够反映该区域地表通量的分布特征,但是与地面观测数据相比存在一定的误差,不同下垫面的均方根误差在50~100W·m~(-2)之间。     

应用MODIS资料对城乡地表热通量的估算

为了揭示城市热岛形成机制,基于MODIS资料,结合自动气象站实测的气象资料,利用地表能量参数化方法估算了地表热通量,分析了城乡地表热通量的空间分布及变化特征.结果表明城乡地气热交换差异明显,与相关文献对比证明该方法是可行、有效的.     

基于MODIS遥感产品估算西北半干旱区的陆面蒸散量

在中分辨率成像光谱仪(MODIS)数据产品的基础上,使用MODIS地表温度替代净辐射及蒸散模型中的气温进行计算,并利用兰州大学半干旱气候与环境监测站(SACOL)的实测资料对模型进行修正。将模型修正前后估算的SACOL站和定西干旱气象与生态环境试验站(定西站)的净辐射和蒸散与实测值进行对比。结果表明,MODIS的白天\夜间地表温度与日最高\最低气温之间具有很好的相关性,相关系数都超过0.75,使用其替代气温进行净辐射和蒸散的估算是可行的,且各参数具有明确的物理意义。通过比较模型估算值与地面通量观测站的实测值发现:使用实测资料对模型进行修正后,净辐射和蒸散的估算结果较修正前有了明显的改善。净辐射估算值与实测值之间的均方根误差减小到25.93 W·m~(-2);修正后模型估算的SACOL站和定西站蒸散更接近于实测值,均方根误差分别减小到0.81 mm和0.68 mm,相关系数都增加到0.6以上,通过了0.01显著性水平检验;且遥感估算的区域蒸散分布特征与地表覆盖特征相符,说明利用修正后模型估算的净辐射和地表蒸散是合理的,由于这种估算净辐射和蒸散的模型不需要任何实时地面资料的辅助,可以为观测资料缺乏地区的辐射及蒸散研究提供一种新的思路。     

利用MERIS和AATSR资料估算黄土高原塬区蒸散发量研究

基于陆面能量平衡原理,通过对搭载在欧洲空间局环境卫星(Environmental Satellite,ENVI-SAT)上中分辨率影像光谱仪(Medium Resolution Imaging Spectrometer,MERIS)2005年6月7,11和27日的遥感观测资料进行大气纠正等预处理后,得到估算瞬时蒸散发量所需要的地表反照率和植被覆盖度等值,并利用分裂窗法和ENVISAT上搭载的先进的沿轨迹扫描辐射计(Advanced Along-TrackScanning Radiometer,AATSR)的观测资料进行了地表温度的反演,进一步估算出黄土高原塬区午间瞬时净辐射、感热通量和土壤热通量。结合与卫星遥感观测资料同期研究区域气象站的太阳辐射、气温、日照时数和风速等气象要素资料,充分考虑到植被冠层和陆地表面对蒸散发量的不同影响,发展了一个可以估算陆面潜热的简化模型,并将瞬时蒸散发量转化为日蒸散发量。对卫星遥感估算的潜热通量,利用黄土高原塬区陆面过程野外观测试验(Loess Plateau land surface process field Experiments,LOPEXs)的地面通量观测资料进行验证,结果表明:二者最大相对差异为10.9%,最小相对差异为4.8%,并对差异误差产生的原因进行了分析和探讨。     

半干旱草地地表土壤热通量的计算及其对能量平衡的影响

利用2008年7月兰州大学半干旱气候与环境观测(SACOL)站的观测资料,对比分析了地表土壤热通量的三种计算方法,即谐波法、温度预报校正法(TDEC法)以及结合自校正热通量板(HFP01SC)测量的温度积分法(ITHP法);进而分析了三种不同方法的计算结果对地表能量平衡的影响。比较5cm深度处谐波法和TDEC法的计算结果与HFP01SC的实测结果,三者的相位基本一致,相互之间均具有很好的线性关系;谐波法与TDEC法的计算值较为接近,但分别比HFP01SC的实测值偏大了2%和6%(主要发生在夜间)。对于地表的土壤热通量(G0),谐波法与TDEC法两者的计算结果仅偏差约1%;TDEC法与ITHP法的计算结果之间也具有很好的线性关系(R2=0.99),但偏差达到9%左右。相对于HFP01SC的实测结果,由谐波法和TDEC法计算的G0可将SACOL站的地表能量闭合率分别提高6%和7%左右;利用温度积分法将HFP01SC的实测结果校正到地表后,地表能量闭合率也提高了约6%。因此,在对涡动相关通量做了常规订正的情况下,当充分考虑了土壤热存储后,SACOL站的地表能量闭合率可提高6%～7%,达到82%～83%左右。     

大口径闪烁仪及其在地表能量平衡监测中的应用

介绍了LAS仪器的测量原理及数据处理方法, 并利用中荷合作项目CEWBMS中获得的河南郑州LAS测站2000年的观测资料, 同时结合其它辅助资料, 对观测点附近地区的能量平衡状况进行了分析。分析结果表明, 由LAS测值得到的显热通量值, 以及结合净辐射资料间接得到的潜热通量值, 合理地反映出了当地能量平衡状况的季节变化, 显示出一年之中当地大部分的净辐射能用于潜热通量的释放。其数据结果所表征的当地下垫面干湿程度的变化与同期的降水及土壤相对湿度相比, 表现出了相当好的一致性, 这为拓展LAS在局地地表能量平衡监测中的应用提供了物理依据。     

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.     

Results from One-Year Continuous Operation of a Large Aperture Scintillometer over a Heterogeneous Land Surface

A large-aperture scintillometer (LAS) was operated continuouslyduring a period of more than one year over a heterogeneous land surface in Central Europeat the transition between marine and continental climates. The LAS measurements of the refractiveindex structure parameter, C_N ², were used to estimate the sensible heat flux. Thiswas possible for about 60to 80% of the time under daytime conditions during thesummer, with lower values obtained for the cold season (October to March). Using datafrom a three-week long field experiment, the LAS-based heat flux was compared with a weighedaverage of local heat flux measurements over the main land use classes (forest, agriculture,water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used forcomparison with the heat flux values of a numerical weather prediction model. An over-predictionof the model heat flux was found in summer but the modelled values were lower than the LASderived data during the cold season.     

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.     

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。     

A Variational Method for Estimating Near-Surface Soil Moisture and Surface Heat Fluxes

A variational data assimilation method is proposed to estimate the near-surface soil moisture and surface sensible and latent heat fluxes.The method merges the five parts into a cost function,i.e.,the differences of wind,potential temperature,anti specific humidity gradient between observations and those computed by the profile method,the difference of latent heat fluxes calculated using the ECMWF land surface evaporation scheme and the profile method,and a weak constraint for surface energy balance.By using an optimal algorithm,the best solutions are found.The method is tested with the data collected at Feixi Station (31.41°N,117.08°E) supported by the China Heavy Rain Experiment and Study (HeRES) during 7-30 June 2001.The results show that estimated near-surface soil moistures can quickly respond to rainfall,and their temporal variation is consistent with that of measurements of average soil moisture over 15-cm top depth with a maximum error less than 0.03 m~3 m~(-3).The surface heat fluxes calculated by this method are consistent with those by the Bowen ratio method,but at the same time it can overcome the instability problem occurring in the Bowen ratio method when the latter is about-1.Meanwhile,the variational method is more accurate than the profile method in terms of satisfying the surface energy balance.The sensitivity tests also show that the variational method is the most stable one among the three methods.     

Modelling Regional Scale Surface Energy Exchanges And Cbl Growth In A Heterogeneous, Urban-Rural Landscape

Over the last decade, simple models of theconvective boundary layer (CBL) have beensuggested as an approach to inferring regionallyaveraged land-air exchanges of heat, water and tracegases, because the properties of the CBL respond toan average of the underlying small-scaleheterogeneity. This paper explores the use of anintegral CBL method to infer regionally averagedfluxes in a landscape that has at least three majorsources of heterogeneity – irrigated andnon-irrigated rural land use and a large urban area(Sacramento region, California).The first part of the paper assesses the validity ofthe simple slab model of the CBL – this isintegrated forwards in time using local-scalemeasured heat and water vapour fluxes, to predictmixed-layer depth, temperature and humidity. Of thefour different CBL growth schemes used, the Tennekesand Driedonks model is found to give the bestperformance. Evaluation of the model performancewith different weightings of heat and water vapourfluxes based on the land use characteristics in theregion suggest that the source area for theboundary-layer sonde measurements is larger thanphysically-based estimates would suggest.Finally, measured time series of potentialtemperature are used to infer regionally averagedsensible heat fluxes using an integral CBL (ICBL)method. These ICBL fluxes are compared with thosemeasured at the local scale over the three land usetypes that comprise the region of interest. They arefound to be closest to the heat fluxes calculated byappropriately weighting the measured heat fluxes inthe source area calculated for the ICBL. We concludethat the integral CBL budget method providesadequate estimates of regionally-averaged surfaceheat fluxes in a landscape that is characterised bysurface types with distinctly different surfaceenergy budgets.     

Effects of Crop Growth and Development on Land Surface Fluxes

In this study, the Crop Estimation through Resource and Environment Synthesis model (CERES3.0) was coupled into the Biosphere-Atmosphere Transfer Scheme (BATS), which is called BATS CERES, to represent interactions between the land surface and crop growth processes. The effects of crop growth and development on land surface processes were then studied based on numerical simulations using the land surface models. Six sensitivity experiments by BATS show that the land surface fluxes underwent substantial changes when the leaf area index was changed from 0 to 6 m2 m-2. Numerical experiments for Yucheng and Taoyuan stations reveal that the coupled model could capture not only the responses of crop growth and development to environmental conditions, but also the feedbacks to land surface processes. For quantitative evaluation of the effects of crop growth and development on surface fluxes in China, two numerical experiments were conducted over continental China: one by BATS CERES and one by the original BATS. Comparison of the two runs shows decreases of leaf area index and fractional vegetation cover when incorporating dynamic crops in land surface simulation, which lead to less canopy interception, vegetation transpiration, total evapotranspiration, top soil moisture, and more soil evaporation, surface runoff, and root zone soil moisture. These changes are accompanied by decreasing latent heat flux and increasing sensible heat flux in the cropland region. In addition, the comparison between the simulations and observations proved that incorporating the crop growth and development process into the land surface model could reduce the systematic biases of the simulated leaf area index and top soil moisture, hence improve the simulation of land surface fluxes.     

Determination of Area-Averaged Sensible Heat Fluxes with a Large Aperture Scintillometer over a Heterogeneous Surface – Flevoland Field Experiment

To test the applicability of the scintillation method over a heterogeneous area an experiment was carried out in the summer of 1998 in Flevoland (The Netherlands). In the patchy area only four crops were grown namely sugar beet, potatoes, wheat and onions. From eddy covariance measurements it was found that the heterogeneity was mainly caused by differences in thermal properties. No variations in the aerodynamics roughness length were observed. Two large aperture scintillometers were installed at a height of 11.6 and 20.4 m. A good resemblance was found between the sensible heat fluxes derived from both LAS instruments and the area-averaged fluxes obtained from the in-situ eddy covariance measurements. The slight underestimation of the lower LAS could be assessed using a blending height model and an analytical footprint model. The results also indicated that when scintillometer measurements are made below the blending height the violation to Monin–Obukhov Similarity Theory is small and that reasonable fluxes can be obtained from path-averaged structure parameters.