Evaluating a Model of Evaporation and Transpiration with Observations in a Partially Wet Douglas-Fir Forest   总被引：1，自引：0，他引：1  

Fred C. Bosveld  Willem Bouten 《Boundary-Layer Meteorology》2003,108(3):365-396

The Penman–Monteith equation is extended to describe evaporation of intercepted rain, transpiration and the interaction between these processes in a single explicit function. This single-layer model simulates the effects of heat exchange, stomatal blocking and changed humidity deficit close to the canopy as a function of canopywater storage. Evaporation depends on the distribution of water over the canopy and the energy exchange between wet and dry parts. Transpiration depends on the dry canopy surface resistance that is described with a Jarvis-type response. The explicit functions obtained for water vapour fluxes facilitate a straightforward identificationof the various processes. Canopy water storage amounts and xylem sapflow were measured simultaneously during drying episodes after rainfall in a dense, partially wet, Douglas-fir forest. Estimates of evaporation and transpiration rates are derived from these observations. The analysis shows that evaporation induced transpirationreduction is mainly caused by energy consumption. Changes in water vapour deficit have a minor effect due to a compensating stomatal reaction. The remaining difference between observed and modelled transpiration reduction can be attributed to partial blocking of stomata by the water layer.  相似文献   

SCICEX Investigations of the Arctic Ocean System   总被引：1，自引：0，他引：1  

Margo H. Edwards  Bernard J. Coakley 《Chemie der Erde / Geochemistry》2003,63(4):281-328

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Study Of `Soft' Night-Time Surface-Layer Decoupling Over Forest Canopies In A Land-Surface Model     

Yves Delage  Paul A. Bartlett  J. Harry McCaughey 《Boundary-Layer Meteorology》2002,103(2):253-276

The cause of a night-time land-surface model cold bias over forest canopies at threedifferent sites is studied in connection with various formulations of turbulent transferand the phenomenon of decoupling between the surface and the boundary layer. Themodel is the Canadian Land Surface Scheme (CLASS), a leading internationally knownmodel that has been tested over a variety of instrumented sites. The bias was first attributed to a deficient turbulent transfer and a few formulations were compared. One formulation is the classical log-linear profile with a sharp cut-off of the fluxes at a critical Richardsonnumber around 0.2, while in the other ones the flux decreases less rapidly with increasingstatic stability. While the surface-layer formulations have an impact on the modelled canopy temperature, other causes were found for the negative bias. The CLASS model neglected the heat capacity of the air trapped inside the canopy and its inclusion multiplied theeffective heat capacity of the canopy, by a factor ranging from 2.3 to 3.4 for the canopies studied, and reduced the error. A correction was also made to the air specific humidity at canopy level and the topsoil thermal conductivity was changed from that of organic matter to that of mineral soil. With these modifications, and using the incoming longwave radiative flux instead of the net longwave flux, the bias almost completely disappeared. Using ascheme with more heat transfer at large static stability, obtained by assuming that thefluxes decrease in magnitude with height in the surface layer, reduced the original biaswhile using the log-linear formulation amplified the cold bias. The impact of the turbulent transfer formulations is much reduced when they are applied to model runs in which the other above modifications have been made.The phenomenon of decoupling is presented and its understanding is complementedwith the new notions of `hard' versus `soft' decoupling and complete versus incompletedecoupling, depending on the impact decoupling has on the model and on the effectiveness of the model in achieving the decoupling. The geostrophic wind speed is a determiningfactor in separating cases of hard decoupling (rare) from the soft cases (frequent) while the completeness of the decoupling primarily depends on the form of the turbulent transfer curve as a function of static stability.  相似文献   

北京冬季城市边界层结构形成机制的初步数值研究   总被引：7，自引：1，他引：6  

李晓莉  毕宝贵  李泽椿 《气象学报》2005,63(6):889-902

利用耦合了城市冠层参数化方案的MM5模式对2001年冬季北京地区一次典型的城市边界层过程进行成功模拟的基础上,对北京城市化作用、周边地形以及城市化进程发展对城市边界层结构的影响等问题进行了一系列的数值模拟试验。城市化作用的因子分离试验发现,城市化的总体作用即城市下垫面结构对大气热力及动力的综合影响导致了北京冬季城市边界层结构主要特征的形成。此外,揭示了城市结构的不同影响因子———动力因子、热力因子和热动力因子间的相互作用在北京冬季城市边界层结构形成和演变过程中的不同作用。在夜间,城市结构的动力因子对于城市边界层主要特征如市区悬浮逆温、近地层中小的风速及较强的湍流动能等的形成起着主导作用;在白天,城市结构的热力因子则成为影响市区混合层强度以及湍流运动特征等边界层结构的主导因素;热、动力因子间的相互作用对城市边界层结构的形成和演变也有着重要作用,但其影响特征比较复杂。北京周边地形作用的敏感性试验的结果表明,北京周边的特殊地形条件对城市边界层热力结构特征如悬浮逆温层及城市热岛等的结构及分布特征的形成也有着明显的影响,使其具有特殊的局地化特征,同时,它也是北京地区近地层主要气流特征的强迫源。不同城市化程度的敏感性试验结果揭示,随着北京城市建筑高度和密度的增加,市区风速将减小、湍流动能将加强,夜间城市悬浮逆温层底的高度会有所提高,城市热岛的强度也将加强,并可能在白天出现比较明显的城市热岛效应。  相似文献   

Energy balance above a boreal coniferous forest: a difference in turbulent fluxes between snow‐covered and snow‐free canopies     

Yuichiro Nakai  Tomoki Sakamoto  Tomomi Terajima  Kenzo Kitamura  Tomoki Shirai 《水文研究》1999,13(4):515-529

To evaluate the interactive effects of snow and forest on turbulent fluxes between the forest surface and the atmosphere, the surface energy balance above a forest was measured by the eddy correlation method during the winter of 1995–1996. The forest was a young coniferous plantation comprised of spruce and fir. The study site, in Sapporo, northern Japan, had heavy and frequent snowfalls and the canopy was frequently covered with snow during the study period. A comparison of the observed energy balance above the forest for periods with and without a snow‐covered canopy and an analysis using a single‐source model gave the following results: during daytime when the canopy was covered with snow, the upward latent heat flux was large, about 80% of the net radiation, and the sensible heat flux was positive but small. On the other hand, during daytime when the canopy was dry and free from snow, the sensible heat flux was dominant and the latent heat flux was minor, about 10% of the net radiation. To explain this difference of energy partition between snow‐covered and snow‐free conditions, not only differences in temperature but also differences in the bulk transfer coefficients for latent heat flux were necessary in the model. Therefore, the high evaporation rate from the snow‐covered canopy can be attributed largely to the high moisture availability of the canopy surface. Evaporation from the forest during a 60‐day period in midwinter was estimated on a daily basis as net radiation minus sensible heat flux. The overall average evaporation during the 60‐day period was 0·6 mm day⁻¹, which is larger than that from open snow fields. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

用LAI监测植被信息分析飞蝗生境     

吴彤  倪绍祥  李云梅  周欣欣 《地球信息科学学报》2006,8(2):95-100

东亚飞蝗自20世纪80年代以来在我国再度猖獗危害。本文选择国家一类蝗区河北省黄骅市为实验区,用植被冠层孔隙度反演了该地区不同植被的LAI。从光学模型建立机理及数量分析的角度,分析和对比了四种由植被冠层孔隙度反演LAI的算法。结果表明,在四种估算方法中LAI-2000算法最适用于研究区植被LAI的估算。为了验证分析结果,用实测的植被盖度与四种算法反演的LAI进行了拟合。发现LAI与植被盖度之间呈明显的正相关关系,且LAI-2000算法最能反映研究区的植被特征。在此基础上,建立了LAI与飞蝗发生面积的关系模型,发现两者之间呈负线性相关,即随着LAI的减小,飞蝗的发生面积呈线性增大。研究结果为实时、快速、大面积监测蝗虫种群动态奠定了基础,并为合理、经济地防治蝗灾提供了科学依据。  相似文献   

中小尺度下植被冠层对屋顶表面温度的调控效应分析     

杨若  敖祖锐  张晶  余洁 《地球信息科学学报》2019,21(7):1097-1108

随着城市化进程的加快,城市热岛问题日益严重,对人类健康和城市可持续发展产生了巨大威胁。植被可有效遮蔽阳光直射,并通过蒸腾作用降低气温,是改善局部热环境的重要途径之一。开展植被对建筑物温度的调控效应的研究,对于理解城市热岛成因、缓解城市热环境恶化等方面都有重要意义。然而,当前研究往往是在遥感影像的基础上进行的,缺乏植被结构信息,同时,受制于有限的空间分辨率,研究大多在城市尺度下开展。在中小尺度上定量地研究植被冠层密度对建筑物温度的影响仍然具有一定挑战性。鉴于此,本文使用激光雷达（Light Detection and Ranging, LiDAR）获取的高分辨率冠层密度数据,在楼间尺度和街区尺度下开展圣罗莎市三维植被结构与单体建筑物表面温度之间定量关系的研究,分析不同尺度下植被冠层的降温特征及其在局部环境中的降温贡献。结果表明：植被对建筑物的降温作用与其周围的冠层密度有密切关系：冠层密度需达到17%才能起到有效的降温作用,其中在中小尺度上冠层密度分别高于30%和40%时,能最大限度发挥植被的温度调控功能;当冠层密度相同时,2个尺度下的温度变化显著不同：随着冠层密度的增加,街区尺度下的屋顶温度比楼间尺度下的屋顶温度平均下降了0.89 ℃;中小尺度下的屋顶温度变化不仅受到其周围植被结构的影响,还与整体热环境状况有关。本文的研究思路与结果有助于在有限的城区土地资源上合理规划绿地建设,构建可持续的人类宜居环境。  相似文献   

多模型耦合下的植被冠层可燃物含水率遥感反演     

全兴文  何彬彬  刘向茁  廖展芒  邱实  殷长明 《遥感学报》2019,23(1):62-77

植被冠层可燃物含水率FMC(Fuel Moisture Content)是评估野火风险及估算火灾蔓延速率的重要指标。以中国西部6个典型研究区为例,基于辐射传输模型,利用Landsat 5 TM及Landsat 8 OLI遥感数据,开展草原、森林冠层FMC定量反演研究。为克服基于物理模型的病态反演问题、FMC自身的弱敏感性问题及西南森林多具复杂的双层冠层结构问题,研究中考虑了模型参数之间的相关特征,使用多波段遥感数据及耦合辐射传输模型等方法。反演结果显示,总体植被冠层FMC反演精度R~2为0.64,RMSE为44.86%,其中草地冠层FMC的反演精度(R~2=0.64,RMSE=47.57%)略低于森林冠层FMC的反演精度(R~2=0.71,RMSE=30.82%)。为进一步论证该反演结果对野火风险评估的有效性,研究中选取并分析了2011年3月2日于云南大理白族自治州剑川县金华镇金和村森林火灾爆发前、爆发时及灾后该区域植被冠层FMC的变化特征。结果显示,火灾爆发时该地区植被冠层FMC明显低于火灾发生前后(约一月时间)植被冠层FMC,证明了本文FMC反演结果对野火风险评估的有效性。  相似文献   

Effect of canopy openness and meteorological factors on spatial variability of throughfall isotopic composition in a Japanese cypress plantation   总被引：1，自引：0，他引：1       下载免费PDF全文

Xinchao Sun  Yuichi Onda  Akiko Hirata  Hiroaki Kato  Takashi Gomi  Xueyan Liu 《水文研究》2018,32(8):1038-1049

Spatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio‐temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre‐thinning (from July to November 2010) and post‐thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ¹⁸O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre‐ and post‐thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ¹⁸O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage.  相似文献   

High-resolution mapping of forest canopy height using machine learning by coupling ICESat-2 LiDAR with Sentinel-1, Sentinel-2 and Landsat-8 data     

《International Journal of Applied Earth Observation and Geoinformation》2020

Forest canopy height is an important indicator of forest carbon storage, productivity, and biodiversity. The present study showed the first attempt to develop a machine-learning workflow to map the spatial pattern of the forest canopy height in a mountainous region in the northeast China by coupling the recently available canopy height (H_canopy) footprint product from ICESat-2 with the Sentinel-1 and Sentinel-2 satellite data. The ICESat-2 H_canopy was initially validated by the high-resolution canopy height from airborne LiDAR data at different spatial scales. Performance comparisons were conducted between two machine-learning models – deep learning (DL) model and random forest (RF) model, and between the Sentinel and Landsat-8 satellites. Results showed that the ICESat-2 H_canopy showed the highest correlation with the airborne LiDAR canopy height at a spatial scale of 250 m with a Pearson’s correlation coefficient (R) of 0.82 and a mean bias of -1.46 m, providing important evidence on the reliability of the ICESat-2 vegetation height product from the case in China’s forest. Both DL and RF models obtained satisfactory accuracy on the upscaling of ICESat-2 H_canopy assisted by Sentinel satellite co-variables with an R-value between the observed and predicted H_canopy equalling 0.78 and 0.68, respectively. Compared to Sentinel satellites, Landsat-8 showed relatively weaker performance in H_canopy prediction, suggesting that the addition of the backscattering coefficients from Sentinel-1 and the red-edge related variables from Sentinel-2 could positively contribute to the prediction of forest canopy height. To our knowledge, few studies have demonstrated large-scale vegetation height mapping in a resolution ≤ 250 m based on the newly available satellites (ICESat-2, Sentinel-1 and Sentinel-2) and DL regression model, particularly in the forest areas in China. Thus, the present work provided a timely and important supplementary to the applications of these new earth observation tools.  相似文献