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81.
Heat flux data collected from the Baiyangdian Heterogeneous Field Experiment were analyzed using the footprint method. High resolution (25 m) Landsat-5 satellite imaging was used to determine the land cover as one of four surface types: farmland, lake, wetland, or village. Data from two observation sites in September 2005 were used. One site (Wangjiazhai) was characterized by highly heterogeneous surfaces in the central area of the Baiyangdian: lake/wetland. The other site (Xiongxian) was on land with more uniform surface cover. An improved Eulerian analytical flux footprint model was used to determine “source areas” of the heat fluxes measured at towers located at each site from surrounding landscapes of mixed surface types. In relative terms results show that wetland and lake areas generally contributed most to the observed heat flux at Wangjiazhai, while farmland contributed most at Xiongxian. Given the areal distribution of surface type contributions, calculations were made to obtain the magnitudes of the heat flux from lake, wetland and farmland to the total observed flux and apportioned contributions of each surface type to the sensible and latent heat fluxes. Results show that on average the sensible heat flux from wetland and farmland were comparable over the diurnal cycle, while the latent heat flux from farmland was somewhat larger by about 30-50 W m-2 during daytime. The latent and sensible fluxes from the lake source in daytime were about 50 W m-2 and 100 W m-2 less, respectively, than from wetland and farmland. The results are judged reasonable and serve to demonstrate the potential for flux apportionment over heterogeneous surfaces. 相似文献
82.
内蒙古农牧交错带草地生产力对气候要素的响应——以多伦县为例 总被引:2,自引:0,他引:2
利用内蒙古农牧交错带多伦县50余a的气候资料,基于Miami和Thornthwaite Memorial气候模型,分析了该地区草地生产潜力对气候要素的响应。结果表明,Thomthwaite Memorial模型更能反映草地生产潜力随气候要素的变化规律。温度升高和降水增加均对当地草地生产潜力起正作用。其中,相对温度而言,降水对草地生产潜力的限制作用更为明显。另外,利用多伦县1999~2002年的畜牧业统计资料,对天然草地和人工草地的有效生产潜力和实际产量作了对比,其中天然草地的草地生产潜力开发度很低,仅为26%,具有很大的开发空间。人工草地的实际产量和草地生产潜力开发度相对更高,显示了它作为提高该地区牧草产量有效途径的可能性。最后通过对比多伦县2000~2002年的理论和实际载畜量,提出该地区草原处于高超载状态,平均超载率高达79.3%。 相似文献
83.
Estimation of the Threshold Friction Velocities over Various Dust Storm Source Areas in Northwest China 总被引:1,自引:0,他引:1 下载免费PDF全文
The emission of dust particles into the atmosphere is governed by the aerodynamic and resistant factors,which are quantified by the friction velocity u* and the threshold friction velocity u*t, respectively. The threshold friction velocity u*t influences the vertical dust flux and dust transport. Based on the micrometeorological data obtained in the springs of 2004 and 2006 over Hunshandake desert area, Loess Plateau,and Gobi desert area, the relationship between dust concentration and friction velocity for the dust events that occurred over Hunshandake desert area was investigated, and the threshold friction velocities over the
three different dust source areas were estimated. The results show that the value of dust concentration is low during the preemission stage of a dust storm event, and the rapid increase of friction velocity provides favorable dynamic conditions for dust emission. During the dust emission stage, the dust concentration increases sharply due to mechanical and thermal turbulent mixing. At the calm-down stage, the dust concentration drops nearly linearly with the decreasing friction velocity, on account of the gravitational deposition of larger dust particles. When the dust concentration is higher than 200 μg m-3, it is considered as a dust emission
process. According to the criteria, the values of threshold friction velocity over Hunshandake desert area and Gobi region are 0.6 and 0.45 m s-1, respectively. The threshold friction velocity over Loess Plateau depends on the wind direction, due to the complex terrain and inhomogeneous surface. The northwest wind shows the effects of the Mu Us desert in the northwest. The corresponding u*t is 0.35 m s-1. The south wind exhibits the characteristics of the Loess hilly dunes in the south, and the u*t is 0.7 m s-1.The large roughness length of the Loess hilly dunes and the large inter-particle cohesion for the clay soil texture increases the local friction velocity. Different threshold friction velocities and occurrence frequencies of strong wind account for different dust emission capabilities for various source areas. 相似文献
84.
地面倒槽、华北地形槽和地面弱高压是天津冬季雾日多见的地面气压系统。为了解雾事件在上述三种天气系统下近地面层气象要素的演变规律,利用天津市250m气象铁塔梯度观测和常规气象观测资料,分别选取2002、2003和2004年相应气压场下的雾个例,比较分析了冬季雾天近地面层结构及低层水汽分布特征。结果表明:(1)三种天气系统条件下,均存在近地层逆位温层结和增湿现象,近地面40m以下高度为弱风。(2)地面倒槽形势下的平流雾过程中,逆温层结稳定且厚达千米,近地层呈多层逆温或弱逆位温层结;80m以上,雾前风力较强,雾中风力较弱;低空各层水汽显著上升时间提前于起雾时间约15h,且日夜增速持续均匀,雾中呈现出逆湿特征,雾顶超过250m。(3)华北地形槽和地面弱高压下的辐射雾过程中,日落后近地面浅薄逆温层结生成并于05时(北京时间,下同)左右达最强,日出后减弱,于11时左右消散;仅夜间近地层水汽显著增加,且塔层250m逆温强度达到3.0℃时才开始出现,距起雾时间约2~9h;雾形成后,逆温层底抬升,雾体中逐渐演变呈不稳定层结;雾中呈现下湿上干特征,雾厚分别为80m和60m左右。(4)华北地形槽和地面弱高压下的风廓线演变规律有显著差异,即前者80m以上6m.s-1左右南风和北风呈规律性日变化转换特征,而后者250m低层大气恒为弱风控制。 相似文献
85.
The effect of water temperature on gut mass and digestive enzyme activity in sea cucumber Apostichopus japonicus, including relative gut mass (RGM), amylase, lipase, pepsin and trypsin activities were studied at temperatures of 7, 14,
21, and 28°C over a period of 40 days. Results show that RGM significantly decreased after 40 days at 21°C and markedly decreased
over the whole experiment period at 28°C; however, no significant effect of duration was observed at 7 or 14°C. At 14°C, trypsin
activity significantly decreased over 10 and 20 days, then increased; amylase and trypsin activity significantly decreased
after 40 days at 28°C. However, no significant effect of duration was found on amylase, pepsin or trypsin activities in the
other temperature treatment groups. At 28°C, lipase activity peaked in 20 days and then markedly decreased to a minimum at
the end of the experiment. On the other hand, pepsin activity at 28°C continuously increased over the whole experimental period.
Principle component analysis showed that sea cucumbers on day 40 in the 21°C group and in the previous 20 days in the 28°C
group were in the prophase of aestivation. At 28°C, sea cucumbers aestivated at 30–40 days after the start of the experiment.
It is concluded that the effect of temperature on the digestion of A. japonicus is comparatively weak within a specific range of water temperatures and aestivation behavior is accompanied by significant
changes in RGM and digestive enzyme activities. 相似文献
86.
基于案例推理的统计地图表示方法智能选取中的应用 总被引:1,自引:1,他引:0
地图符号的选择在空间信息可视化中至关重要。本文基于案例推理的研究范式和专题制图过程,提出了一种智能化专题制图的CBR框架,它依赖的知识主要是系统所存储的专题地图设计过程中解决问题的具体记录。本文从案例库内容设计、案例表达和数据库模式、案例相似性计算和案例检索等方面重点研究了CBR在统计地图表示方法智能选取中的应用,并以《中国人口地图集》为蓝本,以myCBR为工具,展现了CBR在专题地图设计知识获取、表达和推理中的应用潜力。 相似文献
87.
Hongsheng Zhao 《Celestial Mechanics and Dynamical Astronomy》1999,73(1-4):187-194
Tremendous progress has been made recently in modelling the morphology and kinematics of centers of galaxies. Increasingly
realistic models are built for central bar, bulge, nucleus and black hole of galaxies, including our own. The newly revived
Schwarzschild method has played a central role in these theoretical modellings. Here I will highlights some recent work at
Leiden on extending the Schwarzschild method in a few directions. After a brief discussion of (i) an analytical approach to
include stochastic orbits (Zhao 1996), and (ii) the "pendulum effect" of loop and boxlet orbits (Zhao, Carollo, de Zeeuw 1999),
I will concentrate on the very promising (iii) spectral dynamics method, with which not only can one obtain semi-analytically
the actions of individual orbits as previously known, but also many other physical quantities, such as the density in configuration
space and the line-of-sight velocity distribution of a superposition of orbits (Copin, Zhao & de Zeeuw 1999). The latter method
also represents a drastic reduction of storage space for the orbit library and an increase in accuracy over the grid-based
Schwarzschild method.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
88.
89.
提出了渐进式简化建筑物群目标的算法,该算法考虑了目标之间的分辨距离和密度,能够渐进式得到任意比例尺的地图,可以从几何和语义两方面最大限度地保持原有建筑物群的特征,同时可保持综合前后不同分区之间的密度差异。 相似文献
90.
采空区积水透水事故是龙岩市地方小煤矿主要水害,透水事故的典型实例表明有法不依,有章不循的违法行为,是地方小煤矿事故多发的根源;只顾眼前利益,无序开采,是发生采空区积水透水事故的重要原因.因此,加强管理力度、提高管理人员素质是防治矿井水害发生的对策. 相似文献