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

ZHU Hao  ZHANG Hongsheng 《Acta Meteorologica Sinica》2010,24(5):548-557

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   

天津城区大雾过程近地面层特征研究     

吴彬贵  张宏升  张长春  朱好  王兆宇 《气候与环境研究》2010,15(2):179-190

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   

Building Galaxy Models with Schwarzschild Method and Spectral Dynamics     

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   

论水致地震   总被引：2，自引：0，他引：2  

赵洪声  王世芹 《华南地震》1994,14(4):73-77

通过对近３０余年来世界上一些Ｍｓ≥６级水库地震的发震特点分析和涝旱剧变与中国Ｍｓ≥７级大震的相关研究，以及１９９２年美国洛杉矶７．５级地震的水—震过程介绍，充分证实了在地壳岩石长期缓慢运动的基础上，地表水和地下水短期内的大面积大幅度涨落，很可能是孕震区应变非线性增长并驱使孕震体进入不稳定状态乃至失稳的重要原因。同时表明，水致地震关系可能普遍存在于全球各地。  相似文献   

基于加权网格简化的建筑物群渐进式典型化     

郭庆胜  李洪省  刘纪平 《武汉大学学报(信息科学版)》2005,30(4):293-296

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   

小煤矿采空区积水透水事故原因及对策     

何洪生 《中国煤田地质》2005,17(Z1):71-73

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   

应力场演化与强震间相互作用的有限元模拟研究     

王建国  董洪军  陈化然  陈连旺  马宏生 《国际地震动态》2006,(12):24-30

针对川滇地区,在大量搜集前人资料和结果的基础上,建立了包括上地壳、中地壳、下地壳、上地幔等4层结构的三维有限元模型。通过反复调试,确定了其边界条件。然后计算了川滇地区背景应力场、断层蠕动产生的应力场和强震触发的应力场,以及它们的动态变化。结果表明后续地震大多发生在前面地震引发的库仑破裂应力正值区,前一地震对后续地震有一定的触发作用,强震是在较高的应力背景下成组发生的。这对判断、识别地震异常,提高地震预测水平有着很大的意义。  相似文献   

超声风温仪测温的误差订正   总被引：3，自引：0，他引：3  

张宏升  陈家宜 《大气科学》1998,22(1):11-17

利用同步进行的风速、温度、湿度湍流观测资料,对超声风温仪温度测量结果作了水平风速和湿度的订正。结果表明:由于超声风温仪测温受空气湿度和水平风速的影响,对其作相应的订正是十分必要的。|z/L|＜1时,湿度和风速对超声风温仪温度方差和感热通量测量值所引起的误差是不可忽视的。而在|z/L|＞1区间,仅计入湿度影响,已够精确。对于温度谱密度,当nSθ(n)/σ2θ低于0.01时,有一高频的噪声频率阈值,高于此频率,nSθ(n)/σ2θ与无因次频率f呈+1次幂关系。  相似文献   

浙江长兴地区长兴组放射虫化石及其指相意义   总被引：4，自引：0，他引：4  

杨美芳  李继亮 《地质科学》1997,32(2):229-232

浙江省长兴地区的长兴组煤山剖面一直作为中国乃至世界研究二叠系-三叠系界线的经典剖面之一。在该剖面,长兴组主要由灰黑、黑色碳酸盐组成,厚度约为30-45m。依据沉积岩性特征大致可划分上下2段:下段以灰黑或黑色的纹层状粒泥岩和泥粒岩为主,厚度约13m;上段由薄一中层状灰黑或黑色泥粒岩和粒泥岩组成,其中夹有极薄层的黑色钙质泥岩或泥岩和细条带状或透镜状硅质岩,厚27m左右。  相似文献   

多波束声纳计量测试系统设计     

柳义成  韩鸿胜  窦春晖  陈允约 《海洋测绘》2019,39(3):63-66

为满足多波束声纳量值溯源与传递需求,设计多波束声纳关键参数的计量测试系统。基于大比尺原型深水港池和多维运行控制机构,通过水下横向测距代替垂向测深的方式对多波束声纳进行了测深准确度与有效条带宽度的计量测试。阐述了测试系统的主要组成和测试方法,给出了本测试系统测深结果的扩展不确定度。参考标准值与示值(或标称值)比对结果表明,被检多波束声纳测深示值误差小于0.2%,条带宽度误差小于7%。为规范水下声纳设备技术指标校准,指导多波束声纳计量标准建设提供参考。  相似文献